Choices in How We Make Representations - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 41

Data combinations When data are combined with other data, as they

usually are, the choices of projection, scale, and coor-
dinate system become especially important. For
example, data from one projection can usually not be
reliably combined with data from another projection.
It is of the greatest importance that water height and river
bank elevation measurements be recorded geographically
using the same coordinate system.

Cartographic Representation

In traditional cartographic map making, which lacked a distinct phase of
working with geographic information collected by others, cartographic rep-
resentation was a choice of map elements, level of generalization, and visual
variables, as well as many of the geographic representation elements. The
measurements and observations could be transformed manually into a dizzy-
ing array of graphical elements, some of which still remain beyond the
means of GIS or remain highly difficult even with more specialized applica-
tions. In most GIS work, some or even all of the data has already been pre-
pared, making cartographic representation often a more distinct part of the
process.
For work with both geographic information and maps, the basic choices
remain the same, but it's fair to say that you will have fewer chances to work
directly with observations and measurements. Because of the availability of
geographic information, in most cases, people are constrained by the geo-
graphic representation. Of course, there is some flexibility, but for the most
part options are decisively limited by choices already made for the geo-
graphic representation. This limited flexibility and the options that go along
with it are discussed in greater detail in the following chapters. The follow-
ing choices serve only to orient you and are related to the river flooding
example from earlier in the chapter to help you better understand the rela-
tionship between geographic and cartographic representations.

Choices for Cartographic Representation

Projection Every map made on a flat sheet of paper or shown

on a flat screen is projected. An infinite number of
projections can be made. Fortunately, in most cases
at least, commonly used projections have been estab-
lished.
Which common projection used in northern Illinois is best
suited for the presentation of river flooding? This issue
needs to be considered together with the coordinate system.