Geography Reference
In-Depth Information
Thus, an approach that seems basically less appropriate to the task does in fact serve us well—especially
since high-quality two-dimensional maps, that is, analogs of the landscape with their innate advantages
in conveying information—can now be produced by computers, at desired scales.
Moving Spatial Data from Maps to Computers:
Forces for Change
Force for Change #1: There Are Difficulties and
Limitations with Using Maps for Decision Making
Maps can depict things beautifully and usefully, so for many applications, a paper map is exactly what is
needed. But for many purposes maps are hard to use, for these reasons:
A map is a compromise between a storage function and a display function. As more and more
information is stored on a map, it becomes more cluttered. At some point, it becomes unreadable.
A map that stored every theme of interest to everyone would be black. Aeronautical charts are a
good example of this problem. The aeronautical charts of the 1950s were pretty simple affairs, showing
terrain, prominent features, and some airport information. As new regulations came into effect, and
new communication facilities were established (whose radio frequencies were placed on the map), as
new types of official airspaces were defined, as new military training grounds were introduced, the
map had to depict more and more. As a consequence, without careful study (not an activity that can
easily take place in the cockpit of an airplane, where the primary activity should be piloting), it is easy
to misread such a cluttered map. In a GIS, the storage function and the display function are separated.
When display is required, a map can be constructed of only those themes wanted by the user.
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It is difficult to analyze a map. Consider a map that shows highways. Suppose you are interested in
knowing the distance from city A to city B. What is meant by “distance”? How about straight-line
distance from city center to city center? Obtaining an approximation of a straight-line distance isn't
too hard. The map has a scale indicating that a certain linear distance on the map corresponds to a
certain number of miles or kilometers along an idealized “Earth” with no bumps. You only need a way
to measure a distance on the map and compare the measurement to the map's scale. Perform a little
arithmetic and you're done. But since the map is a projection of the spherical Earth, the distance won't
be exact. The scale varies over the map's surface, so the scale applies exactly only in very few places on
the map. Another reason the straight-line distance is not exact, even if you could follow the straight-line
distance over the surface of the Earth, you would probably encounter hills, which add to the distance.
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Let's make the problem harder: You want distance from “A” to “B” along a highway route.
Depending on the type of map, you may get some help in this analysis. If you are looking at an oil
company map or an automobile association map, it might have one of those triangular matrices
that indicates distances between selected cities. Here, part of the analysis has been done for
you—provided your origin and destination are represented on the chart. Another approach could be
used if the map has numbers printed beside segments of the road that you can add up to get the total
distance. Again, some of the analysis has been done for you, but you still have a bit of work to do.
If the map doesn't have these features, then you could use the scale of the map to approximate dis-
tances along the route. Based on what the graphics of the map show, you could determine the sum
of all those curvy road segments. The more the road curves, the more arduous the task. Also, even
when you can sum the segments up, you have to remember that the road curves in three dimensions.
Going up and down hills adds miles to the distance a car travels, over and above the distance that
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