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Fusion of sensor data, knowledge sources and algorithms for extraction and classification of topographic objects
Baltsavias, Emmanuel P.

International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 7-4-3 W6, Valladolid, Spain, 3-4 June, 1999
Various reasons for a decreased phase correlation exist. Repeat
pass interferometry is much more sensitive to temporal
decorrelation, than optical stereoscopy to radiometric changes.
Besides temporal decorrelation, thermal noise, SAR processing
artefacts, baseline decorrelation, atmospheric artefacts and
misregistration of the images have a decreasing impact on the
correlation (Zebker et al., 1994). Layover causes total
decorrelation of the signals, making the terrain also visible in
the coherence map (Fig. 1, Fig. 2). In order to reduce statistical
phase variations, the SAR multilook technique and
interferogram filtering is applied.
Fig. 1. DEM of test site 1.
Fig. 2. Coherence map of test site 1. The
coherence, respectively the height error,
follows closely the terrain shape.
Fig. 3. Correlation coefficient of SPOT stereo
pair in test site 1. Correlation gaps occur
randomly all over the site.
To summarise, low cross-correlation indicates error suspicious
parts in both DEMs and can be used as a common weight for
the fusion process, as it leads to inaccurate measurements in
both cases.
In InSAR DEMs, height accuracy generally follows closely the
phase coherence, but in transition areas from lower to higher
coherence, a refinement of this global assumption is necessary.
In case of LS phase unwrapping, isolated phase inconsistencies
will be bridged, but highly correlated values adjacent to large
areas showing low correlation may be error affected. This fact
will be taken into account in the fusion procedure.
Low correlation also leads to problems in stereo DEM
generation, as it complicates the identification of conjugate
points. Still, some mismatched points may have high correlation
values, thus appearing as spikes in the DEM, and must be
removed before the data fusion.
As mentioned above, different types of errors occur in a DEM.
Therefore, a DEM fusion by using cross-correlation would only
reduce the statistic error, but leave systematic errors and
blunders. In addition, unwanted holes, where correlation is low
in both cases, would occur in the fused DEM. Therefore, the
presence of a height estimate of the whole site is required,
which will be refined by the fused InSAR and stereo-optical
heights. A straightforward solution for this terrain estimate
would be the filtered stereo DEM, which has a robust
performance in most types of terrain and shows only small
systematic errors.
Median filtering is a common filter operation for the reduction
of unavoidable outliers in stereo DEMs. Although it works well
for a terrain smoothing, some problems remain: