The International Archives oj the Photogrammetry, Remote Sensing and Spatial Information Sciences. Voi. XXXVII. Part B7. Beijing 2008 
Fig. (2): 3D view of glacier with DEM and Quickbird image color 
composite (3,2,1) 
4.3 Displacement computation using cross-correlation of 
multi-temporal imagery 
In this work ,a pixel-based image matching approach is used to 
derive metric information on glacier displacement .Two well co 
registered orthoimages are matched through correlation analysis. 
The general principle of offset computation by correlation is the 
following: around every reference image point, for which one 
wants to compute the local offset with a secondary image, a so- 
called rectangular “vignette” is constituted, centered on the 
reference point. Having a rough knowledge of the local offset, 
we can center the search on the secondary image around the 
estimated position. We iteratively build a small vignette on the 
secondary image, which has the same size as the primary one, 
and which travels through an area called “research area” .The 
size of this research area is related to the uncertainty on the 
position in the secondary image(H.Vadon et al.,2001). 
In order to measure correlation rates, we used weighted correlation 
co-efficient which is calculated as follows : 
X X M w, )(g 5 (*,, y, ) - g’ )(.g' (*,, yj ) - g' ) 
p* ■„ „ „ ——r 
[Z J 'Z i ™(x l ,yj)(g 3 (x n y J )-g s ) 2 .'£'£ i w(x l ,y J )(g'(x l ,y J )-g') 2 ] 2 
1=1 7=1 1=1 7=1 
First, correlation rates are computed for every vignette position in 
the research area. The best correlation corresponds to the position 
in the research area of the secondary image that maximizes the 
radiometric resemblance between the two vignettes. Then sub 
pixel search with similar principle is iteratively performed until 
the required localization accuracy is reached and afterwards offset 
computation is done(H.Vadon et al.,2001). 
After correlation analysis, the displacement map is produced. 
5. RESULTS AND DISCUSSION 
Fig. (3) Shows the horizontal displacement of the ground surface 
of Alamchal glacier .On this glacier , no overall displacement 
measurement had ever been performed. 
This research shows a significant retreat in glacier front over a 
long period of time, indicating a decrease in glacier reservoir. 
Furthermore, the results demonstrated the occurrence of major 
surface displacements in the central region of the glacier due to 
both accumulation and ablation. 
In the course of this study, some problems appeared. No 
displacement could be measured for areas in shadows and at high 
elevation on the accumulation zone, even with a low gain, the 
noise remained noticeable. 
4gyooo 4ggooo 4ggooo 
Figure. (3): displacement image of glacier displacement 
4 98 000 
4 99 
1 
4gyooo 
Figure. (4): 3D view of displacement of glacier 
The apparent displacement of the shadows is negative and oriented 
mainly parallel to image lines. The strength of the correlation is 
weakened by a secondary correlation peak due to the shifted 
shadows. However, on the glaciers, this shadowing effect does not 
cause a systematic error in our measurements. Shadows are only 
created by the surface debris and the surrounding mountains. The 
best way to ensure a limited bias is to retain only the 
measurements with the highest correlation coefficient. The effect 
could be larger for other applications such as landslides, especially 
if images are acquired during different seasons.