Geography Reference
In-Depth Information
ideal choice is a system for recording elevation independent of water height
and riverbank elevation. Both measurements can be related to each other
and will show that if the water height is higher than the river bank elevation,
a flood results; additional data can also be added later. The elevation refer-
ence needs to be explicitly defined, something a regional, state, or national
mapping agency or geodesy agency generally provides and keeps current.
And what about the question of where the river floods? The geographic
representation could split the river into segments, the smaller the better, and
for each segment record the average water height and river bank elevation.
Using smaller segments is better because that will allow us to more accu-
rately say where the flooding will occur, but this will require more data—
which adds to the project's costs. Additionally, moving on to an issue for car-
tographic representation, segments that are too small will make it difficult to
show where flooding occurs over the 250-mile length of the river and its trib-
utaries on a small screen or paper because the small segments would be hard
to visually distinguish. Before going on to consider more specifically how
measurements and observations are geographically represented, please note
that this example leaves out a number of important details—most impor-
tantly, the measurements and observations involved in determining the
capacity of the river.
Measurement, Observations, and Relationships
We have at this point a simple model to indicate river f looding that consid-
ers the relationship between water height and river bank elevation. Each of
the elevation measurements is related to an independent elevation refer-
ence. The river segments are related to an independent coordinate system,
which can be used to assure that all observations are recorded at the same
location. To place the measurements into a geographic representation used
for geographic information we finally need to create attributes in a database
that records the values for each river segment. The observations (field or cal-
culated data) can then be stored for each segment. These values can then be
compared to determine if and where the river f loods for a particular water
height.
The determination of flooding is based on a relationship: if the water
height is greater than the river bank elevation, flooding results. This can be
modeled mathematically as
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. This relationship will never be stored in the
geographic representation; we have to calculate it using the recorded charac-
teristics. However, the stored geographic representations were only deter-
mined based on an understanding (however simplistic) of the relationship
between water height and river bank elevation. The relationship is central
for understanding flooding, but the information recorded in the geographic
representation lacks this relationship at first—it must be determined and
recorded as data in another step using the measurements.
In the end, measurements, observations, and relationships are all parts
of geographic representations, but relationships are usually separate from
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