User's Guide and Reference

Data types for single-unit features

Use ST_Point, ST_Linestring, and ST_Polygon to store coordinates that define the space occupied by features that can be perceived as forming a single unit:

• Use ST_Point when you want to indicate the point in space that is occupied by a discrete geographic feature. The feature might be a very small one, such as a water well; a very large one, such as a city; or one of intermediate size, such as a building complex or park. In each case, the point in space can be located at the intersection of an east-west coordinate line (for example, a parallel) and a north-south coordinate line (for example, a meridian). An ST_Point data item includes values--an X coordinate and a Y coordinate--that define such an intersection. The X coordinate indicates where the intersection lies on the east-west line; the Y coordinate indicates where the intersection lies on the north-south line.
• Use ST_Linestring for coordinates that define the space that is occupied by linear features; for example, streets, canals, and pipelines.
• Use ST_Polygon when you want to indicate the extent of space covered by a multi-sided feature; for example, a welfare district, a forest, or a wildlife habitat. An ST_Polygon data item consists of the coordinates that define the perimeter of such a feature.

  In some cases, ST_Polygon and ST_Point can be used for the same feature. For example, suppose that you need spatial information about several apartment complexes. If you want to represent the point in space where each complex is located, you would use ST_Point to store the X and Y coordinates that define each such point. On the other hand, if you want to represent the area that each complex covers, you would use ST_Polygon to store the coordinates that define the perimeter of each such area.