Where A is the distance between the centres of the secti 
ons of surface in the horizontal plane - step along the pro 
file • 
In the same way the heights along the axis Y are deter 
mine do 
To minimize the effect of random errors it is expedient 
that the mean value 
do 
be taken as a digital model. 
All the elements of the matrix 
to the value of the first element, 
element is unknown in advance,it is 
are determined relative 
If the height of the first 
considered to be equal to 
zero. 
such 
The relative heights (first approximation) obtained in 
a way shall bear errors due to the fact that we assumed 
the illumination intensity caused only by the direct light 
(formulae 1 and 2) to be the total intensity of surface illu 
mination* But,as was mentioned above,the brightness of the 
surface is the result of the direct sun illumination and the 
radiation diffsed by the atmosphere and reflected by the sur 
rounding elements of the terrain. The problem is to determine 
the contribution of the diffused and reflected radiation to 
the total illumination intensity* 
The total illumination intensity of the element of terra 
in is equal to 
+ E d + E R (12) 
where Eri is the direct sun radiation ; £ o is the radiati 
on diffused by the atmosphere and is radiation reflected 
from neighbouring slopes. 
The flux of direct radiation is determined from the formu- 
la 
- X m_Q 
= E 0 ■ e cos (n ■ l) 
whereto is the spectral sun constant, T 
thickness of the atmosphere along the normal 
(13) 
is the optical 
to the Earth