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4.2 Initial production steps
The first step of the production consists in extracting as much as
possible elevation data from optical HRS archive, built over
years, and then to determine which areas should be covered by
TerraSAR-X data (to be acquired on purpose). This includes the
following:
» HRS pairs integration in the continental space-triangulation
= Pair matching - Raw HRS DEM computation
= HRS DEM merging and mosaicking
= Void Mask extraction => determination of TSX area
= Water Mask delineation
Then the missing parts are collected by TerraSAR-X, and the
following tasks take place:
" Ascending and descending pairs acquisition on TSX area
= Raw TSX DEM computing (radargrammetry matching)
=» Raw TSX DEM merging and mosaicking
1» Water Mask delineation (on areas not already covered by
HRS Water mask)
These steps were performed in their “standard way” by the staff
in charge of DEM production both in Germany for TerraSAR-X,
and in Toulouse (France) for SPOT 5 HRS.
4.3 DEM merging
After this began a more specific processing chain, directly
linked with AJAX:
=" Merging of DEMs from HRS and TSX
= Automatic detection of voids and artefacts
= Patching of remaining voids with SRTM) DEM
from the Internet (resampled to 1 arc.second)
= Edition Phase
= Water flattening on merged DEM, using the
Water Masks previously delineated
« DID (Double-Line Drains) processing: the
rivers are made flowing smoothly downstream
« Final DEM quality control, visual detection of
remaining artefacts => digitisation of uncertified
areas ; registering into the corresponding mask
At the end of the day, the resulting AJAX DEM originates in
balanced parts from TerraSAR-X and SPOT 5, as shown in
Figure 3 below:
Figure 3 - Sources of the AJAX DEM.
TerraSAR-X is shown in black, SPOT 5 HRS in white.
5. QUALITY CONTROL AND VALIDATION
Since the beginning of the production in 2002, Quality Control
and Validation steps represent a significant part of the
Elevation30 production flow (15 to 20% as an average). The
merging of radar and optical DEMs of course introduces the
necessity of a dedicated V&V process.
5.1 Control of the merging
Indeed, we were very pleased that the two different DEMs,
extracted from 100% independent sources (TerraSAR-X and
SPOT 5) by 100% different and independent teams —though
equally skilled and experimented- in Germany and France
proved extremely consistent, as demonstrated by the histogram
of elevation differences (Figure 4).
45000 +
40000 +
35000 +
30390 -
20000 +
15000 +
10300
Figure 4 — HRS and TerraSAR-X Elevation differences.
The overall bias between the two DEMSs is 0.80m (TSX higher
than HRS), and more than 90% of the differences are less or
equal to 4m. Thanks to this excellent result, no unbiasing/
elevation adjustment was applied during the merging process.
5.2 Absolute validation of the DEM vs ICESAT data
From Carabajal and Harding (2005), the Geoscience Laser
Altimeter System (GLAS) on the Ice, Cloud, and land Elevation
Satellite (ICESat) provides a globally-distributed data set well
suited for evaluating the vertical accuracy of digital elevation
models (DEMs). These authors quote a vertical error of 0.04 +
0.13 m per degree of incidence angle. As compared to the
Elevation30 accuracy specifications, these figures are small
enough to entitle ICESat a perfect data source to assess the
AJAX DEM accuracy.
Whatever precise in elevation, the ICESAT measure refers to a
near-circular 70m spot on the ground, far larger that the AJAX
posting interval (1 arc.second, ie approx 30m in Colombia).
Therefore, the first step of the production process is filtering the
adequate ICESat dataset to carefully select elevations that can
be used with reasonable doubt as “ground truth". The filtering
process is based upon the local slope; land cover is not
considered. Please refer to Le Hir et al. (2010) for more details
on the filtering and selection process of ICESAT data.
