surface and lead to a better understanding of geologic
and tectonic histories of Mars. Topographic mapping
of Mars, however, differs in many ways from the
mapping of Earth. It involves solving unprecedented
problems: the absence of ocean to provide a zero-
elevation reference surface, the lack of precise ground
control, methods of data acquisition, or characteristics
of data, and so forth. These unconventional factors
require new approaches and the development of new
mapping techniques.
As Mars has no seas hence no sea level, the most
appropriate definition for its topographic datum, the
zero-elevation surface, is a 6.1-millibar pressure
surface (Wu, 1981) with a gravity field represented by
fourth-degree and fourth-order spherical harmonics
(Jordan and Lorell, 1975, Christensen, 1975.)
Topographic data used for mars topographic mapping
include imaging and non-imaging remotely sensed
data. Imaging data are Mars images taken by the two
narrow-angle cameras on board of the two Viking
missions. Despite the extremely narrow field-of-view
of the Viking Orbiter images, special photogrammetric
techniques have been developed enabling
stereomodels to be established for map compilation
on AS11-AM analytical stereoplotters (Wu et al,
1982). Using these techniques, a total of 140 contour
maps of 1:2,000,000-scale with contour interval of 1
km has been compiled and covers the entire Martian
surface.
Non-imaging topographic data include the Ultraviolet
Spectrometer(UVS), the Infrared Radiometer(IRR),
and the Infrared Interferometer Spectrometer(IRIS) on
board spacecraft of Mariner 9, and the S-band radio
occultation measurements, from both Mariner 9 and
Viking missions. Earth-based radar observations of
Mars have been used and also played an important
role in Mars topographic mapping.
From 1,157 high-altitude Viking Orbiter images
together with occultation measurements and Earth-
based radar data, a planetwide topographic control net
was established and has produced 4,502 control
points for the control of systematically mapping of the
entire Martian surface (Wu and Schafer,1984).
Mars small-scale topographic maps , i. e., 1:5,000,000,
1:15,000,000, and 1:25,000,000 were compiled with
the combination of topographic data extracted from
the 1:2,000,000-scale contour maps, and the non-
imaging topographic data including Earth-based radar
observations of Mars as described above.
By digitizing 1:2,000,000-scale contour maps, a Mars
Terrain Model (MDTM) has been derived with a
resolution of 1/59.226 degree, exactly 1 km per pixel at
the equator.
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Despite the coarse data from limited data sources,
these topographic products are results of series of
Mars projects and are the first comprehensive
topographic maps.
2. MARS TOPOGRAPHIC DATA SOURCES FROM
MARINER 9 AND VIKING MISSIONS
Topographic data used for Mars topographic mapping
are derived from remotely sensed data of various
devices of experiments from Mariner 9 and Viking
missions. For example, two television cameras on
board the Mariner 9 spacecraft photographed the
entire Martian surface (Masursky et al., 1970). But it
was the two Viking Orbiter missions that sent back
almost 60,000 images of Mars to Earth, made it
possible to systematically map Mars' topography in
great detail. Scientific experiments, which have
significance to the topographic data, imaging or non-
imaging, are briefly described in the following two
sections.
2.1. Imaging Data
The primary goal of Mariner 9 was to photographically
map the planet Mars. More than 7,300 images of Mars
were taken by the two cameras (wide- and narrow-
angle cameras). But it is the 60,000 images of Mars,
which were taken by the Visual Imaging subsystem
(VIS) on the two Viking Orbiters, to have been used for
the control network and for stereoscopically compiling
Mars topography. The VIS on each of the two Viking
Orbiters consists of two identical vidicon cameras with
nominal focal lengths of 475 mm. Each image
consists of 1,056 lines (along y-axis) with 1,182
samples (along x-axis) in each line, and thus contains
approximately 1,200,000 picture elements. At a
12.494 mm x 14.021 mm format, each pixel is 11.8
micrometers square. The field of view is 1.538 x 1.592
degrees, or 2.286 degrees diagonally. A resau grid
consisting of 103 resau marks is incorporated in each
camera. The resau marks are 36 micrometers, or 3.1
pixels, on one side. The calibrated coordinates of
resau marks are known to better than 2 micrometers.
Geometric distortion are less than 3 pixels at the
center and increase outward. After the IPL
decalibration is made to the Viking Orbiter images, the
r.m.s. errors are reduced drastically to less than 2 um
(Ruiz,1976). Therefore, optical distortion can be
ignored for images processed by IPL's dicalibration
software. At the altitude of 1,500 km, the ground
coverage of a vertical photograph is about 39.6 x 44.4
km. Each pixel is 37.5 m square.
2.2 Non-imaging Data
Non-imaging topographic data, such as spectroscopic
sensors and Earth-based radar, can contribute
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996