The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part BI. Beijing 2008 
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were longer than the upper limit distance of the instrument (700 
m). 
A DEM was also obtained from radar images. A portion of this 
DEM is represented in Figure 11. 
Figure 11. Portion of the DEM obtained from processing radar 
images 
Since the DEM was generated with radar images, which have a 
cross-resolution that varies with distance, the DEM also has a 
resolution that decreases as the distance increases. The height 
accuracy is of about 5 m and is not comparable with the one of 
the laser scanner DEM, which has a centimetric accuracy. 
3. COMPARISON BETWEEN THE TWO TECHNIQUES 
Thanks to the measurement campaign, the processing of the 
acquired data and the comparison between the achieved results, 
some important aspects of the two techniques can be 
underlined. 
3.1 GB-InSAR technique 
The main negative and positive aspects are reported: 
- only the displacement component parallel to the line of sight 
(LOS) of the radar can be detected; displacements in other 
directions cannot be measured. Hence, it is necessary to 
install the instrument in a useful position to monitor the 
displacement component of interest; 
- the cross-range resolution of SAR images decreases with an 
increase of the measurement distance, and pixels with 
dimensions of some square metres are obtained at 200 m, but 
of more than 10 m 2 at 1 km of distance. This fact limits the 
application of the GB-SAR technique for the study of slope 
movements. Since radar data are obtained through spatial 
averaging on pixels which have an area of several square 
metres, the displacement measurements are only 
representative if they refer to instability phenomenon of large 
volumes or in which differential displacements in the 
landslide body are limited; 
- it is possible to reduce the pixel dimensions of radar images 
by using, for example, higher frequencies of the radar signal 
than the ones employed in the case study (but the 
measurements are more influenced by atmospheric effects); 
- a careful analysis of coherence behaviour of the scenario and 
of the limit values of pixel coherence is necessary in order to 
obtain good results from the interferometric process; 
- the instrumentation is quite heavy and bulky (mainly because 
of the linear rail dimensions, which are usually greater than 2 
m), therefore it is necessary to transport it with all-terrain 
vehicle or helicopter to the installation position; 
- the instrument has to be installed on a stable base, which 
often needs to be specifically built; 
- the GB-SAR instrument has to be installed in such a position 
that no metal objects (poles, panels, ...) are in a range of 10- 
20 m between the instrument and the scenario. If this 
condition is not respected, the radar images could in fact be 
saturated by the high reflectivity of metal objects nearby and 
they cannot be used for displacement monitoring. Motor 
vehicles passing between the instrument and the scenario 
could also create problems and interrupt the monitoring 
process. Furthermore, the presence of thick vegetation or 
stretches of water in the immediate proximity can create 
problems similar to the ones described concerning metal 
objects; 
- the GB-InSAR technique can reach high accuracy in the 
measurement of displacements (as demonstrated in the 
Florence test site); movements of the order of a fraction of 
the wavelength, corresponding to sub-millimetric 
displacements per day, can be appreciated during continuous 
monitoring. Instead, on a long term time scale, i.e. a few 
centimetres per year, it is not possible to reach similar 
accuracy because of phase decorrelation. In these cases, it is 
necessary to turn to different techniques, such as the 
“coherent points” technique, with a decrease in accuracy; in 
these cases, the displacement measurements are limited to 
many sparse points and the broad information provided by the 
interferometric displacement maps obtained with continuous 
monitoring is, in this way, lost; 
- this technique allows remote monitoring up to distances of 3- 
4 km, without requiring the access of human operators in the 
monitored area, thus reducing risks in the case of dangerous 
ground instabilities; 
- the displacement measurements can be realized almost in real 
time, with a minimum time for radar image acquisition of the 
order of tens of minutes; 
- radar data acquisition can be carried out independently of the 
weather and lighting conditions, even though, in the case of 
heavy rain or snow, it would be better to stop the acquisition 
because of data deterioration; 
- the displacement maps obtained through the interferometric 
process are easily and immediately interpretable 
(displacements are usually displayed with colorimetric scales) 
and they allow a global vision to be obtained of the monitored 
scenario, and it is therefore possible to identify and quantify 
the movement extension; 
- this technique allows the generation of DEM of the monitored 
area with an accuracy of a few metres. This accuracy is not 
comparable with that provided by laser scanner techniques, 
but it is sometimes sufficient for interferometric displacement 
map focusing. 
3.2 TLS technique 
The main negative and positive aspects are reported: 
- laser scanner acquisitions have to be carried out in good 
atmospheric conditions (no precipitation); in the case of 
acquisition of digital images, a good illumination of the 
scenario is also necessary; 
- TLS technique can only reach the same measurement 
accuracy as the GB-SAR technique over small ranges (tens of 
metres), but it provides information on each acquired point 
and non averaged data on pixels of some square metres; 
- laser scanners allow millions of points of the monitored 
scenario to be quickly acquired and provide a complete 3D 
model of the scenario;