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
Analysis Center (ERSDAC) in cooperation with the University
of Tokyo and Mitsubishi Materials Techno Corporation (under
contract to ERSDAC). As before, the GDEM2 will be
distributed at no charge to users through ERSDAC on behalf of
METI, and at the Land Processes Distributed Active Archive
Center (LP DAAC), located at the USGS Earth Resource
Observation and Science Center (EROS), on behalf of NASA.
2. METHODS
2.1 Japan/ERSDAC validation
The Japanese validation team's methods for evaluating the
GDEM2 is documented in detail by Tachikawa et al. (2011), but
is briefly summarized here. The primary reference used for the
Japan study is the 10-m mesh DEM produced by the
Geographical Survey Institute (GSI) of Japan. The study
focused on 4 GDEM2 tiles in central Honshu Island, spanning
elevations from sea level to peaks exceeding 3000 meters. The
impact of land cover on. GDEM2 elevation errors was
determined by stratifying the GDEM2 against the GSI's
“Subdivision Land Use Data of Digital National Land
Information”, a 100-m land cover grid derived from satellite,
aerial photography and field measurements. This land cover
dataset was most recently updated in 2007. The Japan
assessment included horizontal and vertical accuracy
assessment against the GSI DEM, a horizontal resolution
estimate against the GSI DEM decimated to variable
resolutions, and an assessment of artifacts.
2.2 CONUS validation
The validation over the CONUS by the USGS (Gesch et al.,
2011) is described in another paper within this session and will
not be treated here, other than comparing results between the
various validation efforts. Briefly, the USGS approach
estimated absolute vertical accuracy against global positioning
system (GPS) measurements on over 18,000 geodetic
benchmarks, and compared the GDEM2 against the US
National Elevation Database (NED). This study evaluated the
influences of land cover, especially “tall” cover such as forests,
on the validation results.
2.3 Global SRTM validation
The NGA reproduced much of the work done for GDEMI,
using the same 284 GDEM tiles as before, located at 20
geographic areas globally (Krieger et al., 2011). The results
from the current GDEM2 validation are based on either a
comparison with global 1 arc-second Shuttle Radar Topography
Mission (SRTM) digital terrain elevation data (“DTED level 2",
or “DTED2”), or with the GDEMI. The NGA also did an
extensive visual identification of artifacts in the GDEM2.
2.4 Global ICESat validation
The NASA Planetary Geodynamics group at the Goddard Space
Flight Center (GSFC) evaluated the GDEM2 against data
collected by the Geoscience Laser Altimeter System (GLAS) on
board the Ice, Cloud and land Elevation satellite (ICESat) . The
results are described in another paper in this session, and will
not be treated here, other than in comparison with other
validation studies (Carabajal, 2011).
2.5 Characterization of horizontal resolution, artifacts.
The JPL and ERSDAC teams estimated the horizontal
resolutions of the GDEMs and other GDEMS, and characterized
artifacts in the GDEM2. This study was based on comparisons
to higher resolution DEMs derived from LIDAR and non-
LIDAR sources.
3. RESULTS
In summary, changes in the number of acquired ASTER stereo
pairs and improvements in processing (water masking, smaller
correlation kernel size, bias removal) have produced significant
improvements in GDEM2 as compared to GDEMI. These
improvements include increased horizontal and vertical
accuracy, as compared to both GPS benchmarks and standard
DEMs (GSI, NED, STRM DTED2), and improved horizontal
accuracy and resolution (similar to the SRTM DTED2).
The ERSDAC Japan study is summarized in table 1 below:
GDEMI GDEM2
Horizontal error 0.82* west 0.13* west
0.47 “south 0.19 * north
Flat Offset -4.8m -0.7 m
Ver- terrain | Std Dev | 6.2 m 59m
tical RMSE -- 6.1 m
error Forest | Offset 22m 7.4 m
terrain | Std Dev | 15.4 m 12.7 m
RSME -- 15.1 m
Horizontal resolution 3.8“ (114 m) 2.4” (72 m)
Table 1: Results from the ERSDAC study (note: horizontal
resolution estimates assume 1" — 30 m)
This study determined:
* The voids in northern areas have decreased due to new
ASTER acquisitions.
* The artifacts are significantly reduced as a result.
* All lakes in the Japan study are rendered flat by the new
water body detection algorithm (although inland water
body problems exist elsewhere, as determined by JPL).
The US/CONUS validation raised several important
observations about the quality of elevation measurements
contained in GDEM2, some of which are shown here in
comparison to other results:
* The overall RMSE of nearly two-thirds of a meter (8.68 m
vs. 9.34 m of GDEMQ over GDEMI, along with an
improvement in overall mean error (bias) in GDEM2 when
compared with GDEMI (-0.20 m vs. -3.69 m), largely
agrees with the ERSDAC study.
* The influence of the GDEM2 by above ground features
(tree canopies and built structures) is in agreement with the
Japan study that also noted a positive bias over forest cover
types.
* The GDEM has elevations that are higher in the canopy
than SRTM. This observation is based on both the
comparison of GDEM2 with GPS benchmarks, as well as
the GDEM2-SRTM differencing. Once again, this finding
was reinforced in the Japan study, although the latter had a
larger bias for tall cover types: 8.68 meters, compared to
3.10 meters for the CONUS study (see Meyer et al. ,
2011).
* The improvement in accuracy due in the number of
"stacking" scene DEMS used to derive elevation valuse is
minimal beyond about 15 scenes, largely in agreement
with the ICESat findings.
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