Geography Reference
In-Depth Information
point is 1023 feet and a checker with an altimeter set on the spot finds 999 feet, what happened? Was the
problem in measuring altitude? Are the positional coordinates off? Is there some systematic or random
error in the encoding process or equipment?
It is vital to understand that all large databases contain errors. If a variable in a database is a
continuous quantity, such as elevation, there will be values outside the established accuracy
standards. If the base is one of classifications, such as land use activity, some uses will be misclassified.
If the base is geographically referenced, there will be disagreements of actual locations between points
of the Earth's surface and where the system has them located. The purpose of validating data is to
develop an understanding of how great these error rates are and, if they are too great, take steps to
reduce them.
7. Repeat Steps 4, 5, and 6 for the complete set of data for the base.
Realize, however, that the database will be a growing, evolving entity, supplying useful information to
decision makers for years to come. These steps are simply the first ones. By the time the pilot data have
been satisfactorily collected or acquired, encoded, and validated, the database developers should have
a firm grasp on the associated problems, costs, and techniques. Serious thought should be given to the
differences between the pilot project and the major data-gathering effort.
There should be no letup in the testing of data as they come in and encoding. And if you really want to
put your data collection techniques on the line, recollect some data from the pilot area and compare it
against that originally collected. The results of that may be very instructive, if disheartening.
One of the worst losses at this point can be of key personnel, now much more valuable than when the
pilot data collection began. Their importance to the success of the system should be recognized and
appropriately compensated, if possible.
8. Employ techniques for monitoring and updating the database.
Even as the major database collection effort is going on, the world will be changing, and new problems
will be appearing; updating must be a constant activity to keep the database reasonably current.
GPS and GIS
In Chapter 1 you learned a bit about the NAVSTAR Global Positioning System. Let's explore some of the
main reasons for making GPS a primary source of data for GIS.
Availability
—in 1995, the U.S. Department of Defense (DoD) declared NAVSTAR to have “final
operational capability.” Deciphered, this means that the DoD has committed itself to maintain
NAVSTAR's capability for civilians at a level specified by law, for the foreseeable future, at least
in times of peace. Therefore, those with GPS receivers may locate their positions anywhere on the
Earth.
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Accuracy
—GPS allows the user to know position information easily and with remarkable
accuracy. A receiver operating by itself can let you locate yourself within 2 to 4 meters of your
true position. (And using two GPS receivers, when one is positioned over a known (accurately
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