and Satellites give the best current solutions to define 
the Mars north pole an prime meridian by à. (right 
ascension), and 0, (declination), and W (hour angle), 
respectively. The hour angle is measured from Q, the 
node defined by the intersection of the J2000 Earth 
equator and the Mars equator, along the Mars equator 
to the Mars prime meridian. The right ascension, 
declination and hour angle of Mars are computed as 
the following: 
a (2000.0) =317.681°-0.108°T 
0 (2000.0) = 52.886° - 0.061° T 
W (2000.0) = 176.868° + 350.891983° d 
Where T is measured in centuries and d is measured 
in days from JD 2451545.0 TDB 
For the computation of map projection, the adopted 
reference spheroid has an equatorial radius A = 
3,393.4 km and a polar radius of B = 3.375.8 km. This 
yields a Martian flattening of 0.0052 or 1/192 and an 
eccentricity (e) of 0.101715. The Working Group also 
adopted the aerographic coordinate system to be used 
for all Mariner 9, later Viking, and map products. The 
origin of the coordinate system is at the center of mass 
of Mars. 
3.2 Map Projections 
For Mars topographic mapping, three conformal map 
projections (Mercator, Lambert, and Polar 
Stereographic) are used. The Mercator projection is 
used for equatorial band, Lambert for medium 
latitudes, and polar stereographic for the polar regions. 
For large-scale maps, 1:1,000,000 or larger, the 
Transverse Mercator is used. Sinusoidal equal-area 
projection is occasionally used, mainly for global-scale 
digital maps. 
3.2.1 Mercator Conformal Projection: For the 
Mercator projection, the x and y coordinate axes are 
straight lines and the origin of y-coordinates is at the 
equator (Thomas, 1964). The projected scales vary 
depending upon the latitude of the point projected. It 
is 1:1 along the equator and becomes greater for 
greater latitudes. The Mercator projection is used 
between the 65? north and 65? south of latitude for the 
1:25,000,000-scale global map and between 30? 
north and 30? south of latitude for both - and 
1:2,000,000-scale .series of Mars maps. For the 
1:25,000,000-scale map , the scale is 1:10,610,713 at 
the 65? of latitude, greater than twice that at the 
equator. 
3.2.2 Lambert Conformal Conic Projection: In the 
Lambert conformal conic projection, the projected 
parallels (latitudes) are arcs of concentric circles with 
radii which are their corresponding projected 
meridians. the common center is also intersected by . 
the projected meridians (Thomas, 1964, Richardus 
and Adler, 1972). The meridian of each quadrangle 
serves as its y-axis, and the intersection of the y-axis 
and its lower parallel (latitude) serves as the origin. To 
minimize scale distortion, two standard parallels are 
used. This allows the latitude difference between the 
two standard parallels to be 23.34? to be two-thirds of 
the latitude difference between the two boundaries, so 
that scale errors are more uniformly distributed. The 
scale is true only along the two standard parallels. 
It should be noted here that the scale of so-called 
1:5,000,000 maps is not exactly 1:5,000,000. The 
scale of the quadrangles is set to match the scale at 
the lower boundary latitude (30?) of the Lambert 
projection with the scale at the upper boundary latitude 
(30°) of the Mercator quadrangles which is already 
distorted with a scale ratio (with the equator) of 
0.867151. In other words, the scale at the latitude 30? 
at the upper boundary of the Mercator projection, is no 
longer 1:5,000,000, rather, it is 1:4,335,753. 
3.2.3 Polar Stereographic Conformal Projection: 
The polar stereographic projection is a special case of 
the Lambert projection with only one standard parallel 
being the point at the pole. Therefore, the meridians 
are straight lines radiating from a central point which is 
the pole and the parallels are concentric circle about 
this central point (Thomas, 1964). The polar 
stereographic projection is used for the polar regions 
from x55? to the poles for the 1:25,000,000- and 
1:15,000,000-scale maps. and from £65? to the poles 
for 1:5,000,000-scale maps. The scale of the two 
polar quadrangles, MC-1 and MC-2, is determined by 
making the scale of latitude 65? to be the same scale 
as latitude 65? in the Lambert projections. 
3.2.4 Sinusoidal Equal-Area Projection: The 
Sinusoidal is an equal-area projection, i.e., true for 
area scale in the map. It is used for global digital 
maps of Mars. All of the parallels are straight lines 
and the meridians are sinusoidal (sine curves.) 
4. TOPOGRAPHIC DATUM OF MARS 
The purpose of planetary topographic mapping is to 
provide topographic information for the support of 
mission planning and operation, and for geologic and 
other scientific studies of planets, it is vitally important 
that elevations be closely related to actual 
morphologies on the planetary surface such as those 
of lava flows and channel slopes. For instance, Mars 
has no seas and hence on sea water, it is not possible 
to use a sea-level reference for its topographic datum. 
The most appropriate method is to define a datum on 
the basis of Mars' gravity field. This gravity potential 
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996