In this month’s edition of WOM, we going to get down to the “nitty-griddy” on feature class. A feature is a representation of a single entry on a map. For example, a line could represent a road, elevation or a physical object like a river. One can tell what that line represents by the attribute information that is attached to its features. A feature consists of a unique identifier, one or more lists of coordinates, orientation information and a label. Likewise within ArcGIS, a feature class is defined as a collection of geographic features that contain the same geometry type, the same attributes, as well as the same spatial reference. For example a river could be featured as a line on a map. Points representing cities along that river could be used to create a feature class. Or on that same river line, additional line segments could represent roads, latitude/longitude, or elevations. Areas, or polygons, could be associated to that river representing town or property lines. The map could have text features or annotations that describe the visual spatial properties like roads. So now let’s jump into the fine details of feature classes within a geodatabase: points, lines, polygons, and annotation.
Have a GPS coordinate? Then you have a starting point. Points have the least amount of information of any file type, but they all to relate to something else or it would just be dots on a map. So with a spatial reference point, a scale is created with respect to the visual points at their respected coordinates. (If you are a diehard mathematician this would describe points x, y with an associated magnitude).
How are points used? Well a collection of points can classify areas displayed at a small scale, or very zoomed in. If one was a dedicated geocacher they could put in the points of a geocache and have a larger spatial distance but for the majority of the collected points a smaller scale would work best. However you can’t measure points alone, so we have to take advantage of lines and polygons.
Lines or polylines are commonly known and used to describe linear features such as rivers, roads, railroads, etc. They are similar to points in the sense that they have length and direction, however they can measure distance. My favorite math teacher would describe a line as the shortest distance between two points. Likewise the distance in a polyline is additive with respect to the order of each of the shortest distance between the last point and the next point. This is used to approximate the distance of a curve composed of several points, or in a simplest use measure the distance that the archaeologist walks to each of his points. Again, polylines are most useful on a small scale.