The geography and geology of south-eastern Egypt

by John Ball

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.