176 
The essence of the method is as follows• Use is made 
of two photos of the areas of the surface obtained at the 
values of the Sun azimuth differzing from each other by 
about 90 deg. Using the technique (Matiyasevich L.M.,et 
alo,1990) developed in the State Centre "Priroda" we de - 
termine and enter into computer memory the combined mat - 
rices of brightness of elements of the terrain correspon 
ding to the conditions under which the first and second 
photos were obtained.Since clean snow follows the lav/ of 
reflection close to the Lambert ! s law for brigtnesses of 
some element of snow-covered surface measured on two photos, 
it can be written 
cular to the falling rays, £ is the coefficient of bright 
the surface element. Index 1 means that the formula relates 
to the first photo,index 2 - to the second photo. 
The corresponding brightnesses of horizontal sections 
of surfaces shall be expressed as 
Dividing formulae (1) by the corresponding formulae (2) we 
shall obtain the so-called normalized brightnesses 
Hence,the angles of incidence of the sunrays falling on the 
surface element under two different conditions of illumina - 
tion can be expressed as follows: 
It should be noted that the normalization is possible on the 
brightness of arbitrary area of the surface with the known 
slope. 
where E is the illumination intensity of the area perpendi 
ness, t is the angle of incidence of the sunrays falling on 
E 
? C °S L os (2) 
01 
(3)