Spatial Thinking in Planning Practice: An Introduction to GIS

7 grass, and grain of sand. How do we reduce the complexity of the Earth and its inhabitants, so we can portray them in a GIS database and on a map? We do it by selecting the most relevant features (ignoring those we do not think are necessary for our speci!c research or project) and then generalizing the features we have selected. "e image above shows the real world is selectively represented by di#erent features that we are interested in. "ey are also called map layers in GIS. FEATURES AND FEATURE CLASS In ArcGIS, map layers are also called shape !les, or feature classes. Conceptually, there are two parts of a shape !le: a spatial or map component and an attribute or database component. Features have these two components as well. "ey are represented spatially on the map and their attributes, describing the features, are found in a data !le. "ese two parts are linked. In other words, each map feature is linked to a record in a data !le that describes the feature. If you delete the feature’s attributes in the data !le, the feature disappears on the map. Conversely, if you delete the feature from the map, its attributes will disappear too. Figure 1.5. Spatial and attribute data. GIS Commons. http://giscommons.org/introduction-concepts/ Features are individual objects and events that are located (present, past or future) in space. In the above Fig- ure, a single parcel is an example of a feature. Within the GIS industry, features have many synonyms including objects, events, activities, forms, observations, entities, and facilities. Combined with other features of the same type (like all of the parcels in Figure), they are arranged in data !les o$en called layers, coverages, or themes. In the Figure below, three features—parcels, buildings, and street centerlines—of a typical city block are visi- ble. Every feature has a spatial location and a set of attributes. Its spatial location describes not only its location but its extent. Chapter 1: De!ning a Geographic Information System