Geography Reference
In-Depth Information
A Multiplicity of “Storadigms”
ArcGIS supports, or at least recognizes, several different spatial storage paradigms:
1. File and Personal Geodatabases
4
2. Shapefiles
3. Coverages
4. Computer-aided design (CAD) files
5. Vector Product Format (VPF) datasets
6. Raster (GRID) datasets
7. Triangular irregular network (TIN) datasets
8. Terrain datasets
Numbers 2, 3, 4, and 5 of these are based on the concept of the vector. We will examine and work
with the first two extensively. Number 6 is the raster format already briefly discussed and covered in
detail in Chapter 8. Number 7, as briefly discussed earlier, is a technique for storing data where there
is an independent, continuous variable (e.g., elevation) whose values are based on the two dependent
variables
x
and
y,
frequently longitude and latitude. That is, a TIN is used to represent a surface. Terrains
are based on TINs covering areas with closely spaced data points.
Ideally, there would be only a single storage paradigm. We would store all spatial data in this way, and when
we made a query of the database, or asked for a map of a given area and scale, it would be provided. One
problem with this approach is that different sorts of data—representing different aspects of the
environment—have distinctly different characteristics. Pick a point on Earth's surface. It has elevation. If there
is soil there, it has physical characteristics. Someone or some entity probably owns it. At a certain moment
it has a certain temperature, and over a year period it has an average temperature. It may be suitable for
growing some kinds of crops, but not others. It has a particular slope and aspect. The vertical distance down to
bedrock has a certain value, and that bedrock is of a certain type. There may be a volume of coal or oil under
the surface. The atmosphere above it contains some pollutants. We want to represent these facts with datasets.
Some of this information is more easily and effectively stored in one paradigm or format, other
information in a different format. Further, a lot of inventing and creating has gone into the problem of
representing the infinite, continuous environment as numbers, letters, and symbols. So we wind up with
a lot of different paradigms for storing spatial data, or, to coin a term, “storadigms.” One advantage of
using Esri software is that you may easily convert from one method of storing data to another.
Vector-Based Geographic
5
Datasets—Logical Construction
Geodatabases and coverages are the two most sophisticated data structures used by Esri software. They
are sophisticated in different ways, as you will see later. The coverage concept dates back many years
and was the foundation of ArcInfo-lines that represented linear features and that separated polygons
4
Actually, geodatabases can include, or will in the future, other paradigms in this list.
5
Or, should you hail from the United Kingdom: Geographical instead of Geographic.
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