How projection works

How projection works.

Projection is the way to project the points on the earth (globe) to the plane.  For different projection systems, like NAD27 and NAD83, they use the same mathematical formula to do the projection, but use the differnt referece system. The NAD 1927 use the Ellipsoid 7008 (Clarke 1866), and the NAD 1983 use the Ellipsoid 7019 (GRS 1980).

The other difference between NAD 1927 and NAD 1983 are the defining constants like the false northing, false easting, etc..