How features, spatial information, spatial data, and geometries fit together

Spatial functions let you obtain facts and figures that pertain to things that can be defined geographically, that is, in terms of their location on earth, or within a region of the earth. Such facts and figures are referenced as spatial information, and such things as geographic features (called features here, for short).

For example, you could determine whether any populated areas overlap the proposed site for a landfill. The populated areas and the proposed site are features. A finding as to whether any overlap exists would be an example of spatial information. If overlap is found to exist, the extent of it would also be an example of spatial information.

To produce spatial information, the database must process data that defines the locations of features. Such data, called spatial data, consists of coordinates that reference the locations on a map or similar projection. For example, to determine whether one feature overlaps another, the spatial function must determine where the coordinates of one of the features are situated with respect to the coordinates of the other.

In the world of spatial information technology, it is common to think of features as being represented by symbols called geometries. Geometries are partly visual and partly mathematical. Consider their visual aspect. The symbol for a feature that has width and breadth, such as a park or town, is a multisided figure. Such a geometry is called a polygon. The symbol for a linear feature, such as a river or a road, is a line. Such a geometry is called a linestring.

A geometry has properties that correspond to facts about the feature that it represents. Most of these properties can be expressed mathematically. For example, the coordinates for a feature collectively constitute one of the properties of the feature's corresponding geometry. Another property, called dimension, is a numeric value that indicates whether a feature has length or breadth.

Spatial data and certain spatial information can be viewed in terms of geometries. Consider the example, described earlier, of the populated areas and the proposed landfill site. The spatial data for the populated areas includes coordinates stored in a column of a table in a database. The convention is to regard what is stored not simply as data, but as actual geometries. Because populated areas have width and breadth, you can see that these geometries are polygons.