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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
afterwards also in the overlapping areas. Again, the blunders are
detected firstly in ray intersections only, and secondly with refer-
enc to the MOLA DTM (Spiegel, 2007b).
For the evaluation of the bundle adjustment results the mean inter-
section error is calculated only for point lying in an overlapping
area. This reveals the improvement of the bundle adjustment of
blocks more clearly than the usage of all points of a sub-block
would do.
4 RESULTS
4.1 Systematic bundle adjustment of single orbit strips
The method for systematic bundle adjustment of single orbit strips
was used to process the HRSC data received in the almost 7 years
from January 2004 until November 2010. During this time the
Mars Express mission has surrounded the planet 8761 times. In
about 2800 of these orbits the camera took imagery theoretically
suitable for stereo processing. HRSC image data sets of 2535
strips were succesfully processed. In 2329 strips (91.996) the ex-
terior orientation was significantly improved by the bundle ad-
justment. For a number of 123 strips further improvements of the
exterior orientation were achieved by modeling spacecraft oscil-
lations through a shorter orientation point distance (Bostelmann
and Heipke, 2011).
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mean intersection error per strip [m]
Figure 5: Number of strips grouped by their mean intersection
error before (light gray) and after (dark gray) the improvement of
the exterior orientation by bundle adjustment
In Fig. 5 the mean intersection error for a total of 2535 suc-
cessfully processed strips is presented. The strips are classified
according to their intersection error in ranges of 10m. The light
gray bars represent the quality of the exterior orientation data be-
fore, and the dark gray bars after the process of bundle adjust-
ment. It is shown that the number of strips with a mean intersec-
tion error « 10m distinctly increases after the bundle adjustment
(more precisely from 47 to 437). Also the quantity of strips with
a mean intersection error lying between 10m and 20m increases
by a number of 200. On the other hand the number of strips with
higher errors obviously decreases. E.g. the number with an er-
ror > 100m is reduced from 119 to 38 strips. Probably because
there was no exclusion of strips with obvious image degradation,
the mean intersection errors presented here are somewhat higher
than in other statistics of HRSC DTM processing (e.g. Gwinner
et al, 2010). In general the mean intersection error presented
in this paper should not be taken as an absolute criterion for the
quality of HRSC data, but it is a good indicator for a relative im-
provement.
4.2 Multi orbit block processing
The analysis of single orbit strips reveals the internal photogram-
metric accuracy of HRSC image data. To connect different strips
to a stable block, in addition a sufficient number of accurate tie
points in the overlapping area is needed. The previously de-
scribed method of adjusting the exterior orientation of a block
was used for a set of 21 overlapping HRSC strips. The selected
area is a region called Nepenthes Mensae between 1°S to 17°N
and 111°E to 135°E. The footprints of the nadir channel images
are shown as an overlay to the color coded MOLA DTM in Fig.
6.
Figure 6: Color coded MOLA DTM with footprints of 21 HRSC
nadir channel images constituting a photogrammetric block
The criteria for the selection of these strips were:
The 21 strips overlap to form a single block.
e All five panchromatic channels are available.
e The single strip bundle adjustment succeeded with a small
mean intersection error (less than 20m).
e The resolution of the nadir channel is better than 30m.
e The image information allows tie points distributed over the
whole image.
e The overlapping area between neighboring strips is rela-
tively wide.
Table 1 lists the selected strips. They are ordered going from
west to east. The data was aquired in four time frames. 14 of
the strips were taken in the year 2005. Strip h3193 and h3160
were taken 9 months later in 2006. Three strips (h5212, h5230
and h5248) were taken in 2008. The newest two strips (h8362
and h8383) from 2010 fill the remaining gaps. Fig. 7 shows the
mean intersection error of the strips before and after the single
strip bundle adjustment. The use of nominal exterior orientation
results in a mean intersection error of 30m or higher in some
strips. With the adjusted orientation data it is reduced for all strips
to a mean of 11.3m.
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