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International cooperation and technology transfer
Mussio, Luigi

by correlation. The flight plan can be adjusted to
minimize their impact in two ways: modifying the
overlap and carefully considering terrain morphology.
Quoting Ackermann, in digital photogrammetry
“redundancy should replace intelligence”: taking and
processing several images of the same area rather than
a single model is therefore acceptable, if it pays off in
terms of better overall accuracy, reducing the number
of mismatches. The new flight has therefore been
designed with 80% overlap and 60% sidelap, in order
to derive models with a less favourable height to base
ratio, but with smaller perspective distortions. They
will be processed first, to provide an initial DEM to be
later refined with the 60% overlap models.
Using sidelap up to 60% yields four-fold model
coverage of the same terrain patch. The advantage here
is that each model has a different viewing direction,
increasing the possibilities that occluded areas are
visible in at least one model. Currently, virtually all
DEM generation software is stereo-based; thought the
use of several images is envisaged and now generally
recognized as beneficial, this can be implemented in
two significantly different ways: repeating DEM
generation in different stereomodels over the same area
is not as effective as actually performing DEM
generation with all images at the same time, like in
MGCM (Gruen and Baltsavias, 1985) or similar
approaches. Using several models yields elevation data
at nominally coincident grid knots. The question
therefore arises what is the best weighting of each
elevation value at a given location. A simple averaging,
thought optimal in most cases, would certainly fail in
problem areas, where more than one value may be
grossly falsified. Even more robust estimation
methods, with a possible outliers percentage larger than
50%, may give incorrect results.
As far as flight direction is considered, if the
excavation front of the quarry is elongated and follows
roughly the contour lines, then mimimization of the
occlusions may be achieved by using only half image
and flying just over the lower edge of the front. Parallel
strip may be flown, if necessary, maintaining a 60%
side lap, covering the quarry from the lower edge
Illumination may affect the results, given the strong
terrain reflectance, either because of the shadows as
well as for the dynamic range of the scene.
Minimization of the disturbances is achieved by flying
at midday and/or prescribing a flight in a cloudy day.
Good image quality make it necessary a FMC system:
its effect is nevertheless limited because of the large
heigth differences in the model, which leaves a
significant residual motion.
2.2 Reference data
The contour lines file of the 1997 map 1:1000 has been
made available by the Comune di Botticino. The
accuracy of elevation data therein may be assumed to
range from 20 to 50 cm. Data have been interpolated
either by Kriging and by Delaunay triangulation over a
2m grid, to be used to compute the differences with the
elevations estimated by the DPWs. In order to provide
a common reference system for the comparison, we
used the ground coordinates of the tie points from the
analytical AT of the strip, that is just the same points
used for the absolute orientation in the map restitution.
A topographic survey with a motorized laser scanning
theodolite will also provide (sparse but precise)
reference data for the second flight.
2.3 Using orthoimages to check a DEM
To provide an additional check of the accuracy of a
stereo DEM, a technique may be used which employes
two orthophotos, derived from each image of the pair,
by using the DEM to be controlled. By comparing the
(nominally fully) corresponding images so obtained,
errors in the DEM can be traced according to
(Baltsavias, 1996). The underlying concept is depicted
in Fig. 1: if you select a feature in one orthophoto, its
counterpart in the other should show up at the same
(X,Y) location in object space. If not, this is because
the DEM elevation in the area is wrong. The correct Z
value may be determined (and so the error computed)
by going back to the image space and intersecting the
correct pair of homologous point. In digital
photogrammetry this technique is very simple to
implement and automate: extensive testing of DEM
quality (in principle the whole DEM may be checked)
is therefore
Fig. 1 - DTM check by using two orthophotos
feasible. Once the two orthophotos have been
generated, feature points may be selected with an
interest operator or on a grid. Image correlation
techniques can be employed to find the homologous in
the second orthophoto, taking advantage of the
nominally coincident initial location and of the smaller
perspective distortion that the orthophoto should
display, compare to the aerial image. If the discrepancy
between the matched positions, converted into object
space, are within tolerance, the DEM is correct.
Otherwise, each orthophoto point is back projected into
the original image and the two rays are intersected,
yielding the correct ground coordinates.
2.4 Image scanning
Up today image scanning has been performed only on
the 50_51 model of the 1997 flight. Images have been