Glossary for Geospatial Science

projection or map projection:  “A device for representing all or part of a rounded surface on a flat sheet.  Since this cannot be done without distortion, the cartographer must choose the map characteristic [area, shape, scale, direction] which is to be shown accurately at the expense of others.” Map Projections Used by the U. S. Geological Survey, Geological Survey Bulletin 1532, Second Edition, U. S. Government Printing Office, Washington, D. C. , 1982, p. 5.

Every process that establishes or translates map projections offers the same standard selection list of projections.  The practical choice of map projections for ungeoreferenced project materials is normally determined by the kind of projection used by those project materials that are calibrated to a projection.  For example, if you have a scanned USGS topo map in the polyconic projection, you may decide to calibrate satellite images and vector overlays (or translate existing calibration) to the same projection.  (See also: ellipsoid.)