Chapter III
Reading edition
SURVEYING METHODS AND PRINCIPAL GEOGRAPHICAL RESULTS.
Triangulation.
Base-Lines—The triangulation was commenced by measuring a base line[62] near Gebel Muelih by means of a 100-metre steel tape, which had been previously standardised at the Khedivial Observatory. A level tract having been selected, in such a position as to afford easy connection with points already triangulated from the Nile Valley, a line about two and a half kilometres long was ranged out, along which wooden pegs were driven flush with the ground at 100-metre intervals. The inequalities of the ground between the pegs were levelled off by tiny embankments and cuttings, so as to enable the tape to lie flat. On each peg was nailed a zinc plate having a millimetre scale running in the direction of the line. In measuring, the tape was laid on the ground, stretched to a constant tension by a spring balance at each end, in such manner that its end marks fell on the zinc scales, and readings were simultaneously taken on the scales at the two ends. Temperatures were taken in several places along the tape by mercurial thermometers. The operation of measurement was carried out in the early morning, so as to avoid any large difference of temperature between the ground and the air. The levels of the pegs were found by spirit levelling. After correction for the initial error of the tape, temperature, stretch, and inclination, the true length of the base-line reduced to sea-level was found, as the mean of two separate measurements at different tensions, to be 2,482·280 metres. The azimuth of the line was found by observations of Polaris at elongation to be 30° 30′ 6″ E. of N. The geographical position of the west end of the line was found by triangulation-connexion with the Nile Valley by Mr. Villiers Stuart to be latitude 24° 53′ 36″·7 N., longitude 34° 4′ 17″·9 E.
A second connexion to a base-line was made near Gebel Um Harba, where a base had been previously measured by Mr. Villiers Stuart in the course of his triangulation of the western part of the desert. In this case the connexion was not made to the actual base-line, but to another main line tied directly on to it. The data at this point of connexion from the two triangulations afforded a useful check on the accuracy of the work, and were as follows:—
From Mr. Stuart’s triangulation. From my triangulation. Difference.
Latitude N. 23° 36′ 55″·0 23° 36′ 55″·6 0″·6
Longitude E. 34° 30′ 38″·1 34° 30′ 37″·6 0″·5
Length of line to Dagalai beacon 13,170·2 metres. 13,167·9 metres. 2·3 metres.
Reconnaissance for triangulation points was carried out simultaneously with the triangulation itself. The distance of likely looking peaks was determined either by intersecting them from distant stations, or by special small triangulations from short bases, and such selection made as seemed most likely to secure well shaped figures and a good command of surrounding country. As a rule, the highest summits were selected as main (occupied) stations, while all other prominent peaks and other features were fixed by intersection from two or more main stations. The form adopted for the main triangulation net was a series of quadrilateral figures with diagonals, combined with centric polygons, all the angles of the figures being generally measured. The average length of side was about thirty-five kilometres.
Beacons.—Main stations were marked by wrought-iron beacons, consisting of two lengths of stove piping about 15 centimetres diameter by 1½ metres long, the upper length fitting into a faucet made by splaying out the lower tube. Near the top of the tube were affixed four sheet iron wings, bolted on to angle iron cleats. A conical cairn of stones was built up round the tube, nearly up to the wings, so that the beacon when erected was about three metres high, two metres in diameter across the base of the cairn, and about a metre wide across the wings. The beacons were taken down while a station was being occupied, and replaced on leaving.
Intersected points were sometimes marked with a beacon or cairn, but in general the peaks were simply bisected from several stations, as it was found that this gave sufficiently accurate results.
Measurement of Horizontal Angles.—Angles were measured with a 6-inch theodolite furnished with reading microscopes graduated to 10″ and permitting of reliable estimations to 1″. The angles between main points were read on four arcs to eliminate circle errors. Intersected points were observed on one arc only. The average error of closure of main triangles was 3″·3.
Field Computation and Plotting of Triangulated Points.—The triangles were computed by the ordinary method, but the angles were rounded off to 10″ to enable the sines to be taken direct from the logarithmic tables, and the logarithms were only taken to five places. The length of the sides having been thus found, the geographical positions were found by the ordinary L M Z computation, using, however, only two latitude terms and 5-place logarithms, while azimuths were only taken out to the nearest 10″. The abbreviated form of computation used will be best illustrated by an example:—[63]
Computation of Position of △ No. 260 from No. 275.
l = 20042 m. 275 ⎰ ⎱ φ = 23° 55′ 30″·6
Z = 64° 29′ 20″ E of N. λ = 34° 54′ 36″·9
Log l = 4·30194 Log l2 = 8·604
Log cos Z = 1·63416 Log sin2 Z = 1·911
B 2·51194 C = 9·053
2·44804 = 1·568
dφ, 1st term = + 280″·6 Log l = 4·30194
2nd term = − 0″·4 Log sin Z = 1·95545
dφ = 280″·2 A′ = 2·50948
= 4′ 40″·2 = 2·76687
φ = 23° 55′ 30″·6 Log cos φ′ = 1·96072
φ′ = 24° 0′ 10″·8 = 2·80615
dλ = 640″·0
Whence = 10′ 40″·0
260 ⎰ ⎱ φ′ = 24° 0′ 10″·8 λ = 34° 54′ 36″·9
λ′ = 35° 5′ 16″·9 λ′ = 35° 5′ 16″·9
In the above, it will be noticed that the azimuth is always noted as so much east or west of north or south. If this convention be adopted, one may consider the first term of dφ as always +, and the second term will be + if the azimuth contains the word south,—if it is from the north, while the total dφ is to be added or subtracted according as one is going north or south. A somewhat similar convention is adopted in neglecting the sign of dλ till the actual addition or subtraction is made. It was found in the field that this method prevented any mistake of sign, while being much simpler to work than one involving angles greater than 90°.
The geographical coordinates thus found were plotted directly on to the plane-table sheets, on which the graticule at 10′ intervals was the first thing drawn. The odd minutes and seconds were first converted into minutes and decimals, and then into kilometres by multiplying by the factors appropriate to the latitude, so that the plotting could be done by the ordinary scale of kilometres. To avoid difficulties of paper-shrinkage, as many points as possible were plotted at the time of drawing the graticule, and in general the points had to be plotted as far ahead as possible for controlling the traversing and sketching.
Astronomical Observations.—Astronomical checks on the triangulation were obtained by observations of latitude at certain selected main stations 60-120 kilometres apart, and by azimuth observations for certain main lines.
The method used for latitude was that of observing the times of equal altitudes of three or more stars, selected as near to the meridian[64] as possible. This method presents great advantages over the usual Polaris and circummeridian altitudes, in that the observations are more easily made, and yield much more accurate results, because uncertainties in refraction are largely eliminated and the errors of circle graduation are not involved, the altitudes not being read at all.[65] The theodolite used was the same as was employed in triangulation, and the times were taken by a half chronometer watch, preferably one marking sidereal time with a rate which could be considered negligible during the hour or so occupied by the observation. The first star taken was usually Polaris, and the vertical circle was left clamped at its altitude. For the other stars, any dislevelment was corrected by touching up the levelling screws just before the instant of observation; this was found better, than taking bubble readings and correcting for slight difference of altitude.
The method which I found best in the field for reducing the observations differs somewhat from that described by Chauvenet. Assuming approximate values for the latitude and watch error, I first calculated the altitude of each star from the formula
sin h = sin φ sin ε + cos φ cos δ cos t
If the assumed latitude and watch error were correct, all the stars would give the same value for h. If not, each star would give an equation of the form
h + cos A dφ + cos φ sin A dT − h0 = 0
where A is the star’s azimuth, dφ the required correction to the assumed latitude, dT the required correction to the assumed watch times, and h0 the true altitude common to the three stars. The values of cos A and cos φ sin A were calculated from the ordinary formula
sin A = cos δ sin tcos h
by four-place logarithms (using the approximate values for φ, t, and h, since these are quite sufficiently accurate for the purpose) and inserted into the three star-equations.[66] By then solving the three simultaneous equations for dφ, the required correction to the assumed latitude was at once obtained.
As the method is one not usually treated of in books on practical astronomy, I give on the following pages the reduction of an observation worked out in full.
Latitude by Equal Altitudes of Three Stars.
Station on Gebel Um Heshenib. January 30, 1906.
Approximate φ = 24° 20′ 50″ N.
„ λ = 34° 51′ 0″ E.
Sidereal watch U. and C. 30811, approximately 2m 22s fast on L.S.T., rate negligible.
Observed times of equal altitudes by watch:—
Polaris 3h 33m 54s·2
α Columbæ 3 42 40 ·8
ε Canis majoris 4 24 35 ·8
Polaris.
Watch time 3h 33m 54s·2
Watch fast 2 22 ·0
L.S.T. 3 31 32 ·2
Star’s R.A. 1 25 9 ·1
t 2 6 23 ·1 = 31° 35′ 46″·5 W. of meridian.
φ = 24° 20′ 50″ log sin 1·6151769
δ = 88° 48′ 33″·1 log sin 1·9999062
1·6150831 log cos δ 2·3177
Nat. (1) 0·4121764 log sin t 1·7193
2·0370
φ = 24° 20′ 50″ log cos 1·9595488 log cos h 1·9560
δ = 88° 48′ 33″·1 log cos 2·3176870 log sin A 2·0810
t = 31° 35′ 46″·5 log cos 1·9303179 log cos φ 1·9595
2·0405
2·2075537 Nat. = 0·011
Nat. (2) 0·0161270
Nat. (1) 0·4121764 log cos A 1·9999
0·4283034 Nat. = 1·000
log sin h 1·6317515
h = 25° 21′ 35″·8
Whence the equation for Polaris is
35·8 + 1·000 dφ − 0·011 dT − h0 = 0 (1)
α Columbæ.
