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Meeting, April 1998 - home page
We recomputed the Viking Mars Control Point Network and derived
Mars-fixed 3-dimensional coordinates of 3739 selected, globally
distributed landmarks. The Mars Pathfinder coordinates and
data from spacecraft occultation measurements were used as fixed
control information in the analysis. The analysis benefitted
greatly from recently restored Viking Orbiter trajectory data
and new planet rotational parameters derived from Pathfinder lander
tracking. In our attempt to predict the coordinates of the
Viking Lander 1 site from given Pathfinder coordinates, we find
an offset of approx. 4-5 km of this lander location with respect
to what was previously reported. With a revised lander location,
our landmark coordinates have an intrinsic precision of 740 m
anywhere on Mars. This network thus removes previously noted
severe offsets between Mars-fixed coordinates and cartographic
products. The fact that coordinates are available in 3 dimensions
allows us to study the global shape of Mars. We find that the
best-fit sphere about the planet's center of figure has a radius
of 3390.4 km, whereas the best-fit spheroid axes are given by
a=b= 3396.4 km and c = 3376.9 km. The data points, interpolated
to form a gridded DTM, greatly exceed the vertical precision and
spatial resolution of any current global shape model. They
clearly show that the character of topography in the Northern
and Southern hemisphere differ in terms of mean elevation and
surface roughness. The "dichotomy boundary", estimated
to be at an elevation of approx. 1500 m above the 6 mbar Geoid
level, is sometimes -but not always- marked by steep slopes.