The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008
1038
nominal ground resolution (m per
pixel)
HRSC Orbit
nadir
stereo
photometry
53250001
12.5
25
50
53070000
12.5
25
25
52890000
12.5
25
25
5271 0000
12.5
12.5
25
5253 0000
12.5
12.5
25
52350000
12.5
12.5
25
Table 1. Nominal ground resolution of the HRSC orbits (only
panchromatic channels)
3. METHODS
This chapter briefly describes the basic photogrammetric
processes; for more information consult the corresponding
papers. For a detailed overview of HRSC image processing
refers to Schölten et al. (2005).
Chapter 3.1 describes the automatic determination of tie points
by software provided by the Leibniz Universität Hannover.
These tie points are used as input in the bundle adjustment (see
chapter 3.2), provided by the Technische Universität München
and the Freie Universität Berlin, to improve the exterior
orientation for single HRSC orbits and for a bundle block
adjustment with more than one HRSC orbit. These refined
exterior orientations allows us to adapt the HRSC derived data
to the global Mars reference system defined by MOLA and will
be used for the derivation of high resolution DTMs and
ortho-image mosaics, described in chapter 3.3.
3.1 Determination of Tie Points
In order to process large image blocks it is necessary to enhance
the concept for tie point matching presented in Schmidt et al.
(2008). It is not reasonable to process the tie point matching in
all images of the block simultaneously because only
neighbouring strips overlap. Therefore, the block is divided into
parts which consist of two neighboring strips respectively.
Figure 1 shows this concept for a block consisting of three
single strips.
entire block 1. partial block 2. partial block 3. partial block
Figure 1. Concept of partial blocks
The whole block is divided into the same number of partial
blocks as the number single strips where each strip once has to
act as master strip. In case of the block presented in this paper
six partial block have to be build and processed separately. Note
that the last strip of the entire block has no partner. More
information can be found in Schmidt (2008).
3.2 Bundle adjustment
The bundle adjustment approach for photogrammetric point
determination with a three-line camera is a least-squares
adjustment based on the well known collinearity equations. The
approach estimates the parameters of the exterior orientation
only at a few selected image lines, at the so-called orientation
points. Because of Doppler shift measurements to estimate the
position of the orbiter there are systematic effects in the
observed exterior orientation. To model these effects in the
bundle adjustment additional observation equations for bias
(offset) and drift have to be introduced. To use the MOLA DTM
as control information the least squares adjustment has to be
extended with an additional observation equation for each
HRSC point. These observations describe a relation between the
MOLA DTM and these HRSC points. This approach is given in
more detail in Ebner et al. (2004), Spiegel (2007a), Spiegel
(2007b) and Schmidt et al. (2008).
The approach is valid for single orbits as well as for block
configurations. But, there are differences in operational use
concerning blunder detection of tie points and concerning
differences between HRSC points and the MOLA DTM. The
reason for these differences is that the resolution of the MOLA
DTM is lower than the accuracy of HRSC points and depends
on terrain slopes. That circumstance can occur, for example, at
the rims of craters. Another reason for differences is that the
MOLA data does not contain small craters in contrast to the
HRSC data.
The blunder search for single orbits is carried out in two steps.
First, blunders of tie points are detected without using the
MOLA DTM, i.e. only ray intersections are used for this
investigation. In the second step, the DTM is introduced and the
HRSC points are registered to the MOLA DTM. During this
step HRSC points are eliminated that do not fit to the MOLA
DTM surface. After the two steps, we have an exterior
orientation for the orbits and a data set of tie points without
blunders and without big differences between MOLA DTM and
HRSC points.
To compute blocks it is necessary to divide the blocks
temporary into single orbits. With the divided orbits a blunder
search for single orbits can be arranged (two steps). The
resulting set of tie points without blunders is used instead of the
original tie point set for the next steps. In the third step, ray
intersections of tie point located in the overlapping area of two
orbits are investigated. The reason for this is to detect blunders
in HRSC points, that are built from tie points located in two or
more orbits. The fourth step is to register the remaining HRSC
points to the MOLA DTM and compute the block adjusted
exterior orientations.
3.3 DTM derivation
Derivation of DTMs and ortho-image mosaics are basically
performed using software developed at the German Aerospace
Center (DLR), Berlin and is using the Vicar environment
developed at JPL. For our DTM derivation, the main processing
tasks are first a pre-rectification of image data using the global
MOLA-based DTM, then a leastsquares area-based matching
between nadir and the other channels (stereo and photometry) in
a pyramidal approach and finally, DTM raster generation.
Parameters for the derivation of preliminary DTMs are
individually adapted to the image quality and to the initial DTM.
The result is a preliminary HRSC-based DTM which is used for