500 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
face down to solid bedrock. This paper describes the first attempts that have 
been made towards the preparation of such soil engineering maps in South Africa. 
Initial research projects 
Aerial photography is available for practically the whole of Southern Africa, 
in scales varying from 1 : 8,000 to 1 : 50,000. In order to determine the appli 
cability of airphoto interpretation as an aid to rapid soil engineering mapping, 
and involving a minimum of field exploration, four sections of proposed high 
way routes were selected to initiate the research. As shown in fig. 1, the four 
sections were selected in situations more or less towards the east, south, north 
and west of the country, and in areas with mean annual rainfalls of approx 
imately 33-in, 26-in, 18-in, and 7-in respectively, and correspondingly variable 
“climatic N-values”[l]. 
A legend of the mapping units is given in fig. 2. Stereograms of selected 
photographs from each of these projects are reproduced in figs. 3, 4, 5 and 6, 
together with corresponding sketch maps of the delineated mapping units. 
Mapping procedure 
Experience gained on these initial projects has pointed the way to the most 
satisfactory procedure for mapping of this nature under South African con 
ditions. 
At the beginning of a project the mapping units are first selected before pro 
ceeding with the delineation of boundaries on the airphotos. This is done by 
examining exposures and outcrops in the field, and by putting down a number 
of trial holes in a selected key area and recording detailed soil profiles in 
each. [2] The holes are either bored with a 24 inch diameter earth augering 
machine, or dug by hand, so that the materials can be inspected in situ. The 
mapping units which are now selected may be either individual soil or rock 
types or, more frequently, composite soil profiles from surface to bedrock. At 
this stage attention is also paid as far as possible to the practical requirements 
of the engineer: thus, if a particular deposit of gravel would only be economical 
TRANSPORTED SOILS 
PEDOGENIC HORIZONS 
Cl 
CLAY 
CLAYEY 
F [:::| ferrerete 
Si [Щ] SILT 
ЕШ 
SILTY 
CALCRETE 
sd HI 
SAND 
Sd О 
SANDY 
RESIDUAL SOIL 
G 
GRAVEL 
RED CLAY WITH WEATHERING SPHEROIDS, 
(residual from dolerite). 
P 
PEBBLES 
(concentrated on 
surface). 
PEBBLES 
(scattered on 
surface ). 
Aw 
ALLUVIAL WASH (galley soils : variable texture 
dependent on local parent materials). 
ROCKS 
Q. QUARTZITE T. TILLITE M. MUDSTONE 
L. LIMESTONE S. SHALE 0. DOLOMITE 
D. DOLERITE OR DIABASE. 
Fig. 2. Legend of soils and rocks occurring in stereograms