3970617772

photos. In many remote parts of the country this is expensive and time consuming and most of the work has to be completed in one extended visit to the field areas.

The degree of refinement of the air photograph interpretation de-pends a great deal on the time, staff and funds available. In the Red Sea area where there is excellent photography with good overlaps a map far morę accurate than the present 1 :250,000 sheets could be madę by simple tracing of topographic features from the photographs. With scalę and simple tilt adjustments an accurate map could be produced. These maps then serve as (a) a detailed landform map of the area useful for other workers, (b) an accurate geomorphological representation of the area from which deductions regarding processes and geomorphological history can be madę, and (c) a generał locational map yaluable in route finding and generał travelling.

The two greatest problems of working in these conditions are logist-ical and the problem of topographic information. For much of the Red Sea coast water holes are very scattered and travel in the Red Sea hills is limited by the very rugged waterless country suitable only for camel transport. Morę than one yehicle is often essential. The other area work-ed has been within easy distance of the Nile and has few transport problems for desert worthy yehicles.

Two areas have been mapped at present. A 150 mile stretch of the Red Sea coast and Coastal plain extending 15—20 miles inland and the semi-arid areas north-west of Khartoum. In both areas topographic data have been confined to a few very scattered trigonometrical heights. Any height data needed has had to be proyided and this has been achieved by controlled aneroid trayerses. In one case the Nile and in the other Sea Level proyided an additional known datum. Except in the rainy season short closed trayerses can give quite accurate results. Heights can be fixed to within 3 m. under most conditions. Even aneroid work is very time and energy consuming when large numbers of scattered hills have to be climbed and in some cases height data has been confined to detailed trayerses along the plains to record the regional slopes and determina-tions of selected hill heights. Ali this information is best recorded in spot heights on the features. The main data related to a fixed datum is supple-mented wherever possible by recording the height of minor features above the plain. Sand dunes, smali hills, mounds, etc. etc. are all measur-ed in this way. For most purposes in such work local relief is the most important factor and this can readily be illustrated.

For convenience of initial drafting and modification all the field and laboratory work has been carried out on the photo scalę of 1 :40,000 and then the completed drafts have been photographically reduced either to scalę of 1 : 50.000 or 1 :100,000 ready for publication.

The rangę of symbols used in the mapping so far have been deter-mined by the type of country worked in an arid or semi-arid plain and upland area. In these topographic regions there is a very sharp and elear distinction between hill features and plain features and this has been shown in the maps by shading all hill features in red shades or tints. This enables a elear representation of the major units of the landscape. Intermediate features do occur, notably narrow pediments, and alluvial features around the margin of hill features. Rock pediments are shaded as hill features but with broken lines and in the areas considered they represent only a smali proportion of the landscape. Depositional features

24