Geography Reference
In-Depth Information
ACCURACY AND PRECISION
The data collection procedures, tools, and techniques should assure the
highest level of fidelity to the geographic representation and the carto-
graphic representation. If the accuracy of the map materials is known, you
have a great assistance in knowing how accurate the GI is. If not, it becomes
complex. A rule of thumb is to always be more cautious than necessary when
determining accuracy.
Of course, you have to be sure not to mix up accuracy and precision.
“Accuracy” refers to the agreement between the GI or map position and the
ground position, whereas “precision” refers to the number of digits used to
indicate the position. High precision is meaningless without corresponding
accuracy.
CHOICE OF POSITIONAL COLLECTION TECHNOLOGY AND APPROACH
The purpose of and means available for data collection largely will deter-
mine the collection technology. The most important additional factor here is
often cost. If the data collection has to choose between two methods, gener-
ally the lower-cost option will win. The exception would be if the higher-cost
option offers additional information, accuracy, or reliability. Of course, the
lowest cost option can easily end up being the most costly in the end. Far too
often, people collect GI without thinking through the geographic represen-
tation and cartographic representation.
CLOSURE OF AREAS AND CONNECTIONS BETWEEN LINES
A complicated issue for digitization is making sure that an area is closed
(e.g., a county, state, or country) or a line is connected (e.g., a highway, bus
route, or subway). This can be remedied by using a tolerance that moves digi-
tized points together if they fall within a specified distance of each other.
The tolerance is usually based on the accuracy of the GI. This can be diffi-
cult because the proximity of features changes across a map. Buildings are
closer together in urban areas, some areas have long and narrow fields, oth-
ers have very large rectangular fields. The tolerance for connecting points
when digitizing needs to be adjusted to the circumstances.
GENERALIZATION EFFECTS
When digitizing maps, you need to bear in mind that generalization opera-
tions may have moved features on the map to make the map easier to read.
This is common in small-scale maps, but also occurs in large-scale maps. If it
is impossible to find out how features have been generalized, you can at least
use the indicated accuracy of the map as an indicator of how much an indi-
vidual feature could have been moved.
In examining positional collection technology options, you also need to
consider remote sensing, discussed in Chapter 8.
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