GI and Cartography Issues - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 58

data is separated by a huge distance, possibly even many times the size of the
earth. Sometimes—and this is why knowing the projection of geographic
information is so important—the distances between geographic information
objects can be minute, just a few inches or feet. However, because of differ-
ences in projections, what may be minute differences in one place may be
vast differences elsewhere.
Assessing projection distortions and determining the best projection for
an activity and area remains a complex activity that is required for working
with very accurate geographic information. (See Chapter 4.)
Projected or Unprojected Geographic Information
Geographic information or maps for large areas—for example, a continent or
the world—are often projected, but they can also be unprojected. If they are
unprojected, the distortion is very significant because the latitude and longi-
Positional uncertainty when coordinate system datum is unknown in North America
(NAD27 or NAD83).
From Wieczorek, Guo, and Hijmans (2004). Reprinted by permission of Taylor & Francis Ltd.