urning Data into Information Using ArcGIS 10
In order to build a representation of any part of it, you must make choices about what to represent, at what level of detail, and over what time period.
What are geographic data?
Geographic data link place, time, and attributes.
Place, or location, is essential in a geographic information system. Locations are the basis for many of the benefits of geographic information systems: the ability to map, to link different kinds of information because they refer to the same place, and to measure distances and areas. Without locations, data are said to be "aspatial" and have no value at all within a geographic information system.
Time is an optional element. Many aspects of the earth's surface are slow to change and can be thought of as unchanging. Height above sea level changes slowly because of erosion and movements of the earth's crust, but these processes operate on scales of hundreds or thousands of years, and for most applications (except geophysics) we can safely omit time from the representation of elevation. On the other hand, atmospheric temperature changes daily, and dramatic changes sometimes occur in minutes with the passage of a cold front or thunderstorm, so time is distinctly important.
Attributes refer to descriptive information. The range of attributes in geographic information is vast. Some attributes are physical or environmental in nature (such as atmospheric temperature or elevation), while others are social or economic (such as population or income). There are five main types of attributes: nominal, ordinal, interval, ratio, and cyclic:
The simplest type of attribute, called nominal, is one that identifies or distinguishes one entity from another. Place names are a good example, as are addresses of houses and the numbers on a driver's license—each serves only to distinguish the incidence of one particular class of entities from all other classes. Nominal attributes include numbers, letters, and even colors. Even though a nominal attribute can be numeric, it makes no sense to apply arithmetic operations to it: adding two nominal attributes, such as two driver's license numbers, is meaningless.
Attributes are ordinal if their values have a logical order. For example, Canada rates its agricultural land by classes of soil quality, with Class 1 being the best, Class 2 not so good, etc. Adding such numbers makes little sense, because 2 is not twice as much of anything as 1. Ordinal attributes follow a clearly defined sequence, even if the spacing between successive attributes is unknown or unknowable. Averaging makes no sense either, but the median (the value such that half the attributes are higher-ranked and half are lower-ranked) is an effective substitute for the average for ordinal data as it gives a useful central value.
Attributes are interval if they measure the magnitude of difference between one value and another, but not between a value and a true zero point. The scale of Celsius temperature is interval, because 30 and 20 have the same magnitude of difference as 20 and 10. Addition and subtraction are valid between interval values, but multiplication and division are not.
Attributes are ratio if they measure the magnitude of difference between a value and a true zero point. Weight is ratio, because 100 kilograms is a measure of difference from 0 kilograms. Addition, subtraction, multiplication, and division are all valid between ratio values. For example, it makes sense to say that a person of 100 kilograms is twice as heavy as a person of 50 kilograms. Celsius temperature is only interval, because 20 is not twice as hot as 10 (and this argument applies to all scales that are based on similarly arbitrary zero points, including longitude).
How are geographic data represented?
Digital representations of geography hold...
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