ACCURACY EVALUATION OF DEM DERIVED BY TERRASAR-X DATA IN THE
HIMALAYAN REGION
N. Yamane a, *, K. Fujita b, T. Nonaka a, T. Shibayama a, and S. Takagishi aa

PASCO CORPORATION, 2-8-10 Higashiyama, Meguro-ku, Tokyo, Japannaofumi_yamane@pasco.co.jpb
Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan

KEY WORDS: InSAR, Glacier, Elevation, Himalayan, SAR, TerraSAR-X, Monitoring

ABSTRACT:In the past few years, the Himalayan region has experienced a number of floods from glacial lakes, and theneighboring areas were severely damaged. Therefore it is important to continue monitoring glaciers or creating
simulation of floods to prevent the damages in those regions. The DEM is very useful for studying these hazardous
problems. In this study, we tried to generate DEM by TerraSAR-X images for conducting InSAR processing andevaluating the quality of the DEMs by comparing them with SRTM data. DEM generated by TerraSAR-X image
could acquire more detailed surface information of the mountaneous region. The end and side of the moraine wererecognizable with TerraSAR-X DEMs. It is expected that the InSAR DEM would be highly useful for monitoring
purposes in the mountainous regions.

1. INTRODUCTION
During past years, the Himalayan region faced several
floods from glacial lakes, and the neighboring areas
were severely damaged (ICIMOD et al., 2007, J.
Komori 2006., etc). If the floods can be predicted wellin advance, the continuous actions, such as making agood preventive plan and allocating disaster-prevention
facilities, can be adopted well before occurrence of thefloods. Consequently, it is expected that the extent of
the damage of the populated area or infrastructure canbe greatly controlled. The Digital Elevation Model(DEM) is widely used for flood prediction andestimating the affected area and utilized forenvironmental monitoring as well. It is possible todetect the temporal changes of glacial lakes, glaciersand moraines by comparing two DEMs produced at two
different time intervals.
DEMs produced from optical satellite images have beencommonly used for glacier monitoring. However, theproblem is to obtain cloud free images. Most of theglacial lake outburst flood (GLOF) occurs in themonsoon season and therefore, monitoring the glaciersis very difficult due to bad weather conditions andpresence of dense clouds. Data acquisitions,irrespective of weather conditions, by the Synthetic
Aperture Radar (SAR) satellites of C, L and X bands
can create DEMs.

On June 2007 TerraSAR-X satellite was successfullylaunched and presently providing images for the globalusers. The X-band radar is capable of extracting
surface features information better than C-band or Lband sensors due to the short wavelength. Additionaly
DEMs created from X-band sensor satellite images
were not generated by interferometry till recently.
The purpose of this study was to generate DEMs fromTerraSAR-X images utilizing InSAR and evaluating thequalitative accuracy by comparing with Shuttle Radar
Topography Mission (SRTM) data.

2. METHODOLOGY
In this study, we made our attempts to cover DEMs
from a pair of TerraSAR-X images obtained by High
resolution SpotLight (HS) and StripMap (SM) modes
images. After generating the DEMs, we also evaluatedthe qualitative accuracy of the DEMs while comparing
SRTM data for the entire study area.

* Corresponding author. This is useful to know for communication with the appropriate person in cases with more than oneauthor.
203

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008

2.1 Study site
The study site was selected in the periphery of Lake
Imja which is known as one of the most dangerous
glacial lakes in the world due to its rapid expansion andhuge amount of water storage capacity (Figure 1).
The majority of the supraglacial lakes that was formedin the 1960s recently became moraine-dammed lakes
because of the glacier retreat. An example of thoselakes is Lake Imja and has been identified as a potential
threat to the Himalayans region. The lake was formed
within moraines and rapidly stretching towards theglacier snout. The end moraine in Lake Imja measures
600m wide and has a massive dead ice core which isfrequently exposed. Rapid melting of the debris-covered
ice and significant changes in this outlet position has
been reported (Watanabe et al., 1994 and 1995).

