Finland 
5 
The Extension of Horizontal Ground Control 
The use of aerial triangulation for the extension of horizontal ground control 
(x, y) for mapping on 1 : 4 000 — 1 : 100 000 scales has continuously increased 
as can be seen in the Table. 
The geodetic triangulation nets, which serve as the basis for the aerial 
triangulation, are measured combining triangulation and trilatération. Thus a 
significant increase in measuring speed and accuracy has been achieved as 
compared with the classical method with angle observations only. The triangula 
tion points (approximately 4 points/100 sq.kms) are signalized before the 
photography. The elevation control net is established by levelling (in basic 
mapping) at 5 — 10 km intervals. When necessary, the elevation net is condensed 
for aerial triangulation by using the tachymetric method. 
Aerial triangulation is performed using the Elevation Controlled Method, 
in which each model is levelled. Most of the work is done with the C 8 
Stereoplanigraph equipped with an automatic recording device. The transforma 
tion and adjustment of coordinates are performed with the IBM 1620 and 1401 
and National Elliot 803 electronic computers applying second and third degree 
conformal transformations. 
With a geodetic horizontal control point on approximately every third model 
the standard error of the new points is found to be - 0.28 — 0.40 microns 
at the scale of photograph. The results of aerial triangulations during the period 
covered by this report are shown in the Table. 
In addition to the ordinary aerial triangulation, the extension of ground 
control for large scale mapping (1 : 500 — 1:2 000) is performed using the 
Single Model Method. The standard error of the photogrammetrically determined 
coordinates has proved to be 12 — 14 microns, when photographs on the 
scales 1 : 3 000 — 1:9 000 are used. 
Basic Map 1 :10 000 / 1 : 20 000 
The aerial photography is usually performed at the altitude of 4 500 meters 
using simultaneously two cameras: a wide and a normal angle camera. The 
wide angle pictures of 1 : 30 000 are used for the aerial triangulation and 
stereoplotting and the normal angle pictures of 1 : 22 000 for compiling 
aerophotographic maps 1:10 000. Flat areas are photographed only with a 
wide angle camera. 
Aerotriangulated points, approximately 150 points/100 sq.kms are used 
for computing the rectification data for aerial photographs and for compiling 
the aerophotographic map of 1:10 000 and further as control points for 
stereoplotting. 
Copies of the aerophotographic map of 1:10 000 on dimensionally stable 
Correctostat-paper or on plastic sheets are used as a basis for field drawing. 
Rectified aerial photograps on the same scale are prepared for stereoscopic 
interpretation of the terrain. 
In the summer, preceding the actual mapping year, topographical work is 
started with elevation measurements done tachymetrically in order to determine 
the necessary vertical control points for the contour plotting, numerical elevation 
presentation, and for controlling the aerial triangulation. During the following 
winter contour lines are plotted with Wild A 8 Autographs and Wild B 8 
Aviographs from 1 : 30 000 wide angle photographs for such areas, where the 
terrain is not too flat or forest too dense for it. (In the following summer the 
plotted contour lines are checked and supplemented in the field in connection 
with the ground inventory). Property boundaries are placed on the field drawing