6 Finland 
The aerophysical surveys cover about 12 000—15 000 km 2 annually. The total area sur 
veyed during the years 1951 —1955 is 55 700 km 2 . 
Further on the controlled mosaics are used in different geophysical measurements in con 
nection with prospecting. They are used as a basis for planning and marking survey lines 
and as base maps in geologic investigations in connection with prospecting. The scale used 
in this work is usually 1 : 20 000. In detailed explorations of small areas, enlargements of the 
mosaics in the scales of 1 : 4 000 or 1 : 2 000 are also used in some extent. 
In recent years stereo interpretation has been used in investigating the glacial geology of 
the mountain area of Lapland. With the aid of a mirror stereoscope and a parallax bar the 
morphology of the mountain area was investigated and the origin of the soils explained. In 
addition, the limits of different soil areas were determined stereoscopically, thus reducing 
the field work only to checking of this office work. 
For the present, the geological investigations of our country use the possibilities offered by 
photogrammetry chiefly in the form of controlled mosaics. The need of mosaics for this purpose 
is annually about 15 000 — 20 000 km 2 . 
F orestry. 
During the last years the Finnish Forest Service has expanded the use of aerial photography. 
The total area of the mapping and timber surveying projects in North Finland (1947— 1955) 
is 20 470 km 2 . The area of the finished forest maps is so far 6 800 km 2 . 
These projects have been carried out in close co-operation of the Forest Service and the 
Army Topographic Service. The photography has been carried out with a wide angle 
camera on panchromatic film in a picture scale of 1 : 40 000. Because of the known tilt ob 
tained by the two pictures of the horizon line for every exposure, rectified prints can be used 
in every phase of the procedure. 
As to the ground control, a geodetic net is measured using triangulation and traverses. Four 
or five control points are considered satisfactory for forestry purposes in each square of 
20 km X 20 km. 
The ground control is extended by the radial line plot in the scale 1 : 20 000. The rectified 
copies are made in an approximate scale 1 : 20 000. The set of enlargements used is exposed 
on wet paper and treated with glycerin in order to make them unshrinkable, flexible, and 
easy to handle. The ground control points, the nadir points as radial centers, and the pass 
points are marked on enlargements. 
The radial line plot is assembled with mechanical triangulators of »Lazy Daisy»-type, made 
in Finland. The rectified unshrinkable pictures enable a practically errorless nadir point 
triangulation. 
Based on the control prepared by the radial triangulation the exact enlargement ratios 
are calculated and new rectified copies in the scale 1 : 20 000 are made on semimatte paper, 
to be used in the photo interpretation and in the field. 
The net or mapping area to be photo interpreted is outlined on every photograph. Using 
the area around the radial center makes the image displacement in the photo detail transfer 
as small as possible. The photo interpretation is carried out by the aid of mirror stereoscopes. 
In spite of the comparatively small photographic scale, the clear and distinct terrain and 
vegetation features of the northern forests allow a very reliable sketching of the site and 
stand lines. The field checks have shown that 80 — 90 per cent of the photo details needed 
in the final forest map have been correctly interpreted. 
The interpreted photographs are prepared into stereograms which are brought to the 
field where the photo interpretation is checked and completed in connection of the stand 
by stand estimation of the silvicultural condition, the volume, growth, and other data necess 
ary for the timber management plans. 
Transfer of the photo details to the transparent base map sheets is carried out on the light 
table or by transferring instruments, like sketchmasters. The forest maps are printed in the 
scale of 1 : 40 000. 
In South Finland the Forest Service often uses photographs taken for other purposes. 
The photographic scale has been 1 : 20 000 and rectified copies in the scale 1:10 000 have