Spatial Thinking in Planning Practice: An Introduction to GIS

8 Figure 1.6. Each feature in the layers above has a spatial location and attribute data, which describes the indi- vidual feature. GIS Commons. http://giscommons.org/introduction-concepts/ Besides location, each feature usually has a set of descriptive attributes, which characterize the individual fea- ture. Each attribute takes the form of numbers or text (characters), and these values can be qualitative (i.e. low, medium, or high income) or quantitative (actual measurements). Sometimes, features may also have a temporal dimension; a period in which the feature’s spatial or attribute data may change. As an example of a feature class, think of a streetlight. Now imagine a map with the locations of all the streetlights in your neighborhood. In Figure 1.5, streetlights most are depicted as small circles. Now think of all of the di#erent characteristics that you could collect relating to each streetlight. It could be a long list. Streetlight attributes could include height, mate- rial, basement material, presence of a light globe, globe material, color of pole, style, wattage and lumens of bulb, bulb type, bulb color, date of installation, maintenance report, and many others. "e necessary streetlight attri- butes depends on how you intend to use them. For example, if you are solely interested in knowing the location of streetlights for personal safety reasons, you need to know location, pole heights, and bulb strength. On the other hand, if you are interested in historic preservation, you are concerned with the streetlight’s location, style, and color. Now continue thinking about feature attributes, by imagining the trees planted around your campus or of- !ce. What attributes would a gardener want versus a botanist? "ere would be di#erences because they have di#erent needs. You determine your study’s features and the attributes that de!ne the features. ATTRIBUTE DATA TABLE Once you have decided on the features and their attributes, determine how they will be coded in the GIS data- base. "ere are multiple ways to code features in di#erent scale and circumstance. For example, schools can be coded as a point in large scale maps, and a polygon of their campus in small scale maps. You can decide whether to code each feature type as a point, line, or polygon. Together you also need to de!ne the format and storage requirements for each of the feature’s attributes. While thinking about your attribute values, consider where it !ts in the “levels of measurement” scale with its four di#erent data values: nominal, ordinal, interval, and ratio. Stanley S. Stevens, an American psychologist, developed these categories in 1946. For our purposes, these categories are useful way to conceptualize how data values di#er, and it is an important reminder that only some types of variables can be used for certain mathe- matical operations and statistical tests, including many GIS functions. "e di#erent “levels” are depicted in the Chapter 1: De!ning a Geographic Information System