Geography Reference
In-Depth Information
would be traveled by simply following the two-dimensional line. Once data are in a GIS, such
distance calculations are trivial—though they are still an approximation. So the process of finding
the distance—one of the simplest answers you might want from the map—is not simple, and is
guaranteed to be imprecise.
If you are unimpressed with the difficulty of analyzing distances on a map, let's move to a harder
problem. You have a map of a county that has several parks. Suppose your map shows parkland
as green areas. You would like to know the area—in acres, square miles, or square kilometers—that
the parks occupy. How do you determine that? You might use the linear scale of the map to make a
two-dimensional grid of squares on some transparent material, lay this over the parkland polygons,
count squares, and do some arithmetic to estimate the area. Or you could divide each polygon up
into triangles and calculate the area of each triangle.
1
Or you might obtain a remarkable device
called a planimeter—a gadget that is made to measure the area of a graphically represented planar
region—and run its stylus around the boundary to get an approximate value. (You'll find the
planimeter in a museum, next to the slide rule, which is next to the abacus.) Or you could paste the
map down on a thin sheet of aluminum, use tin snips to cut out the green areas, and compare the
weight of all the cutouts to that of a known area of the aluminum sheet. Again, none of these pro-
cesses is easy, or particularly accurate. The point is this: Maps are hard to analyze. In a GIS, once the
data are in the computer, an excellent approximation of the area is a quantity that you get for free.
❏
It is difficult to compare maps. If I didn't convince you how difficult it is to analyze a single map,
look at the issue of comparing maps, or analyzing multiple maps to get information from the
combination of them. Suppose a municipality wants to build an airport to serve its region and you
are to advise the government on how to find adequate locations. What factors must be considered
regarding the several hundred square kilometers around the municipality? Here is a partial list:
❏
Topography (of the potential site and of the surrounding area)
❏
Geology
❏
Environmentally sensitive areas
❏
Soil characteristics
❏
Land cost and land availability
❏
Access to ground transportation facilities
❏
Weather patterns (e.g., tendency for fog to occur)
❏
Obstructions in the airspace (e.g., towers, wires)
❏
Existing land use
❏
Surrounding structures and their heights
❏
Habitat of endangered species
❏
Proximity to populated areas
❏
and several more.
1
Knowing the three sides of a triangle, you can calculate the area as the square root of
s(s-a)(s-b)(s-c)
, where
a
,
b
, and
c
are the lengths of the sides and
s
is the semi-perimeter:
(a+b+c)/2
. I mention this because it seems now to be lost
knowledge as far as high-school geometry teaching is concerned.
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