32 
Figure 4. Voyager 2 pic 
ture 1715s2-001, of Sa 
turn's moon, Enceladus. 
The perspective graticule 
is based on the predicted 
spacecraft trajectory, 
and was used to control 
manual map compilation on 
a Mercator projection. 
Selection of scale, projection, and the scheme for segmen 
ting a planet into a series of map sheets is based on such 
considerations as the resolution of pictures in the data 
set, the maximum tolerable map-sheet size, and the number 
of map sheets necessary for a series of maps at selected 
scales and sheet sizes. 
The mapping of Mars illustrates the full spectrum of ap 
proaches to planetary cartography. The first available 
data, from Earth-based telescopes and flyby missions, has 
such low resolution that map sheets large enough to be use 
ful result in excessive magnification of the data. The 
maps were drawn manually by cartographers who extracted all 
possible information from all possible data sources. At 
the other extreme, the Viking Orb iter missions produced 
images covering large areas of the planet that resolve more 
detail than can be mapped with available resources. Plane- 
tary maps thus tend to evolve from excessively magnified 
portrayals of all known surface information to maps showing 
only the level of detail that can be preserved by the 
lithographic printing process. 
The first program for systematically mapping landforms and 
surface structures on Mars was tailored to the Mariner 9 
mission. This mission was intended to produce photographic 
coverage of the planet with a nominal resolution of one 
kilometer per picture element. Assuming that five picture 
elements are normally required to delineate a natural fea 
ture, and that the smallest features would cover about one 
millimeter on a map, a scale of 1:5,000,000 was selected. 
Map image dimensions of approximately 0.5 X 0.5 meters were 
considered optimum for general use and as bases for geolo 
gic mapping. Conformal projections were specified not only 
to preserve true landform shapes, but to allow the maximum 
number of maps to be joined as required for regional geo 
logic analysis. Mercator, Lambert conformal conic, and 
polar stereographic projections were specified instead of 
transverse Mercator projections, to minimize edge-join 
problems and because of precedents set by the Lunar mapping 
program. The resulting set of 30 1:5,000,000 quadrangles 
was used to map Mars with Mariner 9 data. That series is 
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