# Map Projection Info Map Projection Type = Simple Cylindrical for Vesta

## Description

The global mosaics are presented in a simple cylindrical map projection. This
projection is neither equal-area nor conformal.  The meridians and parallels
are equidistant straight lines, intersecting at right angles (See Snyder 1987,
Ch 12. Equidistant Cylindrical Projection, pp 90-91).                   
                                                                        
In general the transformation from the cartographic coordinates X, Y to line and
sample in the PDS image is as follows:                 
                                                                        
sample = INT(SAMPLE_PROJECTION_OFFSET + SAMPLE_LAST_PIXEL + 1 +         
 (lon - CENTER_LONGITUDE)*MAP_RESOLUTION) mod SAMPLE_LAST_PIXEL 
                                                                        
line   = INT (LINE_PROJECTION_OFFSET - lat*MAP_RESOLUTION) +1           
                                                                        
The use of a non-standard center longitude definition for Vesta, coupled with
the PDS range limitations on longitude, introduces the modulo in the sample
equation above.                                
                                                                        
LINE_PROJECTION_OFFSET is the line number minus one on which the map projection
origin occurs. The map projection origin is the intersection of the equator and
the projection longitude. The value of LINE_PROJECTION_OFFSET is positive for
images starting north of the equator and is negative for images starting south
of the equator.  
                                                                        
SAMPLE_PROJECTION_OFFSET is the nearest sample number to the left of the
projection longitude. The value of SAMPLE_PROJECTION_OFFSET is positive for
images starting to the west of the projection longitude and is negative for
images starting to the east of the projection longitude.                
                                                                        
CENTER_LONGITUDE is the value of the projection longitude, which is the
longitude that passes through the center of the projection.
                                                                        
MAP_RESOLUTION is measured in pixels/degree.                            
                                                                        
There are four PDS parameters that specify the latitude and longitude
boundaries of an image.  MAXIMUM_LATITUDE and MINIMUM_LATITUDE specify the
latitude boundaries of the image, and EASTERNMOST_LONGITUDE and
WESTERNMOST_LONGITUDE specify the longitudinal boundaries of the map. 
                                                                        
Definitions of other mapping parameters can be found in the PDS Data dictionary.

## References

IAU-WGCCRE-Coordinate-System-for-Vesta,
(http://astrogeology.usgs.gov/search/details/Docs/WGCCRE/IAU-WGCCRE-Coordinate-System-for-Vesta/pdf)

Snyder, John P., Map Projections -- A Working Manual,
    U. S. Geological Survey Professional Paper 1395, 383p., 1987.