VIATEK CONSULTING GROUP, Tapióla, Finland 
The Finnish Digital Terrain Model and Experiments performed 
with it. 
Civ.eng. E Viita, Helsinki 
1. Theory of the Digital Terrain Model 
The Finnish Digital Terrain Model is a part of a larger system 
of road planning, the so-called Finnish Surface System. The de 
velopment of this system was commenced in 1964. 
The basis of the Digital Terrain Model is formed by terrain points 
that can be arranged into triangular or square patterns or given 
as arbitrary random points. 
The interpolation of elevation is in principle carried out by 
simulating the terrain surface with plane triangles. The routine 
of the elevation computation is determined by the form of the 
points of the model. In the designing of the Digital Terrain Mod 
el the simplicity of solution and rapidness of use have particu 
larly been stressed without, however, comprising the demands of 
accuracy. The Digital Terrain Model functions in the following 
manner: 
If the terrain points form equilateral triangles, the triangle 
that is formed of the three nearest points is used. This form 
of Terrain Model is the most rapid in computer calculation and 
it can be used for instance when the points are taken from 
aerial photographs. 
If the terrain points form squares the four nearest points are 
used in such a way that the basis of the triangle is formed 
by the two nearest points and the third and fourth points of 
the square are the apexes of the triangles. The elevation of 
the point is determined from both the triangles that are formed 
in such a way and the mean value is used. 
If the basic points of the terrain model are random points, the 
method of calculation is a combination of the elevation deter 
mination methods described above. The programme first tests 
if the point to be measured falls within the triangle that is 
described by the three closest points. If this condition is not