Watch time 3h 42m 40s·8
Watch fast 2 22 ·0
L.S.T. 3 40 18 ·8
Star’s R.A. 5 36 15 ·6
t 1 55 56 ·8 = 28° 59′ 12″·0 E. of meridian.
φ = 24° 20′ 50″ log sin 1·6151769
δ = 34° 7′ 45″·7 log sin 1·7490118
1·3641887
Nat. (1) 0·2313070 log cos δ 1·9179
log sin t 1·6854
φ = 24° 20′ 50″ log cos 1·9595488 1·6033
δ = 34° 7′ 45″·7 log cos 1·9179112 log cos h 1·9560
t = 28° 59′ 12″·0 log cos 1·9418753 log sin A 1·6473
1·8193353 log cos φ 1·9595
Nat. (2) 0·6596830 1·6068
Nat. (1) 0·2313070 Nat. = 0·404
0·4283760
log sin h 1·6318196 log cos A 1·9523
Nat. = 0·896
h = 25° 21′ 51″·1
Whence the equation for α Columbæ is
51·1 − 0·896 dφ + 0·404 dT − h0 = 0 (2)
ε Canis majoris.
Watch time 4h 24m 35s·8
Watch fast 2 22 ·0
4 22 13 ·8
Star’s R.A. 6 54 57 ·1
t 2 32 43 ·3 = 38° 10′ 49″·5 E. of meridian.
φ = 24° 20′ 50″ log sin 1·6151769
δ = 28° 50′ 52″·7 log sin 1·6834857
1·2986626
Nat. (1) 0·1989128
φ = 24° 20′ 50″ log cos 1·9595488
δ = 28° 50′ 52″·7 log cos 1·9424561 log cos δ 1·9424
t = 38° 10′ 49″·5 log cos 1·8954602 log sin t 1·7911
1·7974651 1·7335
Nat. (2) 0·6272853 log cos h 1·9560
Nat. (1) 0·1989128 log sin A 1·7775
0·4283725 log cos φ 1·9595
log sin h 1·6318215 1·7370
Nat. = 0·546
h = 25° 21′ 51″·5
log cos A 1·9034
Nat. = 0·801
Whence the equation for ε Canis majoris is
51·5 − 0·801 dφ + 0·546 dT − h0 = 0 (3)
Collecting the equations of the three stars, we have
35·8 + 1·000 dφ − 0·011 dT − h0 = 0 (1)
51·1 − 0·896 dφ + 0·404 dT − h0 = 0 (2)
51·5 − 0·801 dφ + 0·546 dT − h0 = 0 (3)
By solving these equations for dφ, we find
dφ = + 6″·5
whence the latitude is found to be
24° 20′ 56″·5
It may be remarked that the above process was considerably shortened when it was possible to get a pair of observations on the same south star both east and west of the meridian, instead of on two separate south stars. In that case the watch error was found at once from the difference between the star’s R.A. and the mean of the two observed times, and the latitude could be found from two equations instead of three. The condition for this modification of the method was that a nautical almanac star could be found culminating at an altitude slightly greater than that of Polaris at a time convenient for the observation. It is not advisable to select a star which would give too long an interval between the equal east and west altitudes. The best results are obtainable when the interval between the two observations of the south star is about an hour, and when Polaris is near its transit. Under such circumstances the watch correction is obtained quite nearly enough for a good latitude; for, as Gauss[67] pointed out, “the essential condition is not so much that the precise instant when the star reaches a supposed place should be noted, as that at the time which is noted the star should not be sensibly distant from that place.”
The following table shows the latitudes found by observation and triangulation at the various points. It may be remarked that the method used for latitude determination was liable to observational errors of 2″ or so, as well as to errors of possibly more than double that amount due to plumb-line deflection among the mountains,[68] so that the observed latitudes were only taken as checks to prevent any gross error in triangulation being overlooked, and not for any determination of the figure of the earth, for which latter purpose more elaborate observations would have been necessary.
Point. Lat. observed. Lat. computed from Triangulation. Difference computed-observed.
West Peg, Muelih Base 24° 53′ 40″·3 24° 53′ 36″·7 − 3″·6
Beacon on Gebel Um Heshenib 24° 20′ 56″·5 24° 20′ 49″·2 − 7″·3
„ „ Hill near Gebel Selaia 23° 55′ 33″·2 23° 55′ 30″·6 − 2″·6
„ „ Berenice Temple 23° 54′ 39″·5 23° 54′ 40″·3 − 0″·8
„ „ Gimeida Hill 22° 46′ 33″·2 22° 46′ 29″·4 − 3″·8
Azimuths were determined in the usual manner[69] by elongations of close circumpolar stars, Polaris or 51 Cephei being usually selected. The azimuth mark used was an ordinary Egyptian shamadan (candlestick with spring feed) with a glass globe, placed at a distance of one to two kilometres, with its foot firmly bedded in sand and stones to prevent any motion. The azimuths observed at the different stations are shown in the following table:—
Station of Observation. Point to which Azimuth is given. Azimuth observed.
Peg at West end of Muelih Base Peg east end of base 33° 30′ 6″ E. of N.
Beacon on Gebel Um Heshenib Beacon on Gebel Hamata 45° 15′ 34″ E. of S.
„ „ Hill near Gebel Selaia „ „ Abu Gurdi 25° 30′ 35″ N. of E.
„ „ Berenice Temple „ „ Kalalat 61° 10′ 31″ W. of S.
„ „ Gimeida Hill „ „ Hamra Dom 6° 14′ 46″ E. of S.
Centre of Halaib Fort „ „ Elba 83° 25′ 0″ W. of S.
The observed azimuths, unlike the observed latitudes, were more accurate than the results of triangulation, repetition having shown them to be reliable within 2″ or 3″, an error of which magnitude would soon be surpassed in the process of continuing a chain of azimuths with the unadjusted values of the angles of the triangles, which were the only values possible to be used in the field. On arriving at an azimuth station, therefore, a fresh chain of azimuths was begun from the results of the observation, and continued to the next station where astronomical observations were undertaken. The accumulated azimuth error was, however, never found to exceed 10″ in any chain, a quantity which could not sensibly affect the computed positions of points for plotting on the maps.
Connexion with the Sudan Surveys.—At the south end of the area, connexion was made to a number of points triangulated by the Sudan Surveys, but as the Sudan triangulation was commenced as an independent piece of work from an observed latitude and a telegraphically determined longitude, the connexion affords no check on the accuracy of the triangulations. The difference found between my positions and those of the Sudan Surveys was practically constant for all the Sudan points connected, and amounted to 3″·5 in latitude and 26″ in longitude[70]; these figures represent the errors in the assumed latitude and longitude of the starting point of the Sudan Surveys, and will be employed as corrections to the Sudan positions now that a complete chain of triangulation connects Berber with the Mediterranean.
Levels of Triangulation Points.—The altitudes above sea-level of all triangulation points were determined by vertical angular measurements carried out at the occupied stations, an actual sea-level datum being obtained by including rocks awash in the sea among the triangulated points. To secure constancy of atmospheric refraction as far as possible, vertical angles were always read in the middle of the day, where the change of refraction is slowest. For the occupied stations, refraction and curvature were eliminated by reciprocal observations. For intersected points the formula h = d tan θ + 1 − k2rd2 was used, the value of d tan θ being first found by five-figure logarithms and then that of the curvature and refraction correction 1 − k2rd2 by means of the very convenient “Universal” slide rule of Nessler.[71] The value of k found from a discussion of the first few reciprocal observations was found to be very nearly 0·13, corresponding with the mean of European determinations, and this value for the coefficient was employed throughout the work for intersected points.[72] For obtaining the correction 1 − k2rd2 by the slide rule, a mark R was scratched at 1210[73] on the lower scale of the slide; by bringing this mark R opposite to the distance (in kilometres) on the lower scale (or, where the logarithm of the distance was more convenient, by bringing the mark vertically under that logarithm on the log scale of the rule by means of the cursor) the correction could be read off directly on the lowest fixed scale opposite the end-graduation of the slide. Usually four or five values for the altitude of a single point were obtained from a corresponding number of stations, and the mean taken; the various values generally agreed within two metres.
The constant combination of vertical angular measurements with horizontal ones was of great service from another point of view from that of providing altitude data for the maps. It frequently happened that a peak observed at one station could not be identified among a number of similar peaks visible at another station. When this trouble arose, the vertical angles offered a way out of the difficulty. Vertical and horizontal angles were read off to a number of likely-looking peaks; on working out the triangles to the nearest minute of the observed angles, the distance of each peak was obtained on the assumption that it was the one required. Then to find which of the several peaks was the correct one, the elevations were worked out, assuming the distances correct; in only one case would the level agree from the two stations, and this obviously discriminated the peak required. The working out of the triangles for this purpose could be done with sufficient accuracy in a very few minutes by means of the slide rule, and many points were thus saved from rejection consequent on misidentification.
Checks on absolute level were frequently obtained by observing depression angles to the sea horizon, using the formula θ = 107·8 √ h , where θ is in seconds of arc and h is the altitude in metres. But for high stations the horizon is so distant that very small variations in refraction cause rather large errors in the result, so that this method only furnished a rough check.
Summary of Triangulated Positions.
The following tables give the geographical positions and altitudes above sea-level of all points triangulated, arranged in order of diminishing latitude, i.e., from north to south. The list includes the points fixed within the Sudan for connexion with the Sudan Surveys. Stations occupied are indicated by an asterisk against the number of the point. It will be noticed that in some cases several different mountains bear the same name though widely distant from each other; also that where a single mountain mass possesses several peaks or summits, each of these has been fixed separately.
