# Map Projection Info Map Projection Type = Stereographic

## Description

A conformal, azimuthal projection where the central meridian and a particular
parallel (if shown) are straight lines. This is a perspective projection for
the sphere. All meridians on the polar aspect and the Equator on the
equatorial aspect are straight lines. All other meridians and parallels are
shown as arcs of circles.  Directions from the center of the projection are true
(except on ellipsoidal oblique and equatorial aspects). Scale increases away from
the center of the projection (See Snyder 1987, Ch 21. Stereographic Projection,
pp 154-163).      
                                                                
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 (21-2), (21-3), (21-4), (20-14), (20-15) (20-18), (21-15) of [Synder,
1987] USGS Paper 1395 (pp 157-159) were used.  
                                                        
Used keywords and values 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.