, 2012
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August - 01 September 2012, Melbourne, Australia
TOWARDS AN URBAN DEM GENERATION WITH SATELLITE SAR
INTERFEROMETRY
Cristian Rossi^, Thomas Fritz^, Michael Eineder^, Esra Erten^, Xiao Xiang Zhu® © and Stefan Gernhardt®
“German Aerospace Center (DLR), Remote Sensing Technology Institute (IMF), Wessling, Germany ¢ cristian.rossi@dlr.de
bIstanbul Technical University (ITU), Civil Engineering Faculty, Department of Geomatic Engineering, Maslak Istanbul, Turkey
“Technical University Munich (TUM), Remote Sensing Technology (LMF), Munich, Germany
Commission VII/2
KEY WORDS: SAR, Interferometry, TanDEM-X, Urban Digital Elevation Models
ABSTRACT:
This paper analyzes the potentials offered by the TanDEM-X mission for the generation of urban DEMs. Operationally acquired urban
Raw DEMs are validated using as reference a LIDAR DEM. The approach used for the validation is application-oriented as a sub-
product is a derivation of a urban volume map. The mean buildings absolute height error is found to be below 5 meters. Neglecting the
buildings geolocation errors, this value drops below 60 centimeters. Nevertheless, the operational municipal mapping is limited to the
standard resolution of 12 meters. The limitation is attenuated with the use of high resolution spotlight data. High-resolution raw DEMs
over Las Vegas and Berlin are first analyzed. Some modifications to the interferometric processing chain necessary to produce high
quality DEMS are studied. The obtained results open a new perspective to the urban DEM generation with satellite SAR interferometry.
1 INTRODUCTION
The mapping of urban areas is a claimed task for many manage-
ment applications, such as urban development monitoring, urban
climate studies, and renewable energy surveys. Airborne data are
widely used at this purpose for the generation of Digital Elevation
Models (DEMs). LiDAR (Light Detection And Ranging) is a ma-
ture technology for obtaining DEMs in urbanized environment.
Spaceborne Synthetic Aperture Radar (SAR) is on the contrary
not largely exploited due essentially to the lack of suitable data.
In fact, an essential prerequisite for a successful exploitation of
SAR data finalized to urban reconstruction is a resolution capable
to map the desired target. A second one, but not less important,
is the absence of temporal decorrelation in between the two SAR
acquisitions, or, in other words, an interferometric bistatic config-
uration. The TanDEM-X mission, started in June 2010, has as pri-
mary objective the generation of a global DEM following the high
standard accuracy HRTI-3. The X-band sensors employed for the
mission allow an accurate building mapping, hence accomplish-
ing the first prerequisite. Additionally the mission constitutes the
first bistatic SAR interferometer in space, letting to a faithful sur-
face reconstruction free of atmospheric and temporal decorrela-
tions. The acquisition mode used to achieve the HTRI-3 require-
ments is the stripmap one, with a final spatial DEM resolution of
12 meters. This resolution does not assure an accurate mapping
of dense metropolitan areas, nevertheless semi-urban and indus-
trial areas are well mapped as shown in Sec. 2 with a validation
made using reference LiDAR data over Munich (Germany). The
mission foresees however experimental spotlight acquisitions fi-
nalized to the generation of higher resolution DEMs following the
HRTI-4 standards with a 6 meters spatial resolution. Considering
this resolution, a denser urban mapping becomes then possible.
First examples of urban experimental high-resolution DEM are
shown over Las Vegas (USA) and Berlin (Germany) in Sec. 3.
The processing chain adopted for the interferometric DEM gen-
eration, also called Raw DEM, is built in the Integrated TanDEM-
X Processor (ITP). Although the chain is highly optimized in a
global scale, some modifications are necessary for the purposes
of the paper. In particular, the spectral shift filtering, the coreg-
istration, the interferogram generation and the phase unwrapping
73
processing steps are redesigned towards an optimal configuration
for the local municipal zone to map. The limits of an interfero-
metric SAR city modelling are introduced in Sec. 4 as conclusive
remark.
2 OPERATIONAL URBAN RAW DEM VALIDATION
The main goal of the TanDEM-X mission is the generation of
a global DEM, thus including all the cities all over the world.
The processing of Raw Data for the generation of the Raw DEM
is performed by one single processor, the Integrated TanDEM-
X Processor (ITP) (Breit et al., 2010), not described in this pa-
per. ITP is optimally configured in every single processing stage
to provide a Raw DEM close to the HRTI-3 standards (relative
point-to-point height accuracy of 2 meters for moderate terrain
with slope inferior to 20° in a 1° by 1° cell). In the processing
facilities, for a standard 50 by 30 kilometers scene, the processing
time required for the generation of the Raw DEM from the Raw
Data is about 15 minutes. This high optimization follows the al-
gorithms presented in (Fritz et al., 2011). Cities inside the scene
are mapped at the standard posting of 12 meters. Even if the rel-
ative TanDEM-X height accuracy requirements are successfully
accomplished (Rizzoli et al., 2012), they are too coarsely defined
for urban zones. The cell used for the DEM height error spec-
ification is too large for an accurate analysis over a single city.
A new strategy is then adopted for the validation of the abso-
lute height of isolated buildings inside the DEM. It is based on
the comparison between detected buildings in a reference LiDAR
DEM and the TanDEM-X one. The input for the comparison is
the Digital Buildings Model (DBM), derived through a difference
between the DEM and its Digital Terrain Model (DTM). In prin-
ciple a thresholding operation on the DBM would be sufficient
to detect the buildings. In practice, due to the interferometric er-
rors intrinsic to the urban DEM generation, such as geometric
decorrelations or phase unwrapping issues (Rossi et al., 2011), a
thresholding is not sufficient and a morphological operation, nec-
essary to detect the underlying structures, is used. The simplest
structuring element for the morphological operation is the 2-by-
2 square. By this way sharp rectangular edges representing the
buildings - are extracted and noisy areas are discarded. Standard
