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 
Coherence Loss from Coregistration Mismatch 
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4 
Height Difference (m) 
Figure 5: Coherence loss due to a coregistration mismatch in a 
typical TanDEM-X scenario. 
phase noise mitigation is evident comparing the two DEM por- 
tions. The height variance of the flat roof of the four buildings in 
the middle of the portion is 4.72 meters for the operational case 
and 0.68 meters for the experimental one. The improvement is 
great also for the phase unwrapping stage, necessary in this case 
due to the topography visible in Fig. 6. The combined use of 
the adaptively multilooked phase and the refined coherence in- 
creases dramatically the phase unwrapping quality ratio g,. This 
ratio expresses the percentage of the scene not affected by phase 
unwrapping errors (Rossi et al., 2012). Whereas in the opera- 
tional scenario q. — 7696, meaning that about one quarter of the 
scene is affected by phase unwrapping errors, in the experimental 
one q — 99.996, meaning that the adaptive processing totally 
solved the phase unwrapping issue. The false heights in the oper- 
ational scenario are caused by the very low coherent road at the 
center-left of the scene (Fig. 9). The interferometric phase of the 
road is highly noisy, creating several branch cuts. In the phase un- 
wrapping stage, the two sides of the road remains unconnected, 
creating an hight discrepancy between them of one phase cycles 
(35 meters). This problem is solved employing the adaptive mul- 
tilooking technique, as shown from the radargrammetric control 
maps (Rossi et al., 2012) in Fig. 10 and 11. Thus, the combined 
use of the enhanced phase and coherence in the employment and 
cost computation of the MCF algorithm provides a better result in 
the reduction of false heights generated by the phase unwrapping 
stage. 
The noise mitigation algorithms can be used also to reduce the 
theoretical DEM resolution. An example of Raw DEM over Berlin, 
processed with the region growing algorithm (Vasile et al., 2004), 
with a theoretical resolution of 2 meters, is shown in Fig. 12. The 
relative flatness of the scene allows the identification of a great 
amount of details from this two-dimensional view. À portion of 
the DEM over the city center is in Fig. 13. The height jumps over 
the river are caused by the adaptive multilooking algorithm. The 
height depressions are mostly shadow areas in the SAR image. 
Overall, buildings are well represented even at the imposed small 
resolution. 
4 CONCLUSIONS AND FUTURE WORK 
This paper showed the potentials of TanDEM-X in the mapping 
of urban areas, globally acquired by the mission. Even if the 
operational Raw DEM has a coarse 12 meters resolution for a 
dense urban mapping, it has been showed through an applica- 
tion oriented algorithm how the mean absolute height accuracy 
76 
  
Figure 6: Interferometric phase of a portion of an acquisition over 
Las Vegas. In the black box the buildings under analysis. 
  
A 
E 
Figure 7: Three dimensional visualization of the DEM over Las 
Vegas for an industrial area generated using operational algo- 
rithms. 
of detected buildings in the Raw DEM is below 5 meters. Not 
considering the segmentation errors due to the geometrical decor- 
relations, the mean error drops below the meter and the standard 
deviation is kept below four meters. The building geolocalization 
accuracy has planned to be studied with the help of the Permanent 
Scatterer Interferometry high resolution technique. In the paper 
first examples of experimental high resolution urban Raw DEMs 
have been provided. The quality of the phase has been enhanced 
using the SAR amplitude statistics through adaptive multilooking 
techniques. The enhanced phase, together with the enhanced co- 
herence, helped the phase unwrapping stage. This enhancement 
allowed the generation of fine Raw DEMs having a theoretical 
resolution of just two meters. The absolute height and the geolo- 
cation validation of very high resolution DEMs are planned to be 
performed. Nonetheless, this paper didn’t analyze the limitations 
of the building reconstruction from a single aspect TanDEM ac- 
quisition. The greatest limitation to a urban three dimensional re-