Projections - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 73

PRINCIPLES OF GI AND CARTOGRAPHY Projections

Chapter 4

Projections

Building on the overarching discussion of projections in Chapter 3, espe-
cially the discussion of distortion and accuracy, this chapter focuses on the
principles you will need to work with projections—the transformation of
spherical coordinates to planar coordinates. Chapter 4 also provides the his-
torical background for and presents specific details of various projections.
The focus of this chapter is on projections for geographic information; how-
ever, it also considers the use of projections for maps.
In fact, projections occupy one of the most essential roles in cartogra-
phy for geography and geographic information. For some people, this role
may arguably be perhaps the most essential, because most geographic infor-
mation is “projected,” even if it is never shown on a map. This has started to
change as more and more geographic information is collected and stored in
latitude and longitude coordinates, which are not projected. But even if all
the data you need and want is available in latitude and longitude coordi-
nates, you will probably need to project it to make the sort of map that peo-
ple are familiar with.

Maps without Projections

Some people would claim that if a thing or event is shown on a map, it must
be projected. In most cases this is true—and for good reasons. But there are
exceptions. These exceptions are important enough to pay attention to. The
first exception was already mentioned: locations stored in latitude and longi-
tude coordinates are not projected—they are spherical coordinates. It's even
possible to make a map with these coordinates, but such a map is much dis-
torted and can even be misleading. The second exception is all the maps
drawn following artistic rather than scientific concerns. Usually these maps
are used for advertisements, but they can also be used to show transporta-

75