GI and Cartography Issues - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 53

spread use of two-dimensional coordinate systems, and the cost of trans-
forming two dimensional coordinate systems. Chapter 4 will take a look at
some of these systems, including their applications.
Key Concepts of Projections
Projections convert measured locations of things and events in three dimen-
sions to two dimensions. Projections are important but also complicated
because it is impossible using geometric or more complex mathematical
methods to simultaneously preserve both the shape and the two-dimensional
area of any three-dimensional object found either on the spherical surface of
the earth, in the earth, or near the earth, when we depict it in a two-dimen-
sional coordinate system. Each projection is an abstraction of the earth's sur-
face and introduces distortions that affect the accuracy of the geographic
information or map. A projection starts with one of three representations of
the earth's irregular surface (geoid, ellipsoid, or spheroid) and converts it
directly or through intermediary transformations to a f lat, or planar, trans-
formation.
Choosing the right projection is important for controlling these distor-
tions. Thankfully, choosing the right projection for a particular area is a task
that has often been done by institutions and governments and made part of
World map from 1801 using a Mercator projection.
From www.davidrumsey.com . Reprinted by permission of David Rumsey.