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Commission IV Invited paper 113 
Convergent versus Vertical Photography 
by W. SCHERMERHORN 
LT.C., Delft. 
This title may convey the impression firstly that there is some controversy on this 
matter, which is the case, but secondly that it may be intended to pass judgment on which 
of the two alternatives is superior to the other, which would be misleading. As with any 
other technical aspect of photogrammetry, the only generalisation which is always ap- 
plicable is that NO generalisation can be applied to all circumstances. For any compar- 
ison of methods to be of value, that comparison must be related to the suitability of the 
methods for particular requirements. This will be our approach to the matter. 
Consistent with this approach, Prof. Kasper’s suggestion that the question should 
rather be: “In which cases can convergent photography have advantages?" has much 
merit. I would, however, suggest the addition of the further question: *What are these 
advantages?". 
A further preliminary remark is that we must be careful to distinguish between 
purely geometrical considerations and those which derived from the results of productive 
work — precision is important, but so, also, is production time. Base-height ratios and the 
area of coverage per model must obviously be studied, but they will not give us the whole 
answer to the problem. The relation between base-height ratio and the attainable preci- 
sion of height measurement, particularly towards the upper limits of the range of this 
ratio, between 1.0 and 1.4, is something on which there is as yet insufficient evidence. 
This relation can only be determined empirically from tests. Unfortunately we do not 
know such results which can give a reliable answer to these questions. Also the reports 
received from the panel members are, from the point of view of this kind of basic obser- 
vations and results, rather poor. As long as no better and no more material is available, 
decisive conclusions will remain impossible. 
While several other criteria may be proposed, the criterion of precision may be con- 
sidered from numerous viewpoints which are related to economy: 
1. The precision of planimetry, in one model, is generally a decisive factor for large- 
scale maps such as those for cadastral purposes. 
2. The precision of height measurement and of contouring, in one model, is important 
for topographic maps of medium scale and for large-scale mapping for engineering 
purposes in terrain which is not flat. 
3. The precision attainable in the aerial triangulation of a strip of given length (or a 
given number of models) will have a significance, in all types of work where trian- 
gulation is used, quite different from the other factors mentioned. 
From each of these considerations we can derive a minimum value for the scale of 
photography. The largest of these values will determine the necessary scale and thus 
have a decisive effect on the economy of the whole procedure. 
It will be obvious, however, that a general conclusion cannot be reached by consid- 
ering this approach alone, for there will be cases in which criteria which are, more or 
less, unconnected with precision, will play a decisive role, as we will see. Even when 
some aspects of precision are important, it may be that others do not enter the picture 
at all. If we consider, for instance, the photogrammetric production of maps for highway 
engineering in entirely flat terrain, for which purpose convergent photography has been 
in regular use in the Netherlands since 1932, the construction phase of the project 
requires the availability of a network of points known both in planimetry and height, 
and there is, thus, no need at all for aerial triangulation. Efficiency in the project is