CHAPTER VII. PHOTO-INTERPRETATION 
For this period an increasing activity can be mentioned. We must distinguish between 
the application for forestry, for geology and for soil survey and deal with this in separate 
sections. 
VII. I. Interpretation for forestry 
VII.l.a. Activities in the Netherlands 
Training possibilities. As in preceding years a special course was given at the I. T.C. in 
Delft dealing with the application of aerial photographs in forestry. In recent years, in 
addition to indoortraining, importance was attached to giving the students the opportu- 
nity to carry out field work. The forest regions selected for this purpose were situated in 
Austria and Switzerland. In the coming year a forest area in Germany will be selected. 
During the field work the student has the opportunity to check his interpretation work 
done in the office and to collect all further information necessary for the compilation of a 
forest type map of the region. The final results are afterwards worked out as a report to 
be submitted at the end of the course. 
The scale of the aerial photographs used for this purpose varies between 1 : 20.000 and 
1 : 10.000. Provisional results obtained so far indicate that large-scale photography (say 
scale 1 : 10.000) is preferable to smaller scales. Particularly when the local forest contains 
numerous stand-types, as is mostly found in Switzerland, large-scale photographs proved 
to be very necessary. If no large-scale photography was available enlargements also gave 
satisfactory results. During the field check there is the opportunity to make comparative 
tests of interpretation results obtained from different scales and film-filter combinations. 
Also the result of different printing methods can be ascertained. 
Different methods of map compilation are afterwards used in the office. Transfer of 
detail from the photographs to the map is mostly done by Sketchmaster. The instrument 
constructed by Zeiss is normally preferred above other types such as Abrams and Fairchild. 
The use of the Sketchmaster is specially recommended when detailed and reliable forest 
maps are available and further when the local differences in the elevation are relatively 
small. In pronounced mountainous regions the Radial Line Plotter proved to be the 
most suitable instrument. 
Forest management planning. Aerial photographs are used by the State Forest Service for 
the compilation and periodical revision of management plans. Aerial photographs used 
for this purpose have scales varying between 1 : 20.000 and 1 : 10.000. Photographs are 
mainly used as a guide to the identification and delineation of stand differences. During this 
work, which is mainly executed in the field, other data such as occasional forest roads and 
drainage ditches are also indicated on the photographs. This information is afterwards 
transferred to the map by means of a projection instrument. 
National Forest Inventory. To obtain basic information regarding general forest conditions 
over the whole country, a general forest inventory has been started based upon the study 
of aerial photographs. In addition to information on the condition of the forests, data are 
collected dealing with waste land. The scale of the photographs used for this purpose is 
1 : 20.000. Upon these photographs stand boundaries are delineated and afterwards 
transferred to the topographical map 1 : 25.000. Mapping results are afterwards checked 
in the field and at the same time volumes are determined. 
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