©Infoterra GmbH, Distribution [PASCO]

Figure 2. Lake Imja, TerraSAR-X image, HS, 29 Mar 08.

©Infoterra GmbH, Distribution [PASCO]

Figure 3. Lake Imja, TerraSAR-X image, SM, 04 Apr 08.

2.3 InSAR processing
The technique of SAR interferometry is widelypracticed to produce the map of the topography orground deformation. InSAR for the space-borne SAR isconducted by repeat-pass interferogram. In the repeatpass mode, the orbit accuracy depends on the quality ofDEMs. The orbit of TerraSAR-X is in a ±250 m tube
with respect to a reference orbit (Eineder et al, 2003)
and the orbit distance is shorter and hence a good DEMquality is expected. InSAR processing was conductedbySARscape®
softwareinthisstudy(http://www.sarmap.ch/).

©JAXAFigure 1. Lake Imja study site, ALOS/AVNIR-2 image,22 Oct 08.

2.2 Utilized Data
The study was carried out utilizing a pair TerraSAR-X
images (Table 1, Figure 2 & 3). The interval to obtain
the pairs of images was set at 11days (the shortest
inverval of satellite revisit) to achieve good quality ofDEMs. Two different images were acquired at HS andSM for comparing different spatial resolutions. SRTM
90m DEM data was utilized as reference DEM in thisstudy which was generated through C-band single-passinterferometry. The accuracy of this DEM is about
16m (vertical) (E. Rodríguez et al).
ModeDate

Direction

Incidentangle

SM

2008/3/24

ascending

35.36

VV

SM

2008/4/4

ascending

35.36

VVHS

2008/3/18

decending

39.39

VVHS

2008/3/29

decending

39.39

VV

3. RESULS AND DISCUSSIONS
We conducted the qualitative accuracy of the DEMs
generated by the TerraSAR-X images and comparedthem with the SRTM data (Figure 4, 6 and 8). Coherent
images shown in Figures 5 and 7, indicated theinfluence on the quality of the DEMs (SARScape®,Help Menu).

Polarization

Table 1 TerraSAR-X data acquisition dates.

204

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008

3.1 Evaluation of the quality of DEMs
The DEMs generated by the HS and the SM modes
have a range of elevation of 4800m-5550m and 4850m5500m, respectively for the analysis in the presentstudy area. For the same AOIs, the elevation range ofSRTM data is 4800-6050m. From this result, theminimum elevation of TerraSAR-X DEMs is near but
their maximum elevation value is not close to thereference DEM. In the northern part of the image ofHS-DEM, there is also an area with no data value. The
expected cause might have been due to interruptions
from the topography of the radar signal from thesatellite. The end and side of the moraine arerecognizable in HS-DEM and SM-DEM. In addition, it
is also visible that the end-moraine of Lake Imja meetswith other glacier’s side-moraine in SM-DEM.

Figure 6. DEM generated by TerraSAR-X images SM-mode.

Figure 7. Coherence image generated by TerraSAR-X images,SM-mode, SM-CC.

Figure 4. DEM generated by TerraSAR-X images, HS-mode.

Figure 8. SRTM-DEM.

3.2 Validation of the detailed area.
Figure 5. Coherence image generated by TerraSAR-X images,HS-mode, HS-CC.

Detailed validation of the zones were focused on areas
A and B, and we tried to evaluate the qualitative
accuracy of DEMs generated by TerraSAR-X images.
The areas selected for validation were the end moraineof Lake Imja (A) and a flat area located in the westernpart of the lake (B). The reasons for selecting these twoareas were:(i) To confirm whether the end moraine could be
distinguished or not by the DEMs.

