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.