Full text: CMRT09

In: Stilla U, Rottensteiner F, Paparoditis N (Eds) CMRT09. IAPRS, Vol. XXXVIII, Part 3/W4 — Paris, France, 3-4 September, 2009 
Stefan Hinz 1 , Sarah Abelen 2 
'Institute of Photogrammetry and Remote Sensing, Univeristät Karlsruhe, Kaiserstr. 12, 76 128 Karlsruhe 
2 Remote Sensing Technology, Technische Universität München, Arcisstr. 21,81 333 München 
Commission III, WG 5 
KEY WORDS: Space-borne Interferometrie SAR, Building Heights, TanDEM-X. CosmoSkymed 
The great potential of space-borne SAR images for semi- or fully-automatic mapping of topographic features has been shown by 
many approaches. While most of them focus on 2D mapping of topographic features, some preliminary research on the complex task 
of automatic delineation of 3D information in urban environments has been initiated in recent years. In this paper, we analyze the 
capabilities of new space-borne interferometric SAR missions - in particular the German TanDEM-X mission - with respect to their 
potential of deriving building heights. To this end, we summarize the mathematical framework and carry out a thorough analytical 
accuracy analysis involving various sensor and scene parameters. 
The new class of space-borne high resolution SAR sensors such 
as TerraSAR-X, SAR-Lupe or Cosmo-SkyMed is able to 
provide SAR images of l-3m spatial resolution or even below 
in special spotlight modes. Naturally, the development of 
methods to automatically derive detailed cartographic 
information from this kind of data is a major issue driven by 
these missions. Since SAR is largely independent from 
illumination and weather conditions, it is furthermore an 
attractive imaging technology for acquiring area-wide 
information of regions hit by disasters such as floodings, 
landslides, or earthquakes. 
The great potential of space-borne SAR images for semi- or 
fully-automatic mapping of 2D topographic features has been 
shown by many encouraging approaches, e.g., (Negri et al., 
2006; Frey & Butenuth, 2009) for delineation of roads and 
(Jäger et al. 2007; Hänsch & Hellwich, 2008) for classification 
of agricultural features, just to name few recent ones. The 
derivation of 3D features is however more difficult, since these 
current civilian space-borne systems have only limited 
interferometric capabilities. While the acquisition of along-track 
interferometric image pairs is possible by programming special 
RADAR imaging modes (e.g. DRA mode or Aperture 
Switching mode for TerraSAR-X (Runge et ah, 2006)) enabling 
the detection of moving objects (Suchandt et ah, 2008; Weihing 
et ah, 2008), none of the current civilian space systems is 
equipped with an across-track interferometer, which would 
provide the basis for deriving topographic heights (Bamler & 
Hartl, 1998; Cumming & Wong, 2005). The necessary across- 
track baseline is only given when forming an interferogram of 
two SAR acquisitions taken from the same orbit yet at different 
passes of the satellite, and thereby relying on the positional 
variation of the orbits. It is clear that the resulting 
interferograms suffer from decorrelation depending on temporal 
variability of the objects under investigation. 
This situation will change once TerraSAR-X will be 
accompaigned by a second, quasi-identical SAR satellite in late 
2009, leading to the TanDEM-X mission. Both satellites will fly 
almost in parallel forming a helix-like orbit pair (Zink et ah 
2006). This configuration allows to acquiring SAR image pairs 
with variable across-track geometry resulting in a significantly 
improved interferometric coherence. The great benefit of single 
pass across-track SAR interferometers has been intensively 
studied in the context of the Shuttle Radar Topography Mission 
(SRTM). Despite of the limited spatial resolution of SRTM data 
(approx. 25m), it was possible to compute a global digital 
elevation model with standardized height accuracy of few 
meters, see, e.g., the comprehensive overview given in (Rabus 
et ah, 2003). 
TanDEM-X will deliver high coherence interferometric data of 
the meter class. Although the mission is mainly designed to 
generate accurate digital elevation models satisfying HRTI-3 
standards (Zink et ah, 2006), it can be expected that this kind of 
data opens up a much wider field for specialized methods for 
3D mapping of topographic features. The automated derivation 
of building heights or even the detailed reconstruction of 
buildings is certainly an important application amongst these. 
Apart from the improved spatial resolution, a major difference 
between TanDEM-X and SRTM is the variable across-track 
baseline of TanDEM-X, whereas the baseline of SRTM was 
held quasi-constant due to the second antenna mounted at a 
60m boom (and neglecting periodic baseline variations as 
consequence of thrusting). Hence, a thorough analysis of 
accuracy aspects of height estimation under the given flexibility 
of TanDEM-X is a key issue. 
Following questions should be answered by the analysis: 
Which accuracy level in terms of building height 
estimation can be reached with interferometric data as 
it will be provided by TanDEM-X? 
Is this accuracy sufficient to derive object specific 
information for rapid mapping in the context of crisis 
management? Such information may comprise, e.g., 
o the number of floors to estimate the amount 
of people living in a house 
o attached building parts of different height 
o the roof type (flat roof, saddle roof, etc.) 
How would the accuracy improve, if external data 
from GIS is included (e.g. digital building footprints)?

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