CONVERGENT VERSUS VERTICAL PHOTOGRAPHY, SCHERMERHORN 115 
Large scale mapping : 
Here the following camera types are compared: 
21/18 
11.5/18 
10/14 (plates) 
15/23 
with the convergent cameras 18s, 21/18 
308, 21/18 
a. vertical cameras, size 18 X 18 em? (21/18 and 11.5/18) 
The convergent cameras are very much superior to these two types as far as net 
coverage, number of models and number of passpoints are concerned. The figures of 
comparison are as follows: 
flat terr. mountain. terr. 
21/18 to convergent cameras 185, 21/18 1:14 1:14 
30g, 21/18 1:17 1:1.6 
11.5/18 to convergent cameras 185, 21/18 1:14 1:23 
308, 21/18 1:19 1:26 
b. Vertical plate camera 10/14 (size 14 X 14 cm?) 
to convergent camera 18g, 21/18 1:16 1:25 
30s, 21/18 1:19 1:28 
c. Vertical large size 15/23 (size 23 X 23 cm?) 
to convergent camera 18g, 21/18 1 :0.87 1:41.06 
1:1.01 1:12 
From this it can be seen that the vertical cameras of size 18 X 18 cm? and 14 X 14 
cm? are greatly inferior to the convergent cameras, in flat terrain to a ratio of 1.4 to 
1.9 and in mountainous terrain to a ratio of 1.4 to 2.8. In flat terrain the large-size 
cameras are equivalent, but in mountainous terrain they are inferior to a factor of 1.2. 
Whether these ratios change with an accurate determination of “equivalent” scales 
of photography will be shown by the results of the current I.S.P./O.E.E.P.E. tests”. 
Brucklacher considers, in particular, a comparison of Super wide-angle photography 
with American wide-angle convergent photography. He assumes a scale of photography 
of 1 : 60,000 for mapping at 1 : 100,000. He takes two different types of mountainous 
terrain: the first with H,,, — H,,, of 2125 m, and the second with only 1000 m. Here 
again, the convergent system has a greater coverage per model. For flat terrain the 
proportion is 1: 1.4 and for the quoted cases of mountainous terrain it is 1: 1.7 and 
1:2.3. We believe, however, that such figures show only one side of the medal. 
In this respect, the contributing of Mr. M. Thompson, Chief of the Photogrammetric 
Surveys Section of the U.S.G.S. has a greater importance, since it deals with the results 
of topographie mapping for the standard accuracy, 73 minute sheets, at a publication 
scale of 1 : 24,000 and the 15 minute maps at a publication scale of 1 : 62,500. 
Thompson starts with the remark that the advantages of a large base-height ratio 
(1:1.23) decrease with increases in the steepness of the terrain. It is usual practice with 
super wide-angle vertical photography, however, to fly with 8096 longitudinal overlap, 
carry out the necessary bridging with the 60% overlap and, in terrain with very steep 
slopes, to plot from models with 80% overlap. In that case the relatively large contour 
interval makes the loss of precision in height, due to the shorter base length, acceptable. 
Convergent photography can overcome this difficulty by reducing the overlap in the same 
way. It should be appreciated that the difficulty for topographic mapping arises only 
for a very small percentage of the earth’s surface. For large-scale mapping, it is a dis- 
advantage for all built-up and densely populated areas.