Cross
in ori-
amely
)ot on
95
® the planetographic longitude X, this is the angle bet-
ween the meridian through P and the prime meridian
(\' = 0) through the crater Airy-0 (A' counts positive
towards the west).
It is obvious that A' and A. complement each other to 360°,
whereas ¢' and © are more or less similar values.
The reference surface for mapping will be a spheroid, i.e. a
bi-axial ellipsoid.
Axis of rotation S Tangent to ellipse at P'
Normal to
> P ellipse at P'
>
a h
5 ,
: PAM
S R ~
2 <
Equator of the planet 9 q
Equatorial radius
Fig. 4: Planetocentric latitude ©
and planetographic latitude q' of a point P
3.3 Definition of heights
It is still more difficult to define a reference system for height
measurements, i.e. as an elevation reference level for DTM's
and for the derivation of contour lines in maps. From a theo-
retical point of view an equipotential surface — such as the
geoid on Earth — is supposed to be the best solution. How-
ever, the field of gravity on Mars is much more irregular than
the geoid and its present knowledge is still very weak. It is
therefore not sufficient for the definition of a height reference
System.
On the other hand, the use of a mathematically defined re-
ference surface offers some practical advantages. In
particular it makes the height system independent from im-
proved knowledge of the gravity field, and transformation
between different coordinate systems is easy to handle. The
most appropriate mathematical surface for this purpose is a
tri-axial ellipsoid, which fits to the irregular shape of the planet
significantly better than a bi-axial one. This is why it was
decided to use DTM heights based on a tri-axial ellipsoid,
with the height measured along the radius to the center of
the planet.
This definiton implies that heights are not measured along
a plumb line as we do on Earth, and that theoretically (virtual)
water could run up-hill. However, the differences are esti-
mated to be very small, and it is unlikely that this effect will
ever be cartographically important.
4. PHOTOGRAMMETRIC AND CARTOGRAPHIC
PROCESSING OF HRSC/WAOSS IMAGE DATA
In order to provide a wide range of different photogrammetric
and cartographic products a comprehensive and mostly
automated processing line has been set up. It is coordinated
by the Photogrammetry and Cartograpy Working Group
(PCWG) of this camera experiment under the chairmanship
of Prof. Jórg Albertz, Technical University of Berlin (TUB).
Software development has to consider that the image
formats of the HRSC and WAOSS data are fundamentally
different from common sensor sytems. Different comman-
ding strategies allow many variations within one imaging
sequence, which makes the development of adequate
software components complicated. Variable aspects within
one data set are the starting sample positions on CCD, the
number of samples per line and the pixel resolutions because
of the elliptical orbit and also the formation of so-called
macropixels.
4.1 Photogrammetric Point Determination
The photogrammetric investigations in order to generate high
accurate orbit and attitude information derived by photo-
grammetric bundle block adjustment are guided by Prof.
Heinrich Ebner, Technical University Munich (TUM). This
fundamental basis for all further processing operations will
be generated using the original orbit and attitude data as
well as the data of the existing horizontal and vertical control
network.
In order to prepare for this task computer a great deal of
simulations on block triangulation have been performed to
obtain a survey of the attainable accuracy and to give re-
commendations in the planning phase of the Mars96 mission.
Comprehensive simulations on local, regional and global
point determination based on HRSC and WAOSS imagery
and orbit parameters as well have been conducted by Ohlhof
(1995).
It turned out that local point determination from HRSC data
can be considerably improved if the two photometric chan-
nels of the camera are incorporated into the bundle adjust-
ment.
Regional and especially global point determination greatly
benefit from the combination of HRSC and WAOSS imagery.
For the root mean square values of theoretical standard
deviations of adjusted object point coordinates the relations
in Table 2 were derived.
Camera x/y z
HRSC (3 CCD lines) 9.1m 28.7m
WAOSS (3 CCD lines) 42.2m 82.6 m
HRSC + WAOSS (3+3 CCD lines)
* all points 28.7m 56.1m
e HRSC + WAOSS points 3.4m 13.8 m
* WAOSS points 36.0 m 69.9 m
Table 2: Rms values of theoretical standard deviations
of adjusted object point coordinates
(from Ohlhof 1995)
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
