ASTER Global DEM (ASTER GDEM)
--Quick Guide for V2-1. Introduction
The ASTER instrument that was launched onboard NASA’s Terra spacecraft in December 1999 has
an along-track stereoscopic capability using its near infrared spectral band to acquire the stereo data.
ASTER has two telescopes, one for nadir-viewing and another for backward-viewing, with a
base-to-height ratio of 0.6. The spatial resolution is 15 m in the horizontal plane. One scene consists
of 4,100 samples by 4,200 lines which correspond to about 60 km by 60 km ground area.
A number of 1,514,360 scenes (Level-1A products) that was acquired from March, 2000 to August,
2010 was used to generate ASTER global DEM (ASTER GDEM) version 2.0. ASTER GDEM
was created by stacking all cloud-masked scene DEMs and non-cloud-masked scene DEMs, and
statistical selection algorithm to remove abnormal data.
The statistical approach is not always effective for anomaly removal in area with small number of
data stacking. Several existing reference DEMs were used to correct residual anomalies caused by
the insufficient number of stacking.
2. Data Set Container
Each GDEM tile container accommodates two files, DEM file and quality assessment (QA) file,
with a dimension of 3601 samples by 3601 lines correspond to 1 degree by 1 degree data area, and
is zip-compressed. Unit directory accommodate 5 degrees by 5 degrees zip-compressed tile
container. The maximum number of tiles in one unit directory is 25. The details are shown in
Figure 1.
Figure 1 GDEM directory structure
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The names of individual data tiles refer to the latitude and longitude at the geometric center of
lower-left (southwest) corner pixel. For example, the coordinates of the lower-left corner of the
tile ASTGTM2_N00E006 tile are 0 degree north latitude and 6 degrees east longitude.
ASTGTM2_N00E006_dem and ASTGTM2_N00E006_num files accommodate DEM and QA data,
respectively. The unit name also refer to the latitude and longitude at lower left corner of 5 degree
by 5 degree area. The rows at the north and south edges as well as the column at the east and west
edges of each cell overlap and are identical to the edge row and column in the adjacent cell.
The data coverage is north 83 degrees to south 83 degrees. Tiles including at lease 0.01 % land area
were processed. The number of tiles is 22,702 for version 2 data set.
3. Data Formats
Data formats are summarized in Table 1.
Table 1 Data formats
Tile Size
3601 x 3601 (1 degree by 1 degree)
Posting interval
1 are-second
Geographic coordinates
Geographic latitude and longitude
DEM output format
GeoTiff, signed 16 bits, and 1m/DN
Referenced to the WGS84/EGM96 geoid
Special DN values
-9999 for void pixels, and 0 for sea water body
Coverage
North 83 degrees to south 83 degrees, 22,600 tiles for V1
4. Anomaly Correction
The cloud masking and the statistical approach were used to select the effective data for stacking.
The cloud masking can not completely remove the clouds,
In addition, the statistical approach is
not always effective for anomaly removal in case of small number of observed data. At least, three
stacking data will be necessary for it.
Existing DEMs were used to correct anomaly as reference data. Table 2 shows the list of the
reference DEM used for version 1 data. The anomaly data are replaced by the reference data after
adjusting offsets between ASTER GDEM and the reference DEM data.
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Table 2 Reference DEM list used for version 1
SRTM3 V3
(Void filled version)
Posting: 3 seconds
Coverage: north 60 degrees – south 56 degrees
Only about 90 % tiles of SRTM V2 are void filled
SRTM3 V2
Posting: 3 seconds
Coverage: north 60 degrees – south 56 degrees
Posting: 1 second
NED
(US National Elevation Data)
CDED
(Canada DEM)
Alaska DEM
Posting: 3 seconds for latitude
3, 6 and 12 seconds for longitude depending on latitude
Coverage: all Canada territory
Posting: 2 x 2 seconds
Coverage: all Alaska territory
5. QA Plane
The QA plane file that is included in the tile container is used to describe number of stack by
positive values and replaced reference data by negative values as shown in Table 3. The tile size,
posting interval, and geometric formats are consistent with DEM data shown in Table 1.
Table 3 Reference data vs negative number of QA plane
SRTM3 V3
-1
SRTM3 V2
-2
NED
-5
CDED
-6
Alaska DEM
-11
6. Quality Limitation
In some area, the number of data stack is not sufficient to completely remove anomaly. In addition,
the reference data is not necessarily available for anomaly correction in entire area.
Those
situation will give rise to difficulty for completely remove all anomalies. Therefore, users must be
aware that data may be anomalous in pixels with the small number of stacking data equal to 2 or
less.
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7. Key Processing Factors
The change in the key processing factors from Version 1 to Version 2 is listed in Table 4.
Table 4 Key processing factors
Key processing factor
Version 1
Version 2
Number of input scene DEMs
1,264,118
1,514,350
Posting interval
1 arc-second
1 arc-second
Correlation kernel size
9 by 9 pixels
5 by 5 pixels
12 sq. km
1.0 sq. km
Not applied
Applied
40 m
40 m
-5 m offset observed
V1 offset revomed
June 29th, 2009
Scheduled to be middle of
October, 2011
Minimum water body detection size
Water body post processing
Filtering threshold value
Offset
Release data
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