Summary of Triangulated Positions.
Field Number of Point. Name. Mark. Latitude N. Longitude E. Altitude Metres.
° ′ ″ ° ′ ″
21 G. Hamrat Wogud cairn. 25 9 34 34 20 0 1,103
29 G. Iteima „ 25 8 1 34 11 13 849
* 204 G. Igli „ 25 4 6 34 36 16 975
* 26a G. Atut „ 25 0 56 34 23 49 908
* 203 Erf el Fahid „ 25 0 5 34 11 52 579
14 G. Hagar Dungash „ 24 59 12 34 2 33 815
* 218 G. Sukari „ 24 56 50 34 42 50 476
216 G. Um Tundeba summit. 24 55 48 34 47 29 550
* 202 Muelih base E. peg. 24 54 44 34 5 7 406
219 Isolated hill near sea summit. 24 54 23 34 54 21 190
* 201 Muelih base W. peg. 24 53 37 34 4 18 398
19 G. Muelih cairn. 24 52 44 34 0 37 707
* 225 Kurdeman mines „ 24 52 35 34 41 35 525
71 Marwot Rod el Ligah „ 21 51 31 34 8 21 514
206 G. Mudergeg summit. 24 51 0 34 22 1 885
205 G. Hangalia „ 24 50 29 34 38 43 1,241
* 215 G. Ghadir cairn. 24 50 9 34 47 22 636
* 32a G. Nugrus „ 24 48 34 34 35 47 1,505
* 25 G. Migif „ 24 47 23 34 27 30 1,199
* 214 G. Allawi „ 24 46 42 34 49 39 515
211 G. Zabara „ 24 45 21 34 41 53 1,361
224 G. Lewewi „ 24 44 38 34 46 39 654
212 Ridge near G. Zabara N. end. 24 43 48 34 40 56 1,104
221 Hill near sea summit. 24 42 31 35 3 15 —
208 G. Hamrat Selma „ 24 41 58 34 20 58 761
210 Peak in G. Hafafit cairn. 24 40 44 34 37 8 722
240 Wadi Gemal island N. end. 24 40 45 35 9 6 0
* 223 G. Sikait cairn. 24 39 55 34 48 5 771
213 G. Um Moghar summit. 24 39 16 34 41 5 860
24a G. Abu Khrug cairn. 24 38 57 34 16 19 870
20 G. Sufra beacon. 24 38 42 34 4 13 690
241 Wadi Gemal island S. end. 24 38 38 35 10 36 0
207 G. Nahud N. cone. 24 35 36 34 22 14 662
* 209 Peak in G. Hafafit beacon. 24 35 32 34 45 22 744
* 222 Madaret Um Gamil „ 24 34 52 34 56 28 454
226 G. Abu Had summit. 24 34 12 34 36 6 633
242 Low spur of coast tip. 24 33 47 35 10 2 0
235 G. Um Regeba summit. 24 33 36 34 42 29 571
233 G. Nahud S. cone. 24 33 35 34 20 11 662
239 Low spur of coast tip. 24 28 43 35 10 58 0
247 G. Um Suerab summit. 24 26 50 34 42 33 1,024
231 G. Um Sueh „ 24 26 45 34 54 30 781
236 G. el Abiad „ 24 26 27 34 48 56 892
* 227 G. Abu Hegilig „ 24 26 16 34 58 32 607
237 G. Um el Abbas „ 24 26 11 34 56 33 697
232 G. Nukheira „ 24 25 58 34 32 53 876
73 G. Um Goraf cairn. 24 25 33 34 18 28 —
252 G. Durunkat centre. 24 23 43 34 45 51 924
274 S. end of low sandy island — 24 22 41 35 22 56 0
255 G. Um Sedri S. peak of twin. 24 22 0 34 41 3 970
234 G. el Heda summit. 24 20 56 34 30 11 862
248 G. Sarobi (S. peak) „ 24 20 50 35 9 0 471
* 228 G. Um Heshenib beacon. 24 20 49 34 50 53 1,135
243 G. Hefeiri summit. 24 20 22 35 1 21 612
246 G. Khulla „ 24 19 43 34 38 42 978
253 G. Tarfawi N. end of ridge. 24 18 46 34 55 54 1,363
251 G. Marasan summit. 24 17 34 34 44 56 1,261
249 G. Abu Ghusun „ 24 16 1 34 58 17 1,389
245 G. Metawit „ 24 15 51 34 31 48 741
* 230 G. Abu Hamamid beacon. 24 14 41 34 47 38 1,747
250 G. Um Usher summit. 24 14 11 34 53 21 1,487
259 G. Um Hasidok „ 24 12 45 34 54 46 1,497
* 229 G. Hamata beacon. 24 12 17 35 0 16 1,978
258 G. Abarun summit. 24 11 20 34 50 18 1,602
257 G. Abu Argub „ 24 11 1 34 45 43 1,609
75 G. Hamrat Mukbud cairn. 24 9 53 34 23 17 890
254 G. Ras el Kharit peak. 24 9 25 35 1 55 1,661
261 G. Khashir summit. 24 9 14 35 4 50 1,565
262 G. Ras el Kharit peak. 24 9 2 35 0 28 1,564
* 256 G. Kahfa beacon. 24 8 18 34 38 55 1,018
* 399 Hill near Bir Qoleib beacon. 24 8 6 33 42 46 355
400 Bir Qoleib clay basin. 24 6 36 33 41 20 239
287 G. el Anbat summit. 24 5 12 34 54 58 788
263 G. Mikbi S. end of ridge. 24 4 54 35 4 49 1,388
276 G. Um Huk cairn. 24 4 41 35 15 34 517
279 G. Um Sellim E. peak. 24 3 52 35 9 51 947
320 Peak N. of Berenice summit. 24 3 41 35 26 30 276
264 G. Egat peak. 24 3 40 35 3 32 1,277
273 G. Egat summit. 24 2 52 35 2 29 1,422
280 G. Abu Ghalqa cairn. 24 1 7 35 16 47 561
* 260 G. Abu Gurdi beacon. 24 0 11 35 5 17 1,562
288 G. Derhib summit. 24 0 8 35 1 29 1,160
* 315 Hill near Berenice beacon. 24 0 4 35 30 37 196
268 G. el Homur summit. 23 58 54 34 55 16 731
289 G. Um Gunud cairn. 23 57 46 35 12 10 989
* 339 Limestone hill on Ras Benas beacon. 23 57 16 35 43 10 187
271 G. Selaia summit. 23 57 14 34 52 13 787
* 317 Hill on Ras Benas beacon. 23 56 40 35 39 44 193
* 316 Limestone peak, Ras Benas „ 23 56 36 35 36 9 189
* 338 Hill on Ras Benas cairn. 23 56 1 35 40 53 197
291 G. Aidab summit. 23 55 52 35 13 38 848
270 Hill near G. Selaia „ 23 55 46 34 53 58 623
* 275 Low hill near G. Selaia beacon. 23 55 30 34 54 37 563
278 G. Geneina peak near N. end. 23 54 56 34 47 45 548
336 G. Um Maiat N. end. 23 54 45 35 13 35 928
* 313 Berenice Temple beacon. 23 54 39 35 28 26 8
290 Marwot Elemikan summit. 23 54 14 35 6 8 648
319 Peak N. of G. Kalalat „ 23 54 10 35 13 55 870
334 Hill near Wadi Mindeit „ 23 53 57 35 22 0 —
340 Sheikh’s tomb at Ras Benas centre. 23 53 50 35 47 13 3
341 Tip of Ras Benas sandy point. 23 53 42 35 47 5 0
266 G. Abu Derega summit. 23 53 38 34 59 13 831
335 G. Um Maiat S. end. 23 53 34 35 14 48 842
333 High range N. of G. Kalalat N. end. 23 53 3 35 15 1 875
77 G. el Nekeiba cairn. 23 52 34 34 22 10 570
330 Hill near Wadi Kalalat summit. 23 52 29 35 22 18 328
318 G. Um Maiat central peak. 23 52 24 35 15 36 821
332 High range N. of G. Kalalat S. end. 23 51 59 35 16 29 762
331 G. Kalalat minor peak. 23 50 48 35 15 51 743
294 N. end of Mukawar Island — 23 50 47 35 48 31 0
293 G. Batoga central peak. 23 50 7 35 20 46 802
329 G. Kalalat N. peak. 23 49 57 35 17 6 1,080
294a S. end of Mukawar Island — 23 49 53 35 48 19 0
314 G. Batoga (S. peak) beacon. 23 49 37 35 21 9 785
* 282 G. Kalalat „ 23 49 9 35 17 36 1,125
328 G. Batoga minor peak. 23 48 57 35 22 47 413
353 Minor peak of G. Kalalat S. one of pair. 23 47 37 35 18 46 894
355 G. Um Hegilig summit. 23 47 13 35 14 32 966
327 G. Dibag NW. peak. 23 46 47 35 21 23 517
308 G. Dibag SE. peak. 23 46 40 35 22 4 544
* 265 G. Dahanib beacon. 23 45 44 35 11 10 1,270
* 269 G. Um Bisella „ 23 45 34 34 57 39 824
* 277 G. Zergat Naam „ 23 45 28 34 40 34 823
345 G. Reyan N. peak. 23 45 20 35 17 50 740
346 G. Reyan S. peak. 23 44 7 35 17 20 863
* 343 Hill near Shenshef ruins beacon. 23 44 5 35 22 40 290
292 Erf el Gemal W. end of ridge. 23 43 37 34 52 10 673
272 G. Hagar el Fil summit. 23 43 5 34 42 3 845
281 G. Shut „ 23 42 35 35 16 59 930
398 Bir Abu Hashim principal well. 23 41 56 34 4 26 320
* 397 Hill near Bir Abu Hashim beacon. 23 41 44 34 3 33 386
348 Hill near W. Salib Abiad summit. 23 41 5 35 7 3 703
325 G. Um Etli central peak. 23 39 54 35 23 22 795
384 G. Abu Husenat summit. 23 39 45 35 1 15 725
326 G. Um Etli W. peak. 23 39 32 35 21 52 844
