Over the past 100 years or so, cartographers have adopted and tested conventions concerning symbol color, size, and shape for thematic maps. Abstract graphic symbols must be devised that depict, with minimum ambiguity, the quantities and qualities that give locations their meaning. Representing the attributes of locations on maps is sometimes not so straightforward, however. Notwithstanding the intricacies of map projections, it is a fairly straightforward matter to plot points that stand for locations on the globe. Thematic maps are among the most common forms of geographic information produced by GIS.Ī flat sheet of paper is an imperfect but useful analog for geographic space. ![]() Maps created specifically to highlight the distribution of a particular phenomenon or theme are called thematic maps. Many maps, however, are subsets of available geographic data that have been selected and organized in response to a particular question. All maps, but especially so-called reference maps made to support a variety of uses, can be defined as sets of symbols that represent the locations and attributes of entities measured at certain times. Maps are both the raw material and the product of GIS. In this chapter we’ll take a closer look at how attributes are measured and represented. I mentioned, but did not explain, that attribute data also are specified with reference to measurement scales. Coordinate systems, remember, are formed by juxtaposing two or more spatial measurement scales. ![]() In Chapter 2 we compared the characteristics of geographic and plane coordinate systems that are used to measure and specify positions on the Earth’s surface.
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