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importance for the purpose of the survey. A systematic pedological photo-analysis, executed 
with the help of a good stereoscope means a thorough study of the terrain. 
The pedological photo-analysis is of greatest advantage for surveys on scales 1:50.000 
and smaller. Should it be applied to more detailed maps, the quantity of the field work 
necessary to make the exactness and completeness of the map correspond with the scale 
would increase disproportionally. 
For a so-ealled ''semi-detailed map" (1:50.000) it is necessary that after the analysis 
each separate unit and boundary in the terrain is checked. From the analysis it will be 
known with what degree of intensity this has to be carried out. General experience has 
taught that for these map-scales the time necessary for the complete mapping of an area is 
shortened to 1/, or even !/, of that needed without a preliminary photo-analysis. 
For less detailed maps the time may be reduced to about !/6 or sometimes even !/1o 
by limiting the field work to so-called "key areas". For the intermediate areas a direct 
interpretation of the photo-image is then applied by interpolating the knowledge obtained 
in these key areas on the photo-analytical map. Only under certain conditions such an inter- 
polation is justified. Only similar areas occurring in photos of the same survey may be 
interpreted in this way. The key areas must be regularly distributed over the whole territory 
and must be chosen in representative terrains. This interpolation method should in general 
be limited to the production of reconnaissance maps (1:100.000 or 1:200.000), though in a 
few cases the terrain condition permits a representation of the data on a larger scale. 
A third method is that of extrapolating general pedological knowledge, experience in 
surveying and knowledge obtained by means of a reconnaissance in the photo-analytical 
map. One’s own earlier work in the area in question or a similar area will be a great help. 
This method is based mainly on direct interpretation. Detailed data regarding the soil 
condition or the land capability cannot be derived from it. With this method only schematic 
or so-called exploratory maps can be produced (1 : 300.000 or smaller). 
Also for the two first-mentioned methods a reconnaissance in the terrain before starting 
the work is desirable, because with some preliminary knowledge the analysis will give 
better results than without. 
Photo-analysis. 
The aim of the method of photo-analysis is to prepare a provisional map and clas- 
sification for the field work. The analysis is based on the recognition of objects, conditions 
and patterns, which may represent mapping units. The boundaries of these units are 
indicated; the units are grouped in a provisional classification system according to the 
analysed properties. For pedological purposes the analysis is based on pedological elements. 
Knowledge of the relation soil-vegetation or soil-land use, not only in the primitive, but 
also in the more developed agriculture is necessary; the same holds good for those factors 
which influence the processes of soil formation, i.e. climate, parent material, topography, 
vegetation, time and men (Jenny 1941). The mapping units indicated represent soil as- 
sociations, complexes and phases and only sometimes soil units of the lower categories 
such as soil series or soil types. 
It is obvious that not all soil units can be analyzed from the aerial photographs. Not 
every difference in soil condition shows itself in a different terrain condition. Neither 
does every difference in colour tone and terrain condition tally with differences in soil 
conditions. It is therefore that the method of photo-analysis is not suited for detailed surveys 
and that checking in the field is an absolute necessity. These are also other restrictions 
such as imposed by the quality of the photos and many other limiting factors related to 
the photography itself (Frost 1953). One of the most important factors is the season and 
date of the aerial survey. Only such differences can be analyzed which are clearly visible 
and measurable and have a regional extent. 
The photo-analysis is based on various elements. Differences and similarities constitute 
A 2 
X. x 
* * 
y