Geology Reference
In-Depth Information
mapped in standard cartographic series, or quadrangles.
Indexes and availability of cartographic materials are
maintained on the USGS website (
http://pdsmaps.wr.
usgs.gov)
and include geologic maps and a gazetteer of
of
cially named planetary surface features.
Deriving a coordinate reference system for planets is
challenging. Once the spin axis for an object has been
determined, the equator can be set with latitude running
north and south from the equator. The prime meridian is
arbitrarily de
ned, using some recognizable feature, such as
a small crater. Now come some potential problems!
Longitude can run either east or west from the prime meri-
dian, and both have been used in planetary science, even on
the same planet. Thus, some instrument data sets give
coordinates in east longitude, while others use west longi-
tude. In principle, so long as east or west is designated, there
should be no problems, but in practice, especially with some
digital
files where E or W is not designated directly with the
values, errors can arise. For example, both systems have
been used on Venus, and in the early planning stages for a
mission that would have sent small probes through the
atmosphere toward the surface, the scientists used one sys-
tem and the engineers used the other system; had the mis-
sion
flown (it did not) and the error not been caught, the
probes would have descended through a part of the venusian
atmosphere totally different from that targeted.
Today, the generally accepted use on maps follows the
planetographic system, in which west longitudes are
used for objects that spin in the same direction as that in
which the object orbits. For example, looking down on a
planet
availability and the speci
c planet, different systems are
used throughout the Solar System, and, as with longitude,
one must be familiar with the conventions used for the
speci
c planetary object of interest.
Topographic maps are generated using a variety of
techniques, including photogrammetry, photoclinometry,
and altimetry (see
Section 2.6
). Photogrammetry has long
been used in making topographic maps of Earth from
aerial photographs. This technique is based on stereo-
scopic models in which two or more images are taken of
the same terrain but from different viewing angles. From
knowledge of the geometry of the camera system optics
and the altitude fromwhich the images are taken, the relief
of the terrain can be derived. Photoclinometry, also
known as
“
shape from shading,
”
uses the amount of
light re
ected from the surface to determine the slope of
that surface; this technique requires that the illumination
and viewing geometry be known and that the surfaces are
homogeneous with regard to texture, composition, and
other variables that in
uence their re
ectivity. In these
cases, the re
ectivity of each pixel is measured, from
s north pole, the planet would spin in a counter-
clockwise direction and the planet would orbit the Sun in a
counter-clockwise direction. When viewing the planet
toward the equator from a
fixed position in space, the
longitudes would increase in value toward the west as
the planet rotated. For an object that is in retrograde
motion (spinning in the opposite direction to its orbit,
such as Venus), longitudes increase toward the east.
The
“
zero
”
reference elevation also poses problems;
what does one use on a planet not having a sea level?
Different systems have been used on different planets. For
example, on the Moon and Venus, the mean radius of the
planet is taken as the
“
zero
”
contour, while on Mars, the
original reference was based on the triple point of carbon
dioxide, the main component of the atmosphere. The
triple point of a substance is the pressure
'
Figure 2.21. Digital elevation models (DEMs) enable topography to
be portrayed by a variety of techniques, including as shaded-relief
maps, shown here for the Olympus Mons shield volcano on Mars.
Manipulation of DEMs can provide oblique views of terrain, as might
be seen froman airplane window, and the illumination direction and
angle can be changed to include early-morning to mid-day views
(MOLA topographic DEM courtesy of the US Geological Survey).
temperature
condition at which all three phases (solid, gas, liquid)
exist. As discussed in
Chapter 7
, a different system is
used on Mars currently. Thus, depending on data
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