205

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008(ii) To generate relatively good DEMs from the flatarea, because the topographic features can not affectthe image.
(A) End-moraine
DEM section at the end moraine and dead-ice weregenerated (Figure 9). The end-moraine is visible in each
DEMs but not for the SRTM data. Both sides of the end
moraine were depicting lacking value of HS-DEM.
From the optical image it can be seen that this area ismostly composed by water and the water bodies werefrozen and the surface had snow. Since snow reflect theradar weakly, the value can not be acquired.
(B) Flat area
The border between the debris area and the sidemoraine is visible in the SM-DEMs. On the contrary,
they are not distinguishable in the HS-DEM and SRTM
DEM. It is noticed that the glacier covered with sand orsand gravel (known as debris) is highly elevated and it
gradually decreases from the debris to the side-moraine.
On the debris area many variations of colors could beseen (Figure 10). Apparently, such variation indicatethat these DEMs can offer detailed information.

©JAXA

Figure 9. The end-moraine.
(Upper left:HS-DEM, Upper right: HS-CC, Middle left: SMDEM, middle right: SM-CC, lower left: SRTM, upper right:
ALOS/AVNIR-2)

4. CONCLUSIONSIn this study with the use of InSAR processing, wegenerated DEMs by TerraSAR-X images. As a result ofcomparing them with the SRTM data, it was noticed
that DEMs generated in this study can have the same
qualitative accuracy results for the qualitativeevaluation. In addition, both DEMs can show the endmoraine and the side-moraine, which cannot beobserved by the SRTM data. Therefore, HS-DEM and
SM-DEM may be more useful than the SRTM for
monitoring purposes or for analysis of flood simulations.
Glaciers were located on the cliffs and there was aconcern that such topography might have affected thequality of the images. However, the DEMs of almost allareas were generated without any problem in thepresent study. Only in some rare cases the mountainridge affected the image acquisitions. Therefore withthe data fusion of DEMs, generated in both directions(ascending and descending), it is expected to obtaineven more accurate images.
We did not conduct quantitative evaluation which will
be conducted in the near future. Also in the current
study, occasionally, different results between HS-DEMs
and SM-DEMs were obtained. It would be interestingand necessary as a future study to understand the causesof such phenomena.

©JAXA

Figure 10. The flat area.
(Upper left:HS-DEM, Upper right: HS-CC, Middle left: SMDEM, middle right: SM-CC, lower left: SRTM, upper right:
ALOS/AVNIR-2)

206

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008

REFERENCES
Rodríguez , E., Morris, C.S., Belz, J.E., Chapin, E.C.,
Martin, J.M., Daffer, W., Hensley, S. An Assessment ofthe SRTM Topographic Products. Jet Propulsion
Laboratory, D-31639.
ICIMOD., UNEP., 2007. Impact of Climate Change on
Himalayan Glaciers and Glacial Lakes.
Komori, J., 2006. Glacial lake Expansion and present
Situation in Hinku and Hongu Regions, Eastern Nepal
and Bhutan Himalaya. Journal of Geography 115(4), pp.
531-535.
Eineder, M., Runge, H., Boerner, E., Bamler, R., Adam,N., Schättler, B., Breit, H., Suchandt, S., 2003. SAR
INTERFEROMETRY WITH TERRASAR-X. Proc. of
FRINGE 2003 Workshop, Frascati, Italy, December
2003 (ESA SP-550, June 2004)
SARMAP. (http://www.sarmap.ch/)
Watanabe, T., Ives, J.D., Hammond, J.E., 1994 Rapid
Growth of a Glacial Lake in Khumbu Himal,Himalaya: Prospects for a Catastrophic Flood. InMountain Research and Development, 14(4), pp. 329340.
Watanabe, T., Kameyama, S., Sato, T., 1995. Imja
Glacier Dead-Ice Melt Rates and Changes in a Supraglacial Lake, 1989-1994, Khumbu Himal, Nepal:
Danger of Lake Drainage. In Mountain Research andDevelopment, 15(4), pp. 293-300.

207

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008

208