The nature of spatial data

Spatial data is made up of coordinates that identify a location. Spatial Extender works with two-dimensional coordinates specified by x and y or longitude and latitude values.

For example, latitude is a coordinate that denotes an angle relative to the equatorial plane, usually in degrees. Longitude is a coordinate that denotes an angle relative to the Greenwich meridian, also usually in degrees. Thus, on a map, the position of Yellowstone National Park is defined by latitude 44.45 degrees north of the equator and longitude 110.40 degrees west of the Greenwich meridian. More precisely, these coordinates reference the center of Yellowstone National Park in the USA.

The definitions of latitude and longitude, their points, lines, and planes of reference, units of measure, and other associated parameters are referred to collectively as a coordinate system. Coordinate systems can be based