324 G. Um Etli E. peak. 23 39 26 35 23 53 764
295 G. Hendusi sharp peak. 23 39 15 34 58 44 678
344 G. Um Akra beacon. 23 37 58 35 16 41 1,050
* 385 Ruins of Um Eleiga „ 23 37 20 35 3 5 599
302 Peak near G. Abu Dahr summit. 23 37 16 35 6 44 840
* 79 Um Harba beacon. 23 36 56 34 30 38 688
356 G. Um Akra S. peak. 23 36 35 35 16 58 970
347 G. Hindia summit. 23 36 27 35 13 39 873
337 Zeberged Island central peak. 23 36 16 36 11 42 238
* 267a G. Abu Dahr beacon. 23 36 8 35 5 46 1,131
305 G. Abu Sieiyil summit. 23 35 16 35 1 4 833
321 G. Faraid peak. 23 34 8 35 23 18 1,131
323 G. Faraid „ 23 33 29 35 21 50 1,341
361 Hill S. of G. Abu Dahr summit. 23 33 28 35 4 42 912
322 G. Faraid highest peak. 23 33 7 35 22 10 1,366
342 G. Faraid „ 23 31 53 35 22 46 1,068
360 Hill S. of G. Abu Dahr summit. 23 31 43 35 5 15 772
362 Hill S. of G. Abu Dahr „ 23 31 37 35 2 49 793
359 Hill S. of G. Abu Dahr „ 23 31 17 35 6 22 784
* 285 G. Faraid peak. 23 30 53 35 20 25 1,259
* 85 G. Um Khafur beacon. 23 29 54 34 29 19 560
357 G. Faraid minor peak. 23 29 35 35 17 30 862
286 G. Faraid “The Bodkin.” 23 28 58 35 20 35 1,232
350 G. Faraid outlying peak. 23 28 39 35 17 35 875
351 G. Faraid peak. 23 28 35 35 16 31 954
352 Granite peak near Wadi Rahaba summit. 23 27 48 35 13 15 697
349 G. Faraid outlying peak. 23 27 14 35 19 5 904
297 G. Orga beacon. 23 26 21 35 8 17 682
363 Hill near Wadi Rahaba summit. 23 26 18 35 12 38 506
* 296 G. Abraq beacon. 23 25 19 34 46 48 667
365 Hill near Wadi Rahaba summit. 23 25 2 35 11 42 536
389 G. Faraid minor peak. 23 24 57 35 24 42 585
358 G. Faraid peak. 23 22 39 35 22 19 916
395a G. Abraq „ 23 22 26 34 50 18 699
395 G. Abraq „ 23 22 19 34 50 4 705
* 87 G. Awamtib beacon. 23 20 59 34 26 39 793
* 299 G. Um Tenedba „ 23 19 48 35 10 40 656
368 Hill near G. Um Tenedba summit. 23 18 43 35 12 36 555
* 390 Plateau near Abu Saafa beacon. 23 18 40 34 48 30 639
307 G. Saalek peak E. side. 23 18 18 34 31 10 753
* 354a G. Fereyid beacon. 23 17 29 35 22 48 612
394 G. Hodein „ 23 16 20 34 53 25 695
380 G. Hodein corner of scarp. 23 16 18 34 52 5 718
304 Peak W. of G. Um Reit summit. 23 15 26 34 30 14 805
306 Peak W. of G. Um Reit „ 23 15 23 34 31 47 837
366 Hill near Wadi Rahaba „ 23 15 4 35 16 25 420
* 300 G. Um Reit beacon. 23 15 4 34 34 17 857
* 298a G. Harhagit „ 23 14 35 35 12 52 542
393 G. Tibatib summit. 23 12 41 35 1 28 396
387 Hill near Wadi Hodein „ 23 12 24 35 16 51 383
396 Granite peak near Wadi Rahaba „ 23 11 11 35 20 35 355
392 Rock in sea (level datum). 23 10 55 35 35 57 0
310 Isolated hill west of G. Etresia summit. 23 10 35 34 25 23 703
388 Hill near Wadi Hodein „ 23 10 33 35 16 32 309
423 G. Kala „ 23 10 3 34 45 49 808
503 G. Etresia E. summit. 23 9 24 34 30 43 1,037
303 Hill E. of G. Etresia summit. 23 9 15 34 32 23 922
* 375a G. Anfeib beacon. 23 8 24 34 59 19 705
364 Hill NE. of G. el Anbat summit. 23 8 0 35 21 46 359
* 386 Close to Bir Shalatein beacon. 23 7 57 35 36 23 13
502 G. Etresia summit. 23 7 53 34 31 24 1,038
407 G. Kala „ 23 7 42 34 42 58 846
311 Isolated hill north of G. Shigigat „ 23 7 16 34 24 39 901
425 G. Kala „ 23 7 11 34 42 37 783
424 G. Kala „ 23 6 51 34 42 30 739
501 G. Etus „ 23 6 19 34 29 53 997
* 367a G. el Anbat beacon. 23 6 5 35 19 27 390
312 G. Shigigat conspicuous peak. 23 5 37 34 23 39 1,023
406 G. Kala summit. 23 5 16 34 42 19 629
500 G. Aqab el Negum „ 23 3 26 34 27 20 1,149
496 G. Natetiai peak. 23 3 8 34 21 0 1,022
499 Peak near Aqab el Negum summit. 23 2 43 34 26 3 998
498 Peak near Aqab el Negum „ 23 2 22 34 25 36 979
497 Peak near Aqab el Negum „ 23 1 56 34 25 42 974
495 G. Natetiai peak. 23 0 49 34 22 7 1,164
373 G. Niqrub el Tahtani high pinnacle. 23 0 42 35 0 58 828
373a G. Niqrub el Tahtani beacon. 23 0 39 35 0 53 829
* 369a G. Beida „ 23 0 14 35 16 54 716
494 G. Natetiai peak. 22 59 35 34 22 22 977
508 Peak in S. part of G. Feg summit. 22 58 45 34 20 29 885
391 G. Humariai „ 22 58 38 35 9 36 563
382 Hill S. of Wadi Beida „ 22 58 10 35 12 58 631
507 Peak in S. part of G. Feg „ 22 58 2 34 19 15 836
381a Hill S. of Wadi Beida „ 22 57 30 35 16 53 631
379 G. Mismih „ 22 56 56 34 45 32 599
381 Hill S. of Wadi Beida „ 22 56 47 35 17 3 615
510 Conical hill „ 22 54 48 34 40 27 504
* 401 G. Kolaiqo beacon. 22 54 13 35 24 35 320
* 374 G. Niqrub el Foqani „ 22 51 29 34 56 49 1,078
509 G. Waqif summit. 22 51 21 34 38 57 556
466 Tree at Mersa Shab centre. 22 50 56 35 47 3 1
419 G. Um el Kalala N. peak. 22 50 23 34 44 14 672
506 G. Sheyenit „ 22 50 13 34 18 23 853
505 G. Sheyenit S. peak. 22 49 58 34 18 15 887
415 G. Um el Kalala „ 22 49 26 34 44 18 655
418 Hill close to Bir Madi beacon. 22 47 46 35 1 39 556
* 408 G. Meneiga „ 22 47 35 35 11 7 1,032
* 402 G. Meneiga highest point. 22 47 31 35 10 57 1,092
430 Hill near Wadi Tikosha summit. 22 46 29 35 20 11 443
* 410 Gemeida hill beacon. 22 46 29 35 37 49 123
437 Hill SSE. of Bir Meneiga summit. 22 45 58 35 13 3 987
434 Peak near Wadi Ti Ilak „ 22 44 36 35 17 54 774
376 G. Shabih highest peak. 22 44 26 34 50 21 1,117
527 G. Mishbih NW. summit. 22 44 21 34 41 23 1,321
378 G. Mishbih beacon. 22 44 18 34 41 20 1,316
440 Ridge near head of W. Shellal el Gharbi high point. 22 44 15 35 12 18 1,092
447 Qrein Salama summit. 22 44 5 35 24 32 354
435 Peak near Wadi Ti Ilak „ 22 44 4 35 17 46 849
431 Hill N. of Wadi Muqur „ 22 43 55 35 18 52 670
414 G. el Naga smaller hill. 22 43 32 34 27 22 747
91 G. Seiga cairn. 22 43 31 34 16 16 905
377 G. Mishbih E. peak. 22 43 28 34 42 39 1,311
526 G. Mishbih peak. 22 42 52 34 41 27 1,353
413 G. el Naga N. peak. 22 42 42 34 27 46 787
445 G. Tueiwi summit. 22 42 40 35 5 56 836
* 403 G. Gerf high point on ridge 22 42 15 35 11 43 1,339
427 G. Gerf minor summit. 22 42 13 35 13 32 1,327
* 370 G. Gerf beacon. 22 42 6 35 12 16 1,419
412 G. el Naga S. peak. 22 42 0 34 28 25 827
436 Peak near Wadi Muqur summit. 22 41 58 35 18 22 778
493 G. Mishbih S. peak. 22 41 44 34 42 41 988
432 Peak near Wadi Muqur summit. 22 41 40 35 17 35 962
383 G. Muqur „ 22 41 31 35 16 50 1,058
433 Hill near Bir Sararat Seyet „ 22 41 16 35 0 57 797
426 Peak near Wadi Diqdib summit. 22 41 6 35 14 30 1,258
438 Peak near Wadi Qadiloi „ 22 41 3 35 17 54 911
429 Peak near Wadi Eirahimib „ 22 40 51 35 9 55 1,227
371 G. Shweib „ 22 40 43 34 43 57 914
428 Peak near Wadi Eirahimib „ 22 40 38 35 10 29 1,327
531 Mt. near Bir Baaneit „ 22 40 1 35 18 15 909
404 G. Gerf minor summit. 22 39 59 35 10 7 1,318
405 G. Korabkansi „ 22 39 39 35 0 44 1,052
446 G. Korabkansi N. peak. 22 39 35 34 59 32 1,176
614 G. Hamra Dom „ 22 39 34 35 38 19 326
