13 
Photogrammetric plotting from hydraulic models. 
The examination of hydraulic models by means of photogrammetric plotting has proved to 
be a remarkable realisation in the applications of photogrammetry. 
The laboratory of Hydraulic Research, which appealed to the photogrammetric method for 
the study of its models, uses this method for observing the variations of the bottom from models 
with a so-called moving bottom. 
In definite zones of the model the bottom consists of a plastic material of which the granu- 
lar structure represents articicially the bed of rivers, estuaries and such-like. 
The different experiments are made so as to change the bottom relief of the models by ar- 
tificial currents. 
The task of photogrammetry consists in observing these changes of the model continuously 
and in mapping them out quickly. 
The method used is a classic exemple of photogrammetric pratice. From two viewpoints 
photographs are taken simultaneously with a camera on a fixed base. In order to get a regular 
overlap of the negatives, we have the disposal of a roller-bridge to which the apparatus is faste- 
ned. The marks necessary for the plotting are placed on the model and measured prior to pho- 
tographing. : 
The first example shows the plotting of a model of the Scheldt. 
The bottom consists of granulated plastic material, and its changing by imitated tidal cur- 
rents has been controlled. The photographs have been taken after draining the model. 
When plotting the contour lines the accuracy of the classic and the photogrammetric method 
has been tested by direct comparison, the model having been treated by the classic method too, 
which had used the photographs destined for the photogrammetric purposes. 
The second example, which is treated in the annexed text, represents the study of a model 
of the «Inga-Project» with fixed bottom, although the conception is of a different kind, the me- 
thod used is identically the same. 
The main points in this case were : 
1°) To prove the uniformity of the model and of the topographic plans. 
2°) To possess a permanent plan of the testing form of the model. 
3°) To be able to realize this study upon the model of different hydraulic engineering structu- 
res, by means of a high precision plotting. 
After two such experiments the departement of Public Works has planned the systematical 
use of this method in the near future.