+1.5 m, and their mean is 0.27 m. No systematic translation
appears at the regional scale. On the contrary anomalous values
appear in several subgrids (a clear example is provided by D6):
these anomalies require deeper investigation that will be
performed in the next future.
|
tS
Figure 7. Estimates of the translations between the LR
Lombardy and the HR LiDAR DTMs. Individual translations
have been estimated for the subgrids of the regional DTM. Grid
C2 has no overlap with the Lidar DTM.
4. CONCLUSIONS
The presented research is in the framework of the Interreg Italy-
Switzerland HELI-DEM (Helvetia-Italy Digital Elevation
Model) project. HELI-DEM is aimed at creating a transalpine
DTM by merging all the available height information for the
alpine area. Particularly the LR DTMs of Piedmont (50 meters
of planimetric resolution) and Lombardy (25 meters) Italian
regions and the national Switzerland DTM (25 meters) are
available. Moreover, local HR LiDAR DTM (1 meter) is
available for the main hydrographic basins in Italy.
The paper has discussed the first operations needed for the final
merging. At first, all the DTMs have been transformed to the
same 3D reference frame, that is the european ETRF89. Then,
several cross-checks have been performed: in particular, the LR
DTMs have been compared in their overlapping boundaries and
have been compared with the HR DTMs where they are
available.
To these purposes, several algorithms and procedures have been
developed in MATLAB and GRASS.
The cross-check between Lombardy and Switzerland has not
shown any general bias and the standard deviation of the
differences, even if bigger, is comparable with the nominal
accuracy. However, the spatial distribution of the differences
presents an anomalous behavior. The magnitude of the
differences is not correlated with the heights or the slopes and
the biggest differences are localized in particular subgrids of the
Lombardy DTM: this clearly implicates some blunder in the
production of those particular datasets.
A bias of about 1.5 meter exists between Piedmont and
Switzerland DTMs: obviously, the bias is below the nominal
accuracy of both the datasets but it is everywhere present.
Moreover, it is confirmed by the comparison with the Lombardy
DTM.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
In some valley, the comparison between Lombardy LR DTM
and the HR DTM shows an apparent planimetric translation. A
software to estimate the translation between two DTMs by least
squares has been implemented. Globally, a common translation
does not exist: this is also true at the level of the several sub-
grids of the Lombardy DTM. Therefore, no bias in the reference
frames of the two DTMs should be suspected. However, more
investigations should be done on the subgrids and valleys where
the bias is more evident.
In particular, external validation by GPS surveys will be done in
the next months.
5. ACKNOWLEDGMENTS
This research is a part of the HELI-DEM (Helvetia-Italy Digital
Elevation Model) project, funded by the European Regional
Development Fund (ERDF) within the Italy-Switzerland
cooperation program. More details about the project can be
found on the website: www.helidem.eu.
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