Geography Reference
In-Depth Information
Spatial data, then, are discrete symbols (numbers, letters, or special characters) used to describe some
entity; these data are organized according to the location of that entity in the three-dimensional world.
They are data that pertain to the space occupied by objects. It includes cities, rivers, roads, states, crop
coverages, mountain ranges, and so on.
Normally, when it is desirable to describe things in the real world by spatial data, the objects are
abstracted into some geometrical or mathematical form, as discussed in Chapter 1. For example, a fire
tower might be represented by a point, a stream by a set of connected straight lines, and a lake by a
polygon boundary.
Limiting the Scope
Spatial data (again, facts about the real world organized by locational coordinates) can be used to
describe molecular structures, a human central nervous system, positions of topics in a library, or stars
in the universe. Since this topic relates to geography, we now exclude several categories of spatial data.
Usually not considered are data that relate to:
Conditions that change quickly in time—in matter of hours, days, or even weeks. Current pollution
levels, weather, and ties will not be included, although average pollution at a point, climate, and
ranges of times could be included. Exceptions to this rule include using sensors to collect immediate
data about conditions and put those data out to Internet sites for display on the Web.
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Objects that move about in space—such as automobiles, animals, or people. However, data about
flows of these objects past a certain point at a certain time might well be included. Exceptions to this
rule include using GPS to keep up with trucks and cars, or to track animals in the wild.
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Circumstances in which the locational identifiers must be more precise than 1 decimeter (a tenth
of a meter) to ensure that the related data are useful or valid. The smallest distance separating two
adjacent entities that can be distinguished from one another is called the
resolution distance
, or simply,
resolution. If the separation in distance is less than the resolution, the data cannot be used to resolve
any differences in the condition or situation. Again, there are exceptions: Surveyors are making
increasing use of GIS, and they make measurement to within a centimeter.
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Spatial Data for Decision Making
We have said spatial data relate to conditions, facts, and objects in three-dimensional space. Most spatial
data that now exist use a two-dimensional referencing scheme such as latitude/longitude (or projections
thereof) or street addresses. Unless the dataset is specifically one that considers the matter of altitude,
this third coordinate is either included as part of the attribute data (rather than part of the locational
identifier) or is implied by the nature of the data. For example, if the data describe soil characteristics,
one understands that the top few feet of the Earth's crust, regardless of altitude, are being described.
Data types that might be part of spatial datasets are exemplified in Table 2-1.
Sets of Spatial Data: The Database
We have discussed databases in general (a medium containing numbers, symbols, or graphics organized
according to some scheme). And we have commented on the idea of spatial data (data describing entities
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