# Map Projection Info Map Projection Type = Mercator

## Description

A conformal, cylindrical projection where meridians are equally spaced straight
lines. Parallels are unequally spaced straight lines, closest near the equator,
cutting meridians at right angles.  The scale is true along the equator. Poles
are at infinity with great distortion of area in polar regions (See Snyder 1987,
Ch 7. Mercator Projection, pp 38-47).                                  
                                                                       
In general the transformation from the cartographic coordinates X, Y to line and
sample in the PDS image is as follows:                
                                                                       
sample = INT( X + SAMPLE_PROJECTION_OFFSET) +1                         
line   = INT(-Y + LINE_PROJECTION_OFFSET) +1                           
                                                                       
where                                                                  
     X = F1(latitude,longitude)                                        
     Y = F2(latitude,longitude)                                        
                                                                        
                                                                        
and F1(latitude,longitude) and F2(latitude,longitude) are the cartographic
formulas described detailed in (Snyder 1987)               
                                                                        
Equations (7-1), (7-2), (7-2a), (7-4), (7-4a), (7-5) of USGS Paper 1395
(pp 41,44) were used.                                        
Keywords and values used with respect to map projections follow the PDS3
standard reference whereas a detailed description of the parameter's scope and
definition can be found in the PDS dictionary.
LINE_PROJECTION_OFFSET/SAMPLE_PROJECTION_OFFSET are the line/sample values minus
one onto which the map projection origin falls.

## References

Acton, Jr. C.H., Ancillary Data         
     Services of NASA's Navigation and Ancillary Information Facility,
     Planetary and Space Sciences, 44, Number 1, pp. 65-70, 1996.

Greeley, R. and Batson, G., 1990, Planetary Mapping,
     Cambridge University Press.

Planetary Science Data Dictionary Document,
    Document JPL D-7116, Rev. F, October 20, 2008

Snyder, J.P., 1987, Map Projections - A Working Manual,
     US Government Printing Office, Washington, p. 42.

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