The topographical sketching, carried out on plane-tables on a scale of 1:100,000, was based on the points triangulated, these being computed in camp and plotted by their geographical coordinates as much ahead of the lines of march as possible. When marching, plane-table stations were fixed by re-section from three or more triangulated points, the compass being useless owing to abundance of magnetic rocks. Details along the line of march were put in by tacheometric readings from the stations, using specially devised long distance methods. But the greater portion of the sketching was done from the mountain tops while occupying them as trigonometrical stations, and for this again special methods, involving an extensive use of vertical angles, were employed. On an average, about thirty square kilometres were sketched in during each day’s march, and about 400 square kilometres from each main trigonometrical station, where it was usually necessary to remain for at least a week to get sufficiently clear weather for the more distant sights. It was not, of course, possible to sketch every portion of the area in uniform detail, for some parts lay so far from the line of march and from the triangulation stations that little or no detail was visible. The lacunæ were filled in as far as possible in the field from guides’ statements, taking care on the field maps to distinguish parts so filled as only approximate. A few additions near the west limit of the map have been added from reconnaissance surveys by various colleagues of the Survey Department. For details of the upper part of the basin of the Wadi Alaqi, I have taken advantage of an admirable reconnaissance-survey of this tract recently made by Mr. J. Morrow Campbell, B.Sc., F.R.G.S., for the Egyptian Options, Ltd., who kindly placed their map at the disposal of the Government. My triangulation gave the positions and altitudes of all the principal peaks of this region, and thus enabled Mr. Campbell’s map to be accurately adjusted to the Survey positions, and his drainage lines and other topographical details have been inserted with great advantage to the map, which would otherwise have been almost blank in this particular area. Mr. Campbell’s map does not, unfortunately, give any altitudes, and the only altitude data for the orographical sketching were the peaks and other points of connexion with the Survey map; but by taking into account the depth of shading of the different hill masses and the known altitudes determined by triangulation, it is believed that a fairly approximate picture of the orography has been obtained.
The sea coast north of latitude 24° 10′ was adapted from the Admiralty Chart, fitting it on to coast points trigonometrically fixed. South of 24° 10′ the coast was surveyed in detail by means of depression angles from the occupied peaks of the triangulation. The local circumstances, in the existence of high mountains pretty close to the sea, are so favourable to the employment of this method, that it is confidently believed that the maps represent a considerable advance in accuracy over the Admiralty Charts. It was frequently possible to check the accuracy of the coast-line delineation by plane-table rays, and it was invariably found that the errors were insensible, even on the fairly large scale employed in the field-maps. The coast-line shown is the high water-line. The tidal change of level, though it is generally less than a metre, uncovers in many places extensive coral reefs at low water, but no attempt was made to map these.
Very special care was taken with place-names, which are transliterated on the Egyptian Government system. It may be remarked that this system is based on the Arabic spelling, not on pronunciation. Thus, in this part of the country, the letter g is almost always soft, like an English j, and the q is always sounded like a hard g. The same system was used for the Bishari and Ababda names, these being first written by a guide in Arabic characters and then transliterated. It is believed that the orthography is correct in almost all cases.
The geology was examined along every line of march, and additional specimens were obtained by sending guides to collect rocks from mountains otherwise unvisited. Where possible, the principal geological boundaries were drawn direct on the plane-table sheets in the field, and the remainder have been placed from field notes. The limits of the different igneous and metamorphic rocks shown on the geological map aim only at showing general relationships, for in many cases it is difficult, owing to insensible gradations, to decide exactly where granites and diorites end and gneisses and schists begin, while in other places rocks of two or three distinct species are so intimately mixed that the boundaries could not be shown on the small scale used, even if the exact limits were traced by a life-time’s work; in these latter cases the areas are coloured in accordance with what appeared to be the dominant rock within them.
Throughout the three seasons’ work I was accompanied by Dahab Effendi Hassan, who rendered useful assistance in booking angles and in reconnaissance, and in the third season Mr. O. N. Bakewell also accompanied me and gave me much useful help. The Egyptian unskilled staff employed consisted almost entirely of Bedouin and comprised on an average about twenty-five camel drivers (for thiry-five baggage camels), six porters, three guides, and two postmen, all placed under an Arab sheikh.
The total cost of the field-work was approximately L.E. 5,090, or about 150 milliemes for each square kilometre of country mapped in detail. Nearly two-thirds of the entire amount were expended in camel-transport, the remainder representing the expenditure on salaries and allowances of the author and assistants engaged in actual surveying operations.
[1]The references in parentheses after place-names refer to their positions on the map, [Plate I.]
[2]Bruce’s Travels to discover the Sources of the Nile. Halifax, 1845. pp. 78-80.
[3]Cailliaud, Travels in the Oasis of Thebes and in the Deserts east and west of the Thebaid, edited by Jomard. Translated from the French. London, 1822. This work contains two small maps and many engravings. (The map of the Eastern Desert is unfortunately wanting in the Khedivial Library copy, which is the only one I have seen.)