439 Peak near Wadi Diqdib summit. 22 39 17 35 16 11 1,028
* 372 G. Korabkansi beacon. 22 39 16 34 59 55 1,230
420 Peak near Wadi Difoteb cairn. 22 39 11 35 10 21 1,298
* 417 G. Hamra Dom beacon. 22 39 6 35 38 42 388
450 Tibansi Tikam Ankwei summit. 22 39 5 35 32 11 345
475 Peak near Wadi Sherefa „ 22 38 20 35 11 42 1,344
411 G. Hamra Dom central peak 22 38 2 35 39 12 381
473 Hill near Wadi Um Saha summit. 22 37 54 35 17 56 926
532 Hill near Wadi Um Saha „ 22 37 53 35 17 56 932
444 G. Dreb peak. 22 36 54 35 6 20 1,139
615 G. Hamra Dom S. peak. 22 36 48 35 39 36 317
443 G. Dreb peak. 22 36 41 35 7 0 1,148
442 G. Dreb „ 22 36 40 35 7 46 1,137
476 G. Dreb „ 22 36 16 35 9 15 1,176
478 G. Dreb „ 22 36 10 35 8 21 1,224
477 G. Dreb „ 22 36 1 35 8 56 1,191
504 G. Kulyeit summit. 22 35 50 34 16 38 724
441a G. Dreb peak. 22 35 18 35 8 24 1,293
533 Kilia Arib summit. 22 35 2 35 18 46 647
441 G. Dreb peak. 22 35 0 35 8 36 1,288
535 G. Dreb „ 22 34 15 35 9 42 1,095
534 Tibashoi Tomakolat summit. 22 33 4 35 19 40 464
* 449a Kolmanab hill beacon. 22 32 26 35 53 36 137
566 Hill near Wadi Ibib summit. 22 32 2 35 37 36 459
619 Point near Kolmanab — 22 31 55 35 52 40 110
612 G. Medarai minor peak. 22 31 23 35 13 5 1,103
520 G. Medarai summit. 22 30 58 35 12 5 1,299
610 G. Medarai minor peak. 22 30 56 35 14 37 922
485 G. Anweiyib N. summit. 22 30 22 34 53 42 871
* 567a G. Eqrun (W. summit) beacon. 22 30 10 35 37 9 473
567 G. Eqrun SE. summit. 22 30 0 35 37 18 468
665 Adar Aweib Um Bishtit summit. 22 29 28 35 33 2 455
577 Granite hill „ 22 29 19 35 54 6 143
522 G. Medarai S. peak. 22 29 8 35 12 4 1,114
528 G. Anweiyib summit. 22 29 5 34 53 32 864
587 G. Um Rasein N. peak. 22 28 52 35 20 44 791
* 416 G. Um Rasein beacon. 22 28 12 35 20 19 909
* 575 Einiwai hill „ 22 27 50 35 57 59 138
521 G. Abu Hireiq N. summit. 22 27 39 35 14 58 1,116
609 Hill near Wadi Merdiyeb summit. 22 27 21 35 15 51 856
613 Tahaqayet „ 22 26 47 35 40 23 432
596 Hill near Wadi Odruk „ 22 26 47 35 18 42 639
595 Hill near Wadi Odruk „ 22 26 25 35 19 17 615
582 Granite hill „ 22 26 23 35 56 20 155
592 Near Wadi Odruk peak. 22 26 9 35 20 9 627
523 G. Abu Hireiq highest point. 22 25 57 35 14 39 1,319
581 Osnei hill summit. 22 25 38 35 50 42 251
455 Hill near Bir Um Bishtit „ 22 25 14 35 33 19 634
472 Mt. near Wadi Abu Hireiq „ 22 24 43 35 16 13 1,256
611 G. Adatalob Hadal beacon. 22 24 38 35 48 22 381
451 Titailibab summit. 22 24 32 35 38 52 593
459 G. Orgem NW. peak 22 24 31 35 31 1 779
576 G. Orgem SE. peak. 22 24 21 35 31 11 775
488 G. Anweiyib NW. summit. 22 24 16 34 49 46 884
471 Mt. near Wadi Abu Hireiq summit. 22 23 56 35 16 41 1,226
486 G. Anweiyib SE. summit. 22 23 37 34 51 25 921
664 Hill S. of Titailibab summit. 22 23 26 35 39 5 554
422 G. Abu Hodeid beacon. 22 23 18 35 14 9 1,482
572 G. Adatalob Adara summit. 22 23 1 35 49 58 385
603 Near Wadi Tikraneib peak. 22 22 57 35 18 38 732
454 G. Meis-heit-ar summit. 22 22 37 35 34 44 717
470 Mt. near Bir Abu Hodeid „ 22 22 31 35 17 33 1,074
479 G. Mansur Diab „ 22 22 28 35 10 49 1,091
469 Mt. near Bir Abu Hodeid „ 22 22 10 35 17 51 1,078
457 Hill near head of Wadi Qidmib „ 22 22 3 35 32 42 738
583 G. Tishushi Tiboki minor summit. 22 22 0 35 57 12 284
647 Abu Hodeid Oqla summit. 22 21 29 35 17 51 992
492 G. Um el Tiur el Tahtani „ 22 21 26 34 34 46 783
666 G. Meis-heit-ar „ 22 21 15 35 34 48 721
580 G. Tishushi Tiboki highest point. 22 21 14 35 55 46 359
* 569a Ti Keferiai beacon. 22 21 0 35 49 47 494
598 G. Hamra Tit SW. peak. 22 20 6 35 20 44 642
461 G. Qidmib summit. 22 19 55 35 30 45 1,037
* 456 O Shakafa „ 22 19 44 35 34 42 751
597 G. Geror NE. summit. 22 19 31 35 52 4 434
462 G. Qidmib summit. 22 19 13 35 30 23 1,108
464 G. Qidmib „ 22 19 4 35 29 18 1,070
463 G. Qidmib „ 22 18 49 35 30 15 1,099
616 G. Geror high point. 22 18 37 35 50 18 510
584 G. Qidmib summit. 22 18 18 35 29 20 1,089
453 Adar Aqdeib summit. 22 18 6 35 37 51 736
490 G. Um el Tiur el Foqani „ 22 17 56 34 41 14 946
602 G. Geror summit. 22 17 51 35 50 55 516
649 G. el Sela peak. 22 17 37 36 14 17 433
487 G. Hadal Derqa summit. 22 16 40 35 9 55 1,108
* 460 Hadal Aweib Meisah beacon. 22 16 39 35 31 55 1,224
650 G. el Sela peak. 22 16 32 36 12 59 560
491 G. el Adraq summit. 22 16 22 34 35 3 770
421 Qara Saba „ 22 16 7 35 41 34 778
601 G. Um Seleim „ 22 15 58 35 21 35 1,098
480 G. Hadal Derqa SE. peak. 22 15 50 35 10 59 1,090
585 Hadal Aweib Meisah minor summit. 22 15 48 35 30 10 1,092
604 G. Leqaq summit. 22 15 40 35 20 8 1,192
512 G. Heianai NW. summit. 22 14 40 35 1 30 1,007
668 G. Hamida minor summit. 22 14 34 35 46 13 583
620 Qash Amir beacon. 22 14 31 36 12 20 724
651 G. Sul Hamid highest peak. 22 14 16 36 4 53 572
568 G. Balatitda summit. 22 13 56 35 58 5 592
513 G. Hilwit Hasium NE. peak. 22 13 50 35 14 38 1,037
468 G. Adar Qaqa summit. 22 13 47 35 19 0 1,606
482 G. Heianai SE. summit. 22 13 41 35 3 11 1,256
618 G. Hamida minor summit. 22 13 36 35 46 47 701
608 G. Adar Qaqa „ 22 13 33 35 19 21 1,542
448 G. Hamida summit. 22 13 32 35 46 33 754
* 458 Halaib Fort centre of top. 22 13 25 36 38 56 8
556 G. Elba peak near Bir Kansisrob. 22 13 21 36 22 23 820
652 Hill E. of G. Balatitda peak. 22 13 20 35 59 43 491
555 Karam Elba summit. 22 13 13 36 25 40 586
525 G. Shanaiyet „ 22 13 4 34 49 43 907
653 Hill near Wadi Warabeit „ 22 13 4 35 50 12 511
676 G. Elba peak near Wadi Yahameib. 22 13 2 36 20 54 935
662 G. Balatitda minor summit. 22 12 56 35 57 44 493
489 G. el Hateib summit. 22 12 44 34 43 18 854
674 Hill near Wadi Siamtit summit. 22 12 42 36 13 10 507
516 G. Hilwit Hasium SW. peak. 22 12 30 35 13 14 952
669 Mt. south of Hadal Aweib Meisah summit. 22 12 21 35 30 40 1,072
481 G. el Arib „ 22 12 15 35 8 32 1,112
579a G. Um Ein beacon. 22 11 52 35 39 4 901
667 Hill near head of Wadi Warabeit summit. 22 11 40 35 47 6 651
467 G. Soaorib „ 22 11 33 35 20 13 1,469
* 536 G. Elba beacon. 22 11 27 36 20 52 1,428
621 G. Elba minor peak. 22 11 16 36 23 44 1,102
675 G. Elba „ 22 11 10 36 20 35 1,394
483 G. Heleikonti summit. 22 11 7 34 57 59 1,151
573 G. Warabeit „ 22 11 1 35 47 52 794
518 G. Soaorib „ 22 10 54 35 17 48 1,397
* 571 Adar Aweib beacon. 22 10 50 35 54 0 620
538 G. Elba minor peak. 22 10 17 36 19 34 1,217
606 G. Soaorib summit. 22 10 14 35 21 25 1,349
98 G. Muqsim cairn. 22 10 11 34 1 12 825
589 Mt. near Wadi Baueiwai summit. 22 10 10 35 30 45 1,266
537 G. Elba highest point. 22 10 3 36 21 52 1,435
617 G. Mashushenai summit. 22 9 54 35 49 45 634
605 G. Soaorib peak. 22 9 50 35 21 45 1,383
678 G. O Sir Eirab summit. 22 9 24 36 20 56 842
