International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
* the deviations between the check points of the two
orthophoto-mosaics are not significant. In fact, they are within
the accuracy requirements for scales <1:100
e the comparison between orthophotos gave small differences
with the best performance in the Y directon.
Test 1: Point errors (19 check points)
Photogrammetricall | Mx =-7 mm My = 5 mm
y produced O,x = 20 mm Goy = | 7 mm
orthophoto-mosaic Mean square value = 19 mm
Orthophoto-mosaic Mx - 4mm My = 1 mm
from laser scanner Gox = 13 mm Coy = 18 mm
data Mean square value = 16 mm
Test 2: Relative errors (26 points)
Mean of M, = 6 mm My = 1 mm
differences
RMS RMS (dX) = 9mm RMS (dY) = 4mm
Max difference max (dX) = 22mm max (dY) = 11mm
Table 2. Evaluation of accuracy between the orthophoto-
mosaics of data set II (eastern facade)
5. CONCLUDING REMARKS
The contribution of measurements obtained by the Cyrax 2500
laser scanner into the production of large scale
orthophotographs has been examined. It was shown that this
alternative could be a reliable choice even under difficult
circumstances, as it usually happens in case of terrestrial
applications, such as geometrical documentation of monuments.
The capability for full coverage of the object surface with laser
scanner point data, is the most critical factor for the success of
this method.
In terms of the accuracy achieved in the final results of the
application described in this paper it was shown that the use of
laser scanned data does not substantially improve the results in
comparison with the equivalent achieved by the standard
photogrammetric orthophoto production procedures. This is
because the derived DSM from laser scanner data is of similar
or slightly better quality to that derived manually using the
photogrammetric data only. However, the automatic DTM
extraction from photogrammetric data can only guarantee
failure for the production of the final orthophoto. The manual
editing of the data can provide good results with the cost of
lengthy processing procedures.
While the advantage of having a very large number of laser
scanner point data is significant in the production of DSMs, the
management of such dense information is not trivial.
Decimation of 15% for the data set used in the DSM production
has shown to produce results of similar quality with decimated
sets at the level of 50% and 80%. However, when no large
overlaps of scan clouds or single scan clouds have been
acquired, the 15% decimation cannot guarantee reliable results.
Although in both data sets discussed in this paper, the
orientation of stereomodels was performed using independent
premarked points, this can also be performed through the target
network required for the registration of laser scanned data or
through the registered laser point clouds. In this way, it is
possible to eliminate the necessary control data, which will be
derived by using standard surveying techniques, thus reducing
the field work.
ACKNOWLEDGEMENTS
The data presented in this paper were collected during the 2003
4^ year undergraduate photogrammetric summer fieldcourse,
The authors acknowledge the contribution in the orthophoto
production of Ms S. Soile, Surveying Engineer — Research
Associate at the Laboratory of Photogrammetry in NTUA.
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