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XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
* GDEMZ exhibits an apparent "true" negative elevation
bias of about 1 meter, which was revealed through an
analysis of mean error by land cover type. This is
discussed in more detail in the USGS presentation.
The horizontal resolution estimates were similar from the US
and Japan teams when using non-LIDAR reference data sets,
with GDEM2 estimated at 70m and 72m, respectively, and
GDEM | estimated at 118 and 114, respectively. The US team's
additional estimates using LIDAR-derived high-resolution
reference data averaged 82m for GDEM2 versus 121m for
GDEMI1. These higher estimates were the expected result of
using a more precise and accurate reference DEM. The same
LIDAR sites also produced average estimates of 77m for SRTM
l-arc-second data and 102m for SRTM 3-arc-second data.
These results are summarized in Table 2.
DEM Average Non- LIDAR | Average LIDAR
GDEMI 116m 121 m
GDEM2 71m 82m
SRTM 1” 72m 77m
SRTM 3" 97m 102m
Table 2: Comparison of JPL & ERSDAC horizontal resolution
estimates for several GDEMs.
Unfortunately, the addition of higher-frequency topographic
signal in GDEM2 as compared to GDEMI came at the cost of
added, nearly ubiquitous, high frequency noise, as is visually
apparent and as indicated by the higher standard deviation of
differences from benchmark elevations (USGS) and from
SRTM postings (NGA) despite the general reduction of artifacts
such as pits and spikes. However, noisy signal is generally
better than missing signal, and fine noise can be suppressed by
filtering if critical, so even from a signal and noise tradeoff
perspective we conclude that GDEM2 is more versatile than
GDEMI.
While it is fairly clear from NGA’s high-level review that the
quality of GDEM2 is superior to GDEMI (especially above 60
degrees north), the data would still have to be assessed and
edited on a case-by-case basis before use in specific NGA
applications.
The ICESat study concluded that globally (with the exception of
Greenland), the GDEM2 elevations are on average within 3
meters of highly edited altimeter measurements, with standard
deviations and RMSEs under 12 meters. For bare ground, the
GDEM2 was on average within around 2 meters to the altimeter
measurements (with the exception of New Zealand), having
standard deviations and RSMEs under 12 meters. Although the
GDEM2 exhibits large errors over much of Greenland, for those
areas classified as either bare or herbaceous, the errors are on
average within 2 meters of the ICESat elevations.
4. REFERENCES
ASTER GDEM Validation Team, 2009, ASTER Global DEM
Validation Summary Report, 28 p.
Carabajal, C.C. (2011) ASTER global DEM version 2.0
evaluation using ICESat geodetic ground control. Report to the
ASTER GDEM Version 2 Validation Team.
Crippen, R.E. (2009). Spatial resolution of the ASTER global
elevation model (GDEM). Presentation at the 35™ ASTER
Science Team Meeting, Kyoto, Japan.
Gesch, D., M. Oimoen, Z. Zhang, J. Danielson, D. Meyer
(2011). Validation of the ASTER Global Digital Elevation
Model (GDEM) Version 2 over the Conterminous United
States. Report to the ASTER GDEM Version 2 Validation
Team.
Meyer. D, T. Tachikawa, M. Kaku, A. Iwasaki, D. Gesch, M.
Oimoen, Z. Zheng, J. Danielson, T. Krieger, W. Curtis, J.
Haase, M. Abrams, R. Crippen, C. Carabajal (2011). ASTER
Global Digital Elevation Model Version 2 — Summary of
Validation Results. Report to the ASTER GDEM Version 2
Validation Team.
Tachikawa, T., M. Kaku, and A. Iwasaki (2009). ASTER
GDEM validation. Presentation at the 35™ ASTER Science
Team Meeting, Kyoto, Japan.
Tachikawa, T., M., Kaku, A. Iwasaki (2011) ASTER GDEM
Version 2 Validation Report. Report to the ASTER GDEM
Version 2 Validation Team.
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