Full text: International cooperation and technology transfer

To be economically acceptable, DEM generation 
should provide accuracies matching those of manual 
plotting (i.e. between 0.1-0.2 %o of the relative flight 
height h r ) in a shorter time. Since the point 
measurement rate is much higher than that of the 
human operator (and bound to increase further with 
processing power), large DEMs with hundreds of 
thousands points may be generated in an hour time or 
less, depending on hardware and software 
performance. As far as accuracy is concerned, figures 
range from 0.1 to 0.8 h r (Bacher, 1998; Balsavias and 
Kaeser, 1998, Duperet, 1995), therefore not always 
matching manually produced DEM. Off-line editing is 
necessary, clearly pointing out that self-diagnosis tools 
are not as reliable as they should. Indeed, in large scale 
photogrammetry and problem areas the percentage of 
points to edit may be as large as 10% or more, making 
it a key feature of the system the performance of the 
editing tools available to the user. 
Any DEM generation system has self-diagnosis 
capabilities at the matching level as well as at the 
interpolation level. At the former, systems mostly 
provide a quality index of each matched point which 
can be as simple as the correlation coefficient 
(confronted with a terrain and/or image dependent 
threshold) or may be based on a classification scheme, 
reporting or providing hints in case of failure, in more 
sophisticated systems. Overall, though, they are not 
always reliable, since they may label as good wrong 
points or may discard them as wrong when they are in 
fact good ones. At the interpolation level, self- 
diagnosis is understood here mainly as the capability of 
filtering out non-terrain objects, rather than blunders in 
the matching process, which may be best dealt with by 
geometric constraints supporting the matching. 
Despite this drawbacks, the advantage of automation is 
spreading the use of automatically generated DEM. 
Recently we have been involved in a project in the 
framework of the management of tens of marble 
quarries in northern Italy, in the province of Brescia. 
The goal of the investigation is to check the accuracy 
level of automatic DEM generation in these areas and 
what kind of project parameters (image resolution, 
scanning accuracy, additional terrain information, 
matching techniques) are best suited. This paper 
reports on the results of DEM generation in the 
repeated survey of a rock quarry, to figure out the 
volume of material excavated, using aerial images. 
This information is used in planning the exploitation of 
the quarries by the authorities in charge; accuracies of 
some percentage point on the volume’s estimate are 
acceptable. A direct control of the excavation volumes 
to fix taxes has been for the time being ruled out, since 
weigthing the lorries in and out of the quarry is more 
The project goals were set as follows; 
1) to study which selection of the flight parameters 
would be the best so that the performance of DEM 
generation could be improved; 
2) to study the accuracy of the DEM and of DEM 
changes, with respect to DPW system (scanner as 
well as software) and what gains could be made by 
first measuring manually a low resolution DEM. 
The marble quarry in Botticino, a small village near 
Brescia, was chosen as a test area, because a recent 
aerial survey was available and because it is quite 
representative of the quarries in the area; they are 
located in steep hills, with height differences from top 
to bottom up to 400-500 m; their fronts range from 
several hundreds meters to more than 1 km. 
The excavation proceeds in stages, cutting the hill side 
in banks 10 to 30 m high, slightly inclined downhill; 
though more or less you may recognize a main front 
running parallel to the contour lines, large blocks are 
extracted here and there, leading to a pattern of 
“holes”. Debris is spread around in several areas, 
depending on which front is currently active. The hills 
around the quarries are covered by bushes and trees, 
while the excavation area is rather bright, resulting in a 
scene with an overall high dynamic range but (though 
not everywhere) a low contrast within the quarry, in 
full daylight and clear sky. 
The first flight, executed in summer 1997, consists of a 
single strip with 4 images (numbered 49 to 52) at the 
average scale 1:5600 and lead to the compilation of a 
1:1000 map of the quarry, which is contained in the 
model 50_51. Although the flight was flown around 
midday, sharp shadows are projected from the banks, 
making sometimes difficult to identify the exact 
location of the base of the walls. 
2.1 The new flight plan 
As already mentioned, the second part of the project is 
supposed to complete a new flight over the same area, 
to highlight volume changes in the time span and to 
allow an accuracy evaluation thanks to a topographic 
The new flight plan has been designed in order to 
improve the accuracy of automatic DEM generation by 
proper choice of the flight parameters. Three aspects 
have been taken into account: flight height and camera 
focal lenght, endlap and sidelap values along and 
across strip and finally strip direction versus terrain 
For a constant image scale, a lower flight height means 
using larger focal lenghts, increasing areas prone to 
occlusions and perspective distortions. A longer focal 
lenght would reduce the occlusions and also help 
image correlation to account for perspective 
differences; if image scale is taken constant, this would 
on the other end change the H/B ratio, worsening the 
elevation accuracy. Larger image scales, compare to 
the well established standard values adopted in 
mapping with analytical plotters, would undermine the 
economies obtained by digital methods, as only for the 
larger surveying work implied. Based on this 
reasoning, either image scale and focal lenght have 
been retained to the values of the preceeding flight. 
Though it is hard to figure out their effect in advance, 
experience shows that occlusions and terrain 
discontinuities degrade the accuracy of DEM generated

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