Geoscience Reference
In-Depth Information
established data points? To look more deeply at this topic, follow some of
the links at the end of this chapter or read references cited at the end of the
book.
Set theory terminology also has linkages with standard English language
terminology. It is important to note what these are because mapping soft-
ware uses the mathematically assigned, set-theoretic, meanings for common
words. The word “intersection” corresponds to the word “and.” That associa-
tion appears straightforward to most. The word “complement” corresponds to
the word “not.” That too appears straightforward. Where there is difficulty for
some is in the translation of the word “union” (sometimes called “join”). The
word “union” or “join” corresponds to “or,” where “or” means “either one or
both.” This form of “or” is said to be “inclusive.” In some forms of common
usage, “or” may mean “either one or the other but not both.” That form of
“or” is said to be “exclusive.” In set-theoretic usage, and therefore in mapping
usage, the inclusive “or” is the one that is used. This fact is important because
some of the wizards that offer choices for spatial analysis assume that the user
knows about the use of “or” (or “union” or “join”) in an inclusive context. If
one does not know, then it can be quite difficult to figure out why the results,
of what appeared to be otherwise correct procedure, have become skewed.
Remember—mapping software is built on mathematics!
3.4 Raster and vector mapping: Know your file formats
A Geographic Information System (GIS) is designed to present all types of
geographical data. It rests on base maps and an underlying table of informa-
tion about the points, lines, and areas within the base maps. This table is
referred to as an “attribute” table. GIS technology elevates the art and science
of mapping from a static one to a dynamic one. In GIS, a change in the data-
base produces a corresponding change on the map. Similarly, a change on the
map produces a corresponding change on the database.
Gone are the days when a change in a place name necessitated redrawing
the map! When maps are made on paper, they are made in a static mode.
If an error is made while one is using India ink to draw a country outline,
a serious problem arises. In times past, we pulled out a razor blade to care-
fully scrape the ink off the surface of our expensive high-rag-content paper.
Because part of the finish of the paper also got removed, thereby causing ink
to smear when reapplied to the scraped spot, we restored the finish using a
special restorative powder. Blowing the powder off could result in the intro-
duction of water, causing yet another set of problems. So, we used nice camel
hair brushes that did not shed to remove the excess powder. Finally, we could
consider inking the spot again, perhaps with extra care so as not to create
yet another error on the newly “fixed” area. During the era where maps were
etched with scribing needles onto pieces of film, that were later exposed to
create maps, paint was applied to the erroneous line, allowed to dry, and the
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