Projections - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 77

face. The characteristic of the geographic information or map indicates how
the projection should show geographic relationships and scales. Choosing a
projection that preserves one projection property often leads to other distor-
tions. For an individual state or province, a projection that maintains con-
stant area to make visual comparisons of areas possible is beneficial, even if
some shapes over a larger area may begin to look distorted. Indeed, larger
areas are hard to show without distortion in any case; many projections com-
monly used for world maps compromise and distort both area and shapes.
Why distortion is commonplace for projections, what are the projection
properties and characteristics, and how to choose a projection is discussed
later in this chapter.
Making Projections
Even if you are only going to use maps and will never work with geographic
information, you need to know some important things about projections.
The first is that projections use different models of the earth. Generally, pro-
jections for the entire earth can use a simple spheroid for most purposes.
When dealing with maps or geographic information of the entire world, the
loss of accuracy is slight compared to the resolution of the geographic infor-
mation or detail of the map. Projections needed for more detailed purposes
or smaller areas use an ellipsoid (also called a spheroid) that generally fits
the actual shape of the earth. For very detailed purposes and the highest lev-
els of accuracy, people use a geoid, often optimized for the shape of the
earth in one particular area.
While this may seem needlessly complex, you should remember that
because the earth is constantly changing shape (not only from volcanoes and
earthquakes), different uses need different levels of positional accuracy. On
one extreme, a map showing worldwide the most popular tourist sites needs
very little accuracy; on the other extreme, an engineer's plan of a 2-km tun-
nel needs extremely great accuracy. Most geographic information and map-
ping activities need a level of accuracy somewhere in between—often cost
and budget determine the accuracy.
What makes all this complicated for working with geographic informa-
tion and maps is that there is no standard earth model, nor geoid model, nor
spheroid model, nor ellipsoid used to represent locations on earth. The use
of different models makes it paramount for geographic information users to
know the model used for projecting the geographic information, which is
often called a “datum” (see below for more information about datums).
The Geoid Model
The most accurate model of the earth's surface is the geoid. The earth,
because of its constantly changing shape due to tectonic movements and
undulations of its gravity field, can be described in the most detailed fash-
ion through sets of measurements that are used to produce a geoid. The