After this selection, only 209 ICESAT “truth points” were kept,
only a few dozens of them laying on the Eastern part of the
geocell (Figure 5).
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Figure 5 — ICESAT “truth points” over the geocell. Three
different orbits can easily be identified. Colours show the
magnitude of the elevation difference (in meters) between
ICESAT measurements and the AJAX DEM.
20 T T T T T
Figure 6 — Histogram (in %) of the elevation differences against
ICESAT (in meters).
The comparison of AJAX DEM against [CESAT measurements
gave the following results:
- Mean=0.51m
- . Std deviation = 3.1m
- approx. 5.0m LE90 accuracy.
5.3 “Vertical Accuracy Commitment” mask
As for each and every Elevation30 product, the AJAX prototype
also includes a Vertical Accuracy mask which provides for cach
elevation post the best accuracy commitment from the producer
(Figure 7).
The methodology to build this layer is detailed in Le Hir (2010).
Figure 7 — Vertical accuracy commitment of the AJAX DEM.
As shown in the Figure 7 above, our accuracy commitment
ranges from 6m LE90 in the flat areas (in green), up to 30m
LE90 (in pink) over the extreme slopes. Black dots indicate
elevation posts for which no commitment could be taken.
6. CONCLUSION
A merged DEM was produced over a very "difficult" area in
Colombia, through the merging of two independently produced
DEMs from TerraSAR-X and SPOT 5 HRS data.
Validation against ICESAT data, as well as the very low
differences between both DEMs, show that the resulting
accuracy is in line with our Elevation30 requirements.
Therefore, detailed commitments can be taken towards the users
regarding the vertical accuracy of the resulting DTED level 2
DEM.
References:
Bouillon et al., 2006, SPOT 5 HRS geometric performances:
Using block adjustment as a key issue to improve quality of
DEM generation, ISPRS Journal of Photogrammetry & Remote
Sensing 60 (2006) 134-136.
Carabajal, C. C., and D. J. Harding (2005), ICESat validation of
SRTM C-band digital elevation models, Geophysical Research
Letters, 32, L22S01, doi:10.1029/ 2005GL023957.
Kay, Winkler, 2004, Quality checking of DEM derived from
satellite data (SPOT and SRTM), 10th Annual Conference on
Control with Remote Sensing of Area-based Subsidies.
Budapest, 24-27 November 2004.
Le Hir et al., 2010, Updating and improving the accuracy of a
large 3D database...., [SPRS Congress, Kyoto, August 2010,
Commission VIII.
Yoshino et al., 2008, Building a consistent geometric frame
over Sparse islands using SPOT 5 data, ISPRS Congress,
Beijing, August 2008, Commission VII, WG VII/7.
