Geography Reference
In-Depth Information
explore the various components of geographic and cartographic representa-
tion in relationship to communication in greater detail.
Geographic Representation
To illustrate the issues and concepts related to geographic representation,
we will begin with an example that you may already have encountered your-
self: f looding. You will come back to this example throughout this chapter,
so you need some background information to aid you in making connections
to cartographic representation and communication issues later. After pre-
senting some background information, we will turn to the issues in creating
a geographic representation using a database. As you will see, geographic
representation is fundamental to GIS, but even without a GIS you can
engage in geographic representation.
One river that frequently floods an urban area is located in northern Illi-
nois near Chicago. The Des Plaines River is a muddy and slow river draining
areas that were once prairie and now are largely developed. It flows into the
Chicago River and then into the Mississippi. Children are afraid of the river
because of legends about leeches and snapping turtles supposedly living
there—never mind the pollution; most adults are more afraid of the river
because the river floods frequently in the spring following winter snow melt
and the first rains of spring. Experts say this is because of the greatly
increased development near the river in northern Illinois, which has left
areas that were previously fields and forests covered with impervious sur-
faces including asphalt, concrete, and houses. Previously, open ground had
absorbed most of the rain and melting snow, only slowly releasing it into the
nearby Des Plaines River. Now much of the melting snow and rain flows
through culverts and pipes almost directly into the river, vastly increasing the
flow of water that very quickly enters the river, increasing the volume of
water in the river, and causing the river to go over its banks and flood lower
lying areas, even those areas that previously hadn't seen flooding for many
years.
The primary purpose for the geographic presentation is to conceptual-
ize the necessary data to help answer the questions of when and where the
river can flood. A very simple model may only consider observations of river
bank elevation and water height. With observations of these characteristics,
we can create geographic information that is the basis for analyzing where
the river floods. To make sure that the observations can be related to one
another, special attention to the measurements must be given. First, we need
to consider the relationship between water height and river bank elevation.
For the entire length of the river, we need to have a defined value that indi-
cates the level of water that leads to a flood. This value isn't simply zero
because the river banks become lower as the river flows away from its higher
source to its mouth on the Chicago River. The water height and river banks
need to be modeled in a relationship that also remains valid if there are
changes along the river bank—for example, building a dike of sandbags. An
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