Geography Reference
In-Depth Information
Several ways of representing elevation in GIS have been used. Three are as follows:
Contours
❏
Digital elevation models
❏
Triangulated irregular networks
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These will get more detailed treatment later, but let's look at their essential characteristics.
Contours are familiar to you from you experience with topographic maps. Each contour line represents
a given elevation. That is, if you walked along the path depicted by the contour line, your elevation
would not change. Recalling that a GIS is a marriage of geographic database and an attribute database,
you see that the geographic points along the path form the geographic part of this partnership, while the
elevation of the line is the attribute datum. To obtain an estimate of the elevation at a point between two
contour lines interpolation might be used.
Digital elevation models (DEMs) rely on the idea of a raster. A
raster
is a set of equal-sized squares,
arranged in rows and columns, which cover (tessellate, if you want a highbrow word) the plane. Think of
a chessboard—or square tiles on a kitchen floor. The geographic position of each square (e.g., its center)
can be calculated by its row and column number. Each square has an attribute that might be its average
elevation (or, if the DEM were being constructed for aircraft pilots, it would be better if the attribute were
the maximum elevation!) The reported elevation is, therefore, constant within a given square and usually
changes at each edge of each square, making for a rather lumpy representation. Obviously, DEMs with
more, and smaller, squares potentially represent the surface better. Of course, the value obtained from a
DEM for a given position is almost guaranteed to be somewhat incorrect.
Triangular irregular networks (TINs) represent the surface of a geographic area by a set of triangles
whose vertices are points of known elevation. Since three points determine a plane, the computer can
come up with an estimated value of elevation for any requested point among those points. Of course, the
Earth's surface is not made up of triangles, and poor selection of the points of known elevation
(e.g., midway up a hill rather than at its top) could dramatically, negatively affect the accuracy of the TIN.
Again, the more known points, the more triangles, and the (potentially) better representation of the true
environment. The use of LIDAR from aircraft sensors can result in elevations of points that are perhaps
only two meters apart horizontally. So high accuracy is possible.
Information Systems
This issue of “processing” data in a map form (analog) versus symbol form (digital) brings us to the
matter of processing or handling data in general. The conceptual model we will use is that data are
processed to produce information. Actually, the terms are not absolute, because what is information
to a person filling one role may be data to someone else filling another. However, the idea of a
before-after concept, distinguishing the two states as data and information, turns out to be useful,
so we employ it.
4
4
The idea that data precedes information might occur in the following context sequence: Existence, Awareness,
Observation, Measurement,
Data
,
Information
, Knowledge Understanding, Wisdom.
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