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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
is a need for some means by which others can draw on this experience. Thus 
in addition to the mental library of the individual we require a library of 
annotated air photographs, stereopairs and multiplets with a suitable classi 
fication and storage system. The following proposals are made in the belief 
that physiographic subdivisions of land are implicitly the basis on which ex 
perienced interpreters recognise soils, but that lack of specific classification 
makes it difficult for one interpreter to recognise that his area has its analogues 
in that of another and to transfer experience from one area to another. 
The Facet 
Probably most soil scientists when interpreting air photographs recognise 
the close relationship between soils and physiography, and that a change in at 
least one of the factors, relief, parent material or age, itself linked to physio 
graphy, is likely to produce a change in soil type. 
It is now over thirty years since Bourne [1931] at Oxford, faced with the 
problem of sampling in reconnaissance land survey, observed that areas of this 
sort were not usually unique but were encountered repeatedly and should be 
recognisable on air photographs. He used the term “site” for an area which 
throughout its extent had similar local environmental conditions: climate, 
physiography, geology and soil. From the examples given he clearly envisaged 
that a site defined physiographically could provide this. 
Bourne’s sites then corresponded with the type of landscape unit which 
interpreters commonly recognise and map, and is precisely what is required 
as the fundamental unit in terms of which to subdivide the landscape when 
interpreting air photographs for soils. The term facet is now proposed; the term 
was used by Woolridge [1932] with the same meaning, that is a small face of 
land surface. A facet is a subdivision of terrain which for most practical pur 
poses may be considered uniform, or, if variable, showing definable variations 
which are of the same kind wherever the facet occurs and in the same sense as 
the variation in the larger landscape of which the facet is apart. A facet is 
Fig. 1 
Block diagram showing three facets as they occur in the English Gotswolds