677 Hill near W. O Sir Hadal „ 22 9 7 36 15 56 724
519 Peak near Wadi Kirir „ 22 8 24 35 18 27 1,328
465 G. Soaorib „ 22 8 22 35 23 36 1,431
590 Mt. N. of Wadi Adar Ameit el Sharqi NE. peak. 22 8 16 35 31 2 1,294
670 Hill near Wadi Aqwem summit. 22 8 12 35 43 30 724
578 Hill near Da-aiyob Wushaq „ 22 7 45 35 44 8 833
681 G. Hanquf minor peak. 22 7 45 36 16 54 789
591 Mt. N. of Wadi Adar Ameit el Sharqi SW. peak. 22 7 12 35 29 54 1,299
682 G. Hanquf minor peak. 22 6 54 36 17 18 877
524 G. Um Reddam summit. 22 6 23 34 59 55 1,109
657 G. Miatit W. peak. 22 6 14 35 34 12 1,229
511 Mt. near G. Egat summit 22 5 53 34 53 1 985
484 G. Egat „ 22 5 39 34 52 15 1,145
586 G. Miatit SE. peak. 22 5 36 35 35 34 1,257
659 Mt. near Wadi Adar Ameit el Gharbi peak. 22 5 20 35 25 30 1,440
540 G. Hanquf N. peak. 22 4 52 36 18 45 1,397
452 Hill near W. Di-ib summit. 22 4 14 36 1 32 491
655 G. Suruk E. peak. 22 4 10 35 38 7 1,059
588 G. Suruk highest point. 22 3 50 35 35 28 1,327
673 Hill near G. Suruk peak. 22 3 46 35 40 16 846
656 G. Shendib „ 22 3 40 35 36 1 1,275
515 Mt. near G. Himeitra „ 22 3 17 35 17 52 1,134
685 G. Shendib „ 22 3 8 36 13 44 1,427
539 G. Shendodai highest peak. 22 3 1 36 25 31 1,529
672 Hill near G. Suruk peak. 22 3 0 35 40 25 847
661 G. Shiab „ 22 2 57 35 45 18 855
* 607 G. Hadarba beacon. 22 2 53 36 47 23 217
654 G. Shiab highest point. 22 2 51 35 44 7 987
517 G. Himeitra summit. 22 2 46 35 14 19 1,231
474 G. Is cairn on summit. 22 2 36 35 28 4 1,736
599 G. Is peak. 22 2 24 35 27 35 1,659
627 G. Shendodai S. peak. 22 2 12 36 25 22 1,395
684 Hill near G. Shendib summit. 22 2 12 36 9 6 601
600 G. Is peak. 22 2 11 35 26 56 1,594
671 Hill near G. Suruk „ 22 1 54 35 40 20 903
593 Mt. east of G. Is N. peak. 22 1 38 35 31 56 1,290
570 G. Hanquf highest point. 22 1 32 36 20 14 1,465
640 G. Shendib peak. 22 1 26 36 14 47 1,724
641 G. Shendib „ 22 1 8 36 14 59 1,698
594 Mt. east of G. Is S. peak. 22 1 2 35 31 11 1,474
642 G. Shendib peak. 22 0 51 36 15 0 1,696
543 G. Shendib beacon. 22 0 48 36 16 30 1,912
546 G. Shendib peak. 22 0 46 36 14 49 1,674
544 G. Shendib peak. 22 0 45 36 16 10 1,852
687 Hill near G. Shellal summit. 22 0 40 36 9 2 614
557 G. Shellal peak. 22 0 39 36 30 30 1,269
529 G. Shellal E. peak. 22 0 34 36 30 45 1,279
574 G. Shellal highest point. 22 0 15 36 29 40 1,409
545 G. Shendib peak. 21 59 56 36 17 6 1,863
547 G. Shendib „ 21 59 49 36 15 42 1,668
550 G. Shendib „ 21 59 8 36 14 16 1,565
645 G. Shendib „ 21 59 7 36 13 22 1,227
549 G. Shendib „ 21 59 6 36 14 40 1,525
644 G. Shendib „ 21 59 0 36 15 20 1,552
646 G. Shendib „ 21 58 58 36 13 11 1,196
660 Low hill near W. Shendib summit. 21 58 24 36 4 25 500
541 G. Qeda E. peak. 21 56 24 36 28 11 1,850
542 G. Qeda W. peak. 21 56 16 36 26 12 1,872
514 Hadal Aweib summit. 21 52 40 35 22 26 1,780
628 G. Asotriba highest point. 21 51 55 36 30 26 2,216
551 Eir Aweit peak. 21 50 30 36 22 48 1,715
552 Eir Aweit „ 21 49 56 36 23 1 1,678
564 G. Obkeik N. peak. 21 49 55 35 39 40 1,837
548 G. Asotriba S. peak. 21 49 30 36 30 40 2,082
565 G. Obkeik S. peak. 21 49 6 35 39 29 1,856
553 Eir Aweit highest peak. 21 47 57 36 22 40 1,853
554 Eir Aweit peak. 21 47 38 36 22 32 1,759
658 Adar It sharp peak. 21 47 1 36 19 14 1,170
409 beacon. 21 44 21 34 36 53 —
558 Arit peak. 21 40 26 36 22 50 1,529
559 Arit „ 21 39 25 36 22 32 1,772
663 Karai Awa summit. 21 39 18 36 10 21 1,056
562 Arit peak. 21 38 3 36 18 57 1,410
560 Arit highest peak. 21 36 51 36 23 27 1,810
563 Arit high peak. 21 36 1 36 19 52 1,727
561 Arit W. end of ridge. 21 35 50 36 22 48 1,532
Comparison of Positions determined on the Red Sea with those previously found in Official Marine Surveys.
It is interesting to compare the positions found for certain triangulation points with those determined by the British Admiralty surveyors and by Lieut. Koss, who accompanied the Austrian research-ship “Pola” in 1895-6.[74] The three best defined points of comparison are Berenice temple, the central peak of St. John’s Island, and Halaib Fort. In the case of Berenice and St. John’s Island the observation of the two prior surveys were made at stations a little distance from my triangulation points, but the published charts enable one to scale off the necessary corrections to reduce the positions to those of the triangulated points. At Berenice, the British Admiralty observation point, for which the latitude is given as 23° 56′ 16″, is 1′ 26″ north of the temple, so that the equivalent Admiralty latitude for the temple is 23° 54′ 50″. Similarly Lieut. Koss’s observation point, where the latitude found was 23° 56′ 27″, is 1′ 47″ north of the temple, giving the latitude of the temple as 23° 54′ 40″. My triangulation gives the latitude of the temple as 23° 54′ 39″, thus showing a remarkably good agreement with that found by the “Pola” observer.
At St. John’s Island, owing to the smallness of the scale of the available charts, it is a little more difficult to scale off accurately the Admiralty position for the peak and the reduction to the peak of Lieut. Koss’s point. As nearly as I can scale, the Admiralty latitude for the peak is 23° 36′ 40″, while the point where Lieut. Koss observed his latitude of 23° 35′ 47″, near the south-west shore of the island, is approximately 40″ south of the peak, giving the latitude of the peak as 23° 36′ 27″. I found the latitude of the peak to be 23° 36′ 16″, thus again showing as good an agreement as could be expected, having regard to the fact that it is not easy to scale the latitude of the point much within 10″ from the existing charts.
At Halaib Fort, the Admiralty latitude is 22° 14′ 10″, while the “Pola” expedition found it 22° 13′ 26″, and my triangulation gave the value 22° 13′ 25″. The close agreement between my triangulation value and that observed by Lieut. Koss seems to prove the Admiralty latitude of this point to be some 45″ too high.
Summarizing the latitudes of the three points for comparison we have:—
Berenice Temple. St. John’s Peak. Halaib Fort.
Admiralty Chart 23° 54′ 50″ 23° 36′ 40″ 22° 14′ 10″
“Pola” Expedition 23° 54′ 40″ 23° 36′ 27″ 22° 13′ 26″
Ball (Triangulation) 23° 54′ 39″ 23° 36′ 16″ 22° 13′ 25″
If we make corresponding comparisons of the longitudes of these three points, Berenice Temple, St. John’s Peak, and Halaib Fort, we obtain much more wide divergences, as is only to be expected from the fact that the Admiralty and “Pola” determinations were made by transport of chronometers from Suez, which is over 500 miles distant from Halaib.
