International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
  
In 
6.9E  6.92E  6.94E Another difference with radar interferometry is the fact that 
optical correlation provides us with the two horizontal V 
components of the motion, whereas interferometry only i 
provides us with the motion projection along the line of sight. n 
Se 
  
  
m 
  
  
  
0 5 10 15 20 25 30 35 40 45 50 55 60 
Figure 5: Total horizontal displacement map, 
23 August/18 September 2003 
4 COMPARISON WITH RADAR 
INTERFEROMETRY 
If we compare the optical images correlation technique with 
radar interferometry, for the monitoring of glaciers surface 
motion, we can notice some similarities: being both based on 
space images, they potentially cover the whole area of 
interest, and provide complete maps of measurement instead 
of point measurement, which is a key issue for global 
monitoring. They also share the same limitation: both are 
limited by the surface temporal coherence loss. Note that this 
temporal coherence loss is only a few days for INSAR and a 
few months for optical images of glaciers. 
But there are major differences between the two techniques: 
it may be difficult or even impossible to measure large terrain 
movement by interferometry because, one fringe representing 
half the wavelength of displacement -2.8 cm in the case of 
ERS and Envisat-, the number of fringes in the interferogram 
can rapidly become very high, making unwrapping 
impossible. In the case of optical image correlation, the 
measurement is proportional to the image resolution. For 
example, a Sm glacier motion will represent 178 fringes 
(impossible to unwrap) in the interferogram, whereas it will 
appear as a 2 pixels offset (easy to measure) if we correlate 
two 2.5m resolution optical images. However, the resolution 
of the optical images is a key factor, because the accuracy of 
our measurement is directly proportional to the image 
resolution, which is not the case in interferometry where the 
accuracy is proportional to the wavelength, in all cases 
around the centimeter (Massonnet, 1998). 
  
A displacement along the track, for example, would never be 
measurable in interferometry. 
5 APPLICATION OF THE 
METHODOLOGY TO OTHER 
GEOPHYSICAL PHENOMENA 
Our methodology has been developed on glaciers but its field 
of application is much wider. Any geophysical phenomena, 
which signature is a surface horizontal terrain displacement, 
on a scale compatible with the satellite image resolution and 
spatial coverage, is a potential candidate for the method, 
provided the geophysical event does not make the surface 
totally losing its coherence. 
In particular, the same methodology can be applied to the 
measurement of other earth surface movements such as 
landslides or earthquakes. 
6 CONCLUSION 
This work, still ongoing, seems very promising for glacier 
surface motion monitoring. Acquiring regularly high 
resolution images on glaciers, one can expect being able to 
follow, year after year, the variation of their velocities. 
Images should better be acquired during the summer, snow 
falls being a problem because they induce coherence losses. 
The interest of glacier monitoring from space images is high, 
if one considers the difficulty of field campaigns in such a 
harsh mountainous environment. Regular acquisitions should 
be programmed, if one considers the probability of cloud 
coverage and the problem of coherence loss on long time 
periods for such surfaces. 
We have explained the general methodology applied for 
displacement computation, and we have shown preliminary 
results on the Alps. The same methodology might directly be 
applied to glaciers from other mountain ranges. Future work 
will consist into trying to extract other useful information, 
such as ablation values. 
References 
Carfantan H, 2001, Estimation non biaisée de décalages 
subpixelaires sur les images SPOT. GRETSI Proceedings, 
Sept. 2001 
Goldstein RM, 1993. Satellite radar interferometry for 
monitoring ice sheet motion: application to an Antarctic ice 
stream, Science, 262 (5139)1525-1530 
Latry C, 1995, French patent. « Procédé d’acquisition d’une 
image par balayage pousse-balai ». Patent n° 95 09242 
Massonnet D, 1998. Radar interferometry and its application 
to changes in the earth surface. Review of Geophysics, 36(4), 
441-500 
834