Finland 
5 
by using the control points obtained by the aerial triangulation. For each map sheet about 
1 io — 130 pairs of photographs are to be compiled. The contour sheet is reduced photographi 
cally to the scale of 1 : 20 000 and copied together with the photo mosaic. 
The office compilation, which contains most of the features to be shown on final map, 
is sent to field surveyors for completion and checking. The field-completed mosaics are then 
reduced photographically to the scale of 1 : 50 000 and fitted together to a mosaic assembly, 
from which the color separation drawings are made on transparent plastic sheets. 
Large scale maps. 
For the fundamental maps of urban agglomerations in the countryside, a scale of 1 : 2 000 
and a contour interval of one meter is used. Ordinarily these maps cover areas of 5 — 6 km 2 , 
and they are prepared by the General Survey Office or by some private surveying enterprises. 
The ground control consists of traverse points (about 20 points per km 2 ), which are signali 
zed before aerial photography, and of additional bench marks, determined by precision 
leveling. 
The aerial photographs are taken with a normal-angle camera in the scale of 1 : 10000 
for enlargements and with a wide-angle camera (f = 11 cm) at a flight altitude of 1 300 — 
1 500 meters for the stereoscopic measurement. Depending on the relief and vegetation of 
the region the survey is accomplished by using either the stereoscopic measurement or the 
photo enlargements or these two methods simultaneously. 
The property boundaries are surveyed by field methods. All other features are mapped 
using aerial photographs. 
The pictures of the normal-angle camera are rectified and enlarged into 1 : 2 000. Two 
sets of copies are prepared, the field photographs and corresponding stereo pictures. The 
features to be mapped are compiled on photo enlargements in the field and a leveling is ac 
complished for plotting the contours on the photographs, using stereo photographs as an 
aid to simplify the work. In addition to the signalized traverse points, supplementary conspi- 
cious terrain features are measured in the field until the co-ordinates of about 50 points per 
square kilometer are known. The control points and the property boundaries are plotted on 
a transparent sheet of astralon. The terrain features are then traced on this sheet from 
the field photographs so that only a little area of the terrain is traced at a time and the astra 
lon sheet is oriented by using the nearest control points in order to eliminate the relief 
displacement. In this way about 650 km 2 has been mapped during last 4 years. 
In recent years stereoscopic plotting has been taken into use also for town planning surveys 
and two autographs Wild A 8 have been purchased for this purpose. Stereo plotting is used 
on sparsely wooded areas and rough open areas. In heavily wooded areas and flat open areas, 
where a high elevation accuracy is needed for drainage planning, a leveling is accomplished 
in connection with the other field surveys. 
While checking the contour accuracy it has been realized that the mean difference between 
the elevations obtained by leveling and by stereo plotting is 0.5 meters in wooded areas and 
0.2 m in open areas. 
Prospecting and geological mapping. 
The results of the geological surveys are published in form of rock and soil maps at the 
scale of 1 : 100 000 with accompanying written explanations. 
Field surveys of the rock formations are at present carried out by using 1 : 20 000-scale 
maps or, preferably, controlled mosaics, as a basis. These controlled mosaics are mainly used 
in prospecting, especially in the aeromagnetic surveying, which started in 1951. This is ac 
complished by flying over the region in straight lines with a spacing of 400 meter at a flight 
altitude of 150 metres. Controlled mosaics are used for orienting the aircraft. For determining 
the actual course of flight the terrain is photographed on a continous strip of 35 mm film 
with a strip camera built in Finland. Thus also the necessary connection points for location 
of the results of the magnetic survey are obtained. A mean deviation of ^ 20 meters from 
the planned flight line has been observed.