148
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962
Discussion
Dr. Maignien (France) asked on which photo scales the interpretation was carried out and,
considering these scales, at which level of classification were the units described as “facets”
by the author? Mr. Webster answered that the photo scales were from 1 : 20,000 to 1 : 30,000.
In some physiographic situations the soils were very similar, whilst in others there was consid
erable heterogeneity. Four cases have been recognised: (1) a facet may correspond to a single
series; (2) a facet may embrace several similar series; (3) a facet may be a soil complex;
(4) a soil series would rarely be mapped physiographically as two or more facets.
The sense and form of soil variation within the facet is one of the criteria by which the facet
is defined.
Prof. N. W. Radforth (Canada) asked what delineates pattern and facet. One school may
tend to split features (units) that another school might insist on being a single facet. Mr.
Webster answered that so far he had adopted a purely pragmatic approach - whether further
subdivision would create units which differ significantly in their broad agricultural or engineer
ing potentialities.
Prof. D. W. McKenzie (New Zealand) asked what objectivity or measurement determines the
facets? Are these particular to areas investigated and subjectively determined by the in
vestigator? New Zealand experience has been that the mathematical approach of Strahler
is excessively time consuming for the resultant classification of landscapes. Mr. Webster
answered that he is sure that ideally it would be desirable to describe all parts of the earth’s
surface using sets of numerical parameters. However this does not appear to be practicable:
the determination of the parameters for even a small portion of the earth’s surface is time con
suming and difficult. Whilst the landscape is continuously variable, it is believed that facets, as
defined in his paper, correspond with commonly occurring groupings of parameters.
Mr. J. L. Retzer (U.S.A.) remarked that: (1) It is important to know for which survey the
method of Mr. Webster is to be used. (2) It is necessary to design mapping units to meet the
objective. Toponomic units are not necessarily the same. (3) There is a need for objective
classification of land forms based on shape, the genesis being an inferred opinion which, as
such, cannot be mapped. Mr. Webster answered that: (1) The purpose of the survey may
dictate the scale of mapping, but in general the more fundamental the classification of the
mapping units the wider the application of the map. (2) Facets are defined on the basis of
general morphology and on the nature of the variations of minor relief and soil within them.
They are not defined on their genesis, though it is convenient to give them genetic names
where these are widely used. At a later date it may be possible to put numerical limits to the
attributes of facets. At present for the definition of facets we rely on the precision of verbal
descriptions aided by block diagrams.