For Berenice Temple the Admiralty longitude (obtained by applying the scaled reduction of − 40″ to the observed longitude 35° 29′ 11″ at the observation point) is 35° 28′ 31″. Lieut. Koss’s transport of chronometers from Suez via Jidda leads to a longitude of 33° 30′ 20″ for his observation point, which lies 54″ east of the temple; thus the “Pola” longitude for the temple is 35° 29′ 26″. My triangulation gives 35° 28′ 26″, thus agreeing very nearly with the Admiralty value and differing by exactly 1′ from the value found by Lieut. Koss.
For St. John’s Peak, the longitude scaled from the Admiralty Chart is 36° 10′ 20″. The observations of Lieut. Koss lead to a longitude of 36° 12′ 38″ for his observation point, which is about 35″ west of the peak; thus the “Pola” longitude for the peak is 36° 13′ 13″. My triangulation gives 36° 11′ 42″, being thus between the Admiralty and “Pola” values.
For Halaib Fort, the Admiralty longitude is 36° 37′ 3″, while Lieut. Koss’s figures lead to the value 36° 40′ 38″; my triangulation gives 36° 38′ 56″, thus again showing a value between the two marine determinations.
Summarizing the longitudes obtained for the three points, we have:—
Berenice Temple. St. John’s Peak. Halaib Fort.
Admiralty Chart 35° 28′ 31″ 36° 10′ 20″ 36° 37′ 3″
“Pola” Expedition 35° 29′ 26″ 36° 13′ 13″ 36° 40′ 38″
Ball (Triangulation) 35° 28′ 26″ 36° 11′ 42″ 36° 38′ 56″
There can, of course, be no doubt of the immensely greater accuracy of the triangulation method of determining longitude as compared with that of chronometer transport in voyages lasting for months, no matter how many chronometers are carried nor what care is taken in the work. The above differences of longitude are small when one considers that no absolute control of the chronometer of the “Pola” expedition was obtained from October 23, 1895, when the ship left Suez, till its return to the same port on January 27, 1896. If we examine the difference of longitude between points fairly close together, we find rather better agreement between chronometers and triangulation. Thus, for instance, between Berenice and Halaib, a six days’ voyage, we have:—
Longitude by Chronometers (Koss). Longitude by Triangulation (Ball).
Halaib Fort 36° 40′ 38″ 36° 38′ 56″
Berenice Temple 35° 29′ 26″ 35° 28′ 26″
Difference 1° 11′ 12″ 1° 10′ 30″
showing an error of only 42″, or about three seconds of time in the six days.
The triangulation-positions:—
Latitude N. Longitude E.
Berenice Temple 23° 54′ 39″ 35° 28′ 26″
St. John’s Peak 23° 36′ 16″ 36° 11′ 42″
Halaib Fort 22° 13′ 25″ 36° 38′ 56″
may therefore be taken as practically correct, and the three points may be used as well-determined positions both for further discussion of the “Pola” results and in further surveying expeditions in the Red Sea.[75]
Detail Surveying along Lines of March.
All detail visible along lines of march from camp to camp was recorded on plane-table sheets on a scale of 1:100,000. The usual process was as follows: The plane-table sheet was first provided with a graticule at 10′ intervals of latitude and longitude, and all the triangulation points previously fixed within the area covered by the sheet were marked in their computed places. Stations were chosen along the route at an average distance apart of two or three kilometres, the most commanding hills being selected, and the positions of these were found by plane-table re-section from three or more triangulation points. The compass, being frequently disturbed by magnetic rocks,[76] was only used to get a first approximation to the true orientation of the table. The plane-table station having been fixed on the map, tacheometric readings were taken to all conspicuous points easy of access within a radius of about two kilometres, and plotted at their measured distances along the directions given by the alidade.
In the telemetric measurements a 5-inch tacheometer was used side by side with the plane-table, and two staff-men were employed. As the scale of the map was small, the sights were much longer than is usual in tacheometry, and the maximum distance of 800 metres directly readable by the four-metre staves employed was generally exceeded. For the long distance readings, where the distance between two cross-wires subtended more than the length of the staff, I devised the following process. Bringing the centre wire to the base of the staff, a reading of the vertical circle was taken; next, by the tangent-screw, the wire was brought to the top of the staff, and a second reading of the vertical circle was taken, the difference giving the angle subtended by the four-metre staff. It is clear that the distance is as many times greater than 800 metres, as the angle subtended is less than 17′ of arc, and the distance is thus found by simple proportion.
In the case of very long sights, even this method failed, because the circle could only be read to half-minutes, which was too coarse a graduation to give a good result, and in these cases the method used was one of repetition. The wire being brought to the base of the staff as before, and a first reading of the vertical circle taken, the wire was brought to the top of the staff by the tangent screw, then to the bottom again by altering the levelling screws slightly, again to the top by the tangent screw, and so on, three or four times, and then a second reading was taken on the vertical circle. The slight alteration of level had no sensible influence on the result, and it is obvious that by automatically summing up, say, four intercepts in this way, a very much more accurate value of the subtense angle was obtained than was possible from a single measurement. In practice I found it was best to carry in the waistcoat pocket a card giving the distance corresponding to any number of minutes of difference of reading after a four-fold repetition, and it was quite practicable to measure up to three kilometres of distance within one hundred metres of the truth; as this only represented a millimetre on the sheet, and as, moreover, errors were not cumulative, owing to the independent fixation of each successive station by re-section, the accuracy was all that could be desired, and the rapidity of measurement was very great. In this long distance type of tacheometry, finely graduated staves were of no use; the form of staff employed was a broad-faced one, fifteen centimetres wide, bearing fifty-centimetre divisions painted alternately black and white right across the whole breadth of the staff.[77]
An average of about six or eight conspicuous points having been telemetrically fixed from a station, the detail was sketched in around them, and other more distant points were at the same time fixed by plane-table intersections from several stations. At the stations the pencil sketching of relief was by form-lines which were subsequently replaced by hachure-rendering when inking up the sheets in camp.
Occasionally, when a high hill-station was employed overlooking a long wadi, time was saved by reading only two distances, both in the same wadi, one very near to the station and the other two or three kilometres away, at the same time observing the depression-angles to these points. The slope of the wadi being found in this way, the depression-angles to intermediate points gave the distances of such points without the necessity of staff reading at the intermediate points at all. Thus, suppose the near point was close under the station, with a distance of 500 metres and a depression-angle of 18°, while the remote point up the wadi had a distance of three kilometres and a depression-angle of 2° 30′. By means of a slide rule or three-figure logarithms, the near point was found to be 163 metres below the station, and the distant one 130 metres. A point in the wadi estimated roughly as half way between the two would be about 146 metres below the station; so that if the observed depression-angle to it was, say, 4° 40′, its distance would be 146tan 4° 40′, or 1·8 kilometres. Any possible error of preliminary estimation of the distance in order to find the level would be without sensible influence on the resulting true distance.
The process of traversing between stations was seldom resorted to, as the method of fixing stations by re-section from triangulation points is much more accurate (the errors not being cumulative) besides being more rapid. But in certain tortuous cañon-like wadis, where great and time-consuming climbs would have had to be undertaken in order to see any triangulation points, the method of traversing with the tacheometer had to be employed.
Levels along the line of march were measured trigonometrically whenever possible; the vertical angles to one or more triangulation points being read with the tacheometer, and the distances scaled off the map, the differences of height, corrected for refraction, were found by the slide rule in the same manner as in the triangulation already described. Heights of passes and camps whence no triangulation points were visible were determined by barometer-comparisons between them and points of precisely determined altitude.
Names of places were written down by the guides in Arabic characters on the spot where they were ascertained, and transliterated on the Egyptian Government system for insertion in the map. Most of the place-names were checked by getting the guides to give them from several different stations.
Detail Surveying at Occupied Triangulation Stations.
It was generally necessary to remain for at least a week on the mountain summits which formed the main triangulation stations, for only on about one day in seven was the air clear enough for sighting the beacons on the longer lines. Occasionally the entire landscape was blotted out for ten or more consecutive days by clouds surrounding the summit, while at other times it was possible to see only for a limited distance round the station owing to haze. Such times were made use of to map all visible detail within a moderate range (say within a radius of twelve kilometres) round the station.
In this work the first stage was to find a small base, one end of which was the station itself. Usually a minor peak of the same range, 500 to 1,000 metres away from the station, was fairly easily accessible, and was chosen for the other end of the base. The six-inch theodolite being at the main station, the five-inch tacheometer was set up at the auxiliary station, and all noteworthy hill tops, as well as a few points along each main line of drainage, were triangulated off this small base. The length of the base was found by including one or two main triangulation points in the round of angles. These minor triangles were conveniently reduced by the slide rule, and the points plotted at once on the plane-table by means of the alidade and the calculated distances. The base being short, it was necessary to observe to fine marks; cracks in the rocks, and the droppings of birds on the peaks, and the centres of selected tree trunks in the wadis, were usually chosen. The levels of these minor points were determined by vertical angulation in the ordinary way. Usually about thirty points were thus fixed round each high station. Once a number of points were fixed in the wadis, the levels of these gave the slope, and the difference of height between any other parts of the wadis and the station could be estimated to within a few metres by means of the knowledge thus obtained. A sketch being now made of the wadis, which appeared spread out almost like a map below the station, a hundred or more points along them were selected, and their depths below the station being very approximately known from the wadi slope, their distances were found by observing depression angles to them and reducing the vertical triangle by means of the slide rule on the spot. In all, therefore, measurements were usually made of the distances of from 100 to 200 conspicuous points in the area round the station, and when these were plotted with the alidade on the plane-table sheet it was not difficult to sketch in all the detail with considerable accuracy. Usually it was not possible to see all round the mountain from the station itself, so that subsidiary plane-table stations near the main one were necessary. In other cases more than one small base was measured in order to get good angles to various points by the minor triangulation.
By the combined use of minor triangulation for peaks, and vertical angulation for points situated along drainage lines, it was found that far more sketching could be done in a few days at a main station than would have been possible in the same time by tacheometric work on the lower ground, and that of greater accuracy. More than half of the entire detail sketching was in fact done at the main stations.
Survey of the Coast-line.
In the earlier portion of the work, i.e., north of latitude 24°, the work on the coast-line was confined to the fixation of prominent points such as spurs and tips of islands. Some of these were fixed as intersected points during the triangulation, while others were determined by observing the depression angle and azimuth to them from trigonometrical stations of known altitude. In this latter method, taking the mean coefficient of refraction as 0·13, the formula[78] employed was
d = 35·497 (θ − √ θ2 − 11621 h )
where d = distance in metres, θ = depression angle in seconds, and h = altitude of station in metres. This formula is rather tedious to work out, though the work is relatively not so great if a number of points are to be calculated from observations at the same station. Prominent points having been fixed in this way, the coast-line north of 24° was sketched in from the Admiralty Chart, adjusting the longitude to fit the points fixed.
It was sometimes a little difficult, however, to identify the fixed points on the existing charts, and hence I tried to find some process of surveying the entire remaining coast-line de novo in detail. Traversing along the coast was placed out of question by the great expenditure of time and money which it would have entailed. Eventually I was led to devise a new method, by which long stretches of coast-line could be mapped by polar co-ordinates from mountain stations with great speed and accuracy. The directions of a series of points sufficiently close together along the coast were taken by theodolite, and the depression angles simultaneously observed with the vertical circle. Then, instead of computing the distances to the points, they were plotted by a special scale graduated directly in angles of dip; by thus doing away with all calculation at the station, it was possible to lay down the points on the chart as fast as the observations could be taken, usually at the rate of four or five points a minute, and it was frequently possible to map thirty kilometres or more of coast-line in an hour with great accuracy. As I have given a full account of this new method in a separate publication,[79] I shall not go into it further here, but would refer those interested to the publication just mentioned.
With the aid of the new method the entire coast-line from just north of Ras Benas southwards to the parallel of 22°, a distance of over 200 miles, was mapped in detail on the 100,000 scale. I had frequent opportunities of testing the accuracy of the delineation of the coast, both by mapping the same stretch from two widely-distant stations of different heights, and by subsequently surveying small portions of the coast directly by plane-table and tacheometer from triangulation points on or near the coast; and in all cases I found the accuracy to be very high, the differences found rarely exceeding the thickness of a line on the map. The tides in the Red Sea are so small in range (generally only about a metre) that variations of sea-level were practically negligible during the operations. Bearing in mind the great difficulties attending the survey of so inhospitable a coast by the ordinary method of traversing, I believe the resulting outline of the coast on my maps is very much more correct than that shown on any previous charts. An accurate delineation of the coast-line in this region is of course chiefly of value as indicating the extent of land; it is of little importance to the navigator, for whom the positions of the outlying dangerous reefs, mapped by the Admiralty surveyors, are far more important.
Location of the Administrative Boundary between Egypt and the Sudan.
By an Arrêté of the Ministry of Interior issued in 1902 it was enacted that the boundary between the administrative divisions of Egypt and the Sudan should be as follows:—
Commencing at Bir Shalatein on the coast of the Red Sea, the limit runs to Bir Meneiga, thence to Gebel Niqrub, thence to Gebel Um el Tiur and to Deiga. From Deiga the line continues to Bir Esmet Omar, thence to Gebel Bartazuga, and finally to the Nile at Korosko.
This frontier was defined after a commission had sat, at the Mudiria of Aswân, to make enquiries as to the vested rights of the Bedouin tribes, the guiding principle being that all Bisharin tribes should be under Sudan administration, and all Ababda tribes (with one exception) under the Government of Egypt. A map accompanied the decision, but was of a very rough character, and the positions of the points specified were not known within several miles.
Part of my work comprised the precise fixation of the points specified in the Arrêté, with a view to laying down the boundary accurately on a map. As was natural in the case of a boundary settled without careful reference to the ground, certain difficulties arose in the location. In the first place, there are two distinct Gebels Niqrub, and two distinct Gebels Um el Tiur. I assumed the higher mountain of each pair to be the one indicated. Secondly, the precise point of the mountain was unspecified; I assumed the highest peak to be the point referred to. Thirdly, the text of the Arrêté disagrees with the map accompanying it, in that the map shows the line curved to pass close to Gebel Mishbih, which is not mentioned in the Arrêté. I assumed the text of the order to be determinative, and that between the points specified the limit followed great circles on the globe, i.e., practically straight lines on the map. The locality called Deiga was not visited, and can only be approximately fixed; it is said by guides to be a narrow road near Gebel Muqsim, of which mountain several peaks were fixed by triangulation.
The following table gives the positions found for the various points along the boundary from the sea to the meridian of 34° (1) from my survey operations, and (2) as scaled from the map accompanying the Ministerial Arrêté. A comparison of the two sets of positions will show how much the survey has added to our knowledge of the geography of this part of the desert.
Point. Latitude N. Longitude E.
From my Survey. From Arrêté Map. From my Survey. From Arrêté Map.
Bir Shalatein 23° 8′ 5″ 22° 39′ 0″ 35° 36′ 28″ 36° 2′ 30″
Bir Meneiga 22° 47′ 8″ 22° 41′ 30″ 35° 12′ 20″ 35° 2′ 30″
Gebel Niqrub (El Foqani) 22° 51′ 29″ 22° 48′ 0″ 34° 56′ 48″ 34° 51′ 0″
Gebel Um el Tiur (El Foqani) 22° 17′ 54″ 22° 18′ 30″ 34° 41′ 1″ 34° 32′ 0″
Deiga (approx.) 22° 10′ 0″ 22° 9′ 0″ 34° 1′ 0″ 34° 3′ 0″
Variation of the Compass.
Careful observations were made of the variation of the compass at three stations, viz., Berenice Temple, Abu Saafa Springs, and near Halaib Fort,[80] these three points being selected as being well-known places and at the same time likely to be free from local magnetic disturbance. Berenice is on the coast-plain where only coral and sand occur; Abu Saafa is in a sandstone district about 100 kilometres south-west of Berenice, and Halaib is on the calcareous and gypseous rocks of the coast about 220 kilometres south-east of Abu Saafa.
The instrument used was a five-inch theodolite fitted with a good trough compass. A lens was used to bring the needle accurately to zero, and the sun or a star was employed to find the true meridian. In each case several observations, each with an independent setting of the needle, were made, and the mean taken. The values obtained were:—
Place. Date and Time. Compass-Variation West.
Berenice Temple January 6, 1907, 5·30 p.m. 2° 44′
Abu Saafa Springs October 25, 1907, 4 p.m. 2° 37′
Near Halaib Fort May 7, 1908, 10 a.m. 2° 18′
In order to find to what degree of accuracy the results of these determinations might be relied on, the instrument used was tested against the Kew magnetometer at the Khedivial Observatory, Helwân, after the completion of the work. Three observations for the declination at Helwân gave the westerly variation as
2° 41′·2
2° 58′·8
2° 55′·0
Mean 2° 55′·0
while the true declination as given by the Kew magnetometer at the same time was 2° 52′ 5″. Thus it appears reasonable to believe the observed values to be within about 5′ of the truth.
It is rather curious to note that the declination observed at Abu Saafa is less than that at Berenice, which lies further east, even if allowance is made for the secular change in the interval. The difference from what one would expect is, however, possibly owing to errors of observation of the magnitude above-mentioned, or it may be due to a slight local influence of magnetic rocks underground at Abu Saafa, where the ground is near to the base of the Nubian sandstone beds, which rest presumably on eruptive and metamorphic rocks.
It is interesting to compare the observations of declination at Berenice and Halaib with those recorded by Rossler[81] at the same places in 1895. Thus we have:—
Berenice. Mersa Halaib.
Rossler, Nov. 27, 1895 3° 54′ Rossler, Nov. 18, 1895 3° 36′
Ball, Jan. 6, 1907 2° 44′ Ball, May 7, 1908 2° 18′
Diff. in 11·1 years 1° 10′ Diff. in 12·5 years 1° 18′
Yearly decrease 6′·3 Yearly decrease 6′·3
We thus arrive at a yearly secular diminution of declination of 6′ 3″, confirming the value arrived at by Mr. Keeling[82] for the rate of secular change in the Red Sea area from a comparison of other observations. It may be therefore concluded that the rate of 3′ per annum given in the “Admiralty Pilot”[83] is only about half the true value.
Travellers in the Eastern Desert should exercise care in the reliance they place on compass bearings. Bearings taken with a compass are generally normal in granite and sandstone country; but wherever dark igneous and metamorphic rocks abound, disturbances are likely to occur. Local deviations of 5° to 10° are quite common, and in some serpentine areas even 40° of disturbance may be observed. In some places, one can deflect the needle through a large angle by means of a fragment of the rock no larger than a nut, so rich are some of the basic rocks in magnetic minerals. Some lumps of rock even show strong polarity, attracting or repelling the north pole of the needle according as one part or another of the lump is presented to it.