Geography Reference
In-Depth Information
CHAPTER
4
OVERVIEW
IN WHICH you explore the ways geo-
graphic datasets are stored in the
memory and on the disk drives of a
computer. You also learn the rudi-
ments of using ArcToolbox.
Structures for Storing
Geographic Data
Why Is Spatial Data Analysis
So Hard?
Spatial (that is, geographic) datasets are notoriously difficult to
analyze. In other fields of human endeavor, most of the datasets
one wants to analyze are naturally made up of numbers. What
is the history of the stock market's up and downs? Numbers.
What are the statistics relating to the grades of students in
the sophomore class? Numbers. How many parts-per-million
carbon monoxide molecules may be safely tolerated by
different types of air-breathing animals? Numbers. But the chief
way of
storing spatial data
for most of human history has been
the map—whether paper, Mylar, or computer image.
Numbers and text are composed of nicely behaved discrete
symbols. Each symbol may be represented by a bit of ink or
by a few pixels on a computer screen that fit neatly into a
square roughly an eighth of an inch on a side. And, in English,
there aren't very many different symbols: 10 digits, 26 letters
uppercase, another 26 lowercase, and a bunch of special
symbols—in total a maximum of 256. Maps use symbols
also, but they are not nearly so well behaved. For example,
symbolizing a road may result in a wavy line 2 feet long.
As discussed in Chapter 2, maps are difficult to analyze, and it
is hard to compare maps. Also, the map has been the primary
way of
both storing and displaying
spatial data—an idea we
discussed earlier. One of the major advantages of a
computer-based GIS is that we separate the storage function
from the display function.
A physical method of comparing maps involves a set of,
initially, clear plastic sheets, one for each theme in the study
area. Each map is darkened in certain areas to indicate the lack
of suitability of that theme (for some activity or structure) in
the location. A completely clear area of the map might mean
Search WWH ::
Custom Search