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REPORT OF COMMISSION VII 81
same time. The field work required to determine the plan of roads and water-
courses can be reduced considerably by the use of photographs. Vertical photo-
graphs on a scale of 1:10,000 or larger, taken early in spring, are preferred.
From these, enlargements are made on a scale of 1 :2,500. The work is generally
performed on the enlargements. At the same time for large reallotments a rough
mosaic is compiled to provide a general view of the area.
F. HYDROGRAFISCHE DIENST AT THE HAGUE (HYDROGRAPHICAL MAPPING)
In the main the field of operation of this Service is in the open sea, where
only in exceptional cases do the aerial photographs offer sufficient data. The use
is only applied restrictedly and in shallow waters for the defining of the profile
of the sea bottom, for obtaining a view of the course of underwater sandbanks
and gullies, and for tracing coral reefs in tropical waters.
The methods by which multiple film-filter combinations are used when taking
aerial photographs have been studied and appear to produce good results in
shallow water. For peacetime application, however, these methods are too ex-
pensive. Other methods have been studied which define the profile of the sea
bottom from aerial photographs on the basis of the trend of the wave direction.
However, the cost of taking photographs for this purpose is too high, since
information is obtained only from a narrow strip. Existing photographs are
sometimes used for this purpose, and at the same time for mapping coastlines
and the topography of the coastal strip. For the above-mentioned purposes
a scale of 1:40,000 is preferred.
G. DIENST VAN DE ZUIDERZEEWERKEN AT THE HAGUE (RECLAIMING THE ZUYDER
ZEE)
As the application of aerial photo-interpretation in this Service is still in
the first stage, the whole domain of possibilities of application cannot yet be
viewed. Accordingly, in the following only those matters are discussed in which
interpretation has yielded satisfactory results.
1. Supplying of hydrographical data. This has already been practiced both at the
mouth of the Kattendiep in the Ysselmeer and at the Balgzand in the Waddenzee.
2. Studies of the movement of mud. It is a fact that with the aid of aerial photo-
interpretation, data can be obtained regarding the displacement of mud which cannot
be obtained in any other way. Unexpected differences in mud percentages locally
measured can be explained in this way. For this purpose the photos should be taken
when there is a strong wind coming on and they should be obtained at frequent intervals.
3. Ice-drift on the Ysselmeer. Formerly special flights were made over the Ysselmeer
in order to observe during thaw the moving of ice owing to the influence of wind. In the
future this problem will be studied with the aid of photographs taken from great heights.
4. Hydrological studies. Yt is expected that, with the aid of aerial photo interpreta-
tion, data can be derived regarding ooze. Results, however, are not yet obtained.
5. Undulatory motion. Some results have already been obtained which provide a
better view of undulatory motions, wave patterns, interference phenomena, etc. by means
of interpretation. This is of great importance for the building of new dikes.
H. RIJKSDIENST VOOR HET NATIONALE PLAN AT THE HAGUE (GENERAL PLANNING)
In this service as well as in the different provincial subdivisions a very limited
use still is made of the aerial photograph. However some use has been made in
preparing regional plans, in preparing municipal plans for the extension and
purchasing of grounds, in studies of the development of town planning, and in
planning for the agrarian use of soil. In general vertical photos on a scale of
1:10,000 taken in wintertime or in spring are preferred.
