Planar coordinates and map projections
Planar
coordinate systems are independent of any particular map projection—you can use
them with any projection you like. The choice of projection is not irrelevant,
however, and specialized coordinate systems (like the Universal Transverse Mercator and State Plane Coordinate systems) are based on
conformal projections. The reason has to do with the distance and direction
calculations mentioned in the previous concept.
Since a
conformal map preserves local angles, directions are not
distorted (at least not at large scales) as they would be on other
projections. Distance calculations are distorted, but this is true of other
projections, too, since no projection preserves true scale everywhere on the
map.
The
coordinate systems you'll read about next are divided into zones, each with its
own origin and axes. This controls distortion because each grid is applied to a
limited spatial extent for which the projection parameters have been optimized.