sec- 
As seen from Fig.1 , from two photos the 3lope of the 
tion of surface cannot be obtained unambiguously. Two ver 
sions of the position of the normal can exist. 
Then,for the slope of the section of surface we can 
write: 
cos £ = coj L l ■ COS i 0L + Sin i- Sin i 01 ■ cos(yi £) 
However,the ambiguity can be eliminated quite easily. In 
most points one of the values £ > 90 deg. Such a solution 
is discarded. If two solutions remain, then the one solu - 
tion is selected from the considerations of continuity and 
smoothness• 
If because of inaccuracy of the data in some point the 
re is no solution(for instance,when in estimating arc cosine 
the argument in module is in excess of unity),taken as the 
solution is the mean value in those adjacent points v/here it 
exists. 
Once the slope <5 is determined,the azimuth of the sec 
tion of surface relative to the plane of the Sun vertical on 
the first photo is determined from the formula 
Cos 
A = 
COS Li - COS £ ■ cos L 0± 
Sin £ 'Sent 
Oi 
(8) 
Having calculated the slope and the azimuth of the section 
of surface as described above we can determine the slope in 
the directions of the profiles parallel to the axes X and Y. 
(It is conveniently to orient the axes X and Y along the li 
nes and columns of the matrix of values of brightness). 
-ta € i = tp G ' cos (# + Aoi) Ï 
ta £ a = tg* £ ’ $ Ln (ÏÏ + floi ) J 
ta 
(9) 
t 
V/here £,± is the slope along the axis X, tgt 2 is the slope 
along the axis Y. 
The weights of the sections of surfaces,(for instance 
along the profile parallel to the axis X) are determined 
from the formula: 
x x a 
2. =■ Ï : - Y 
Lit L 
(^f Z,,L + 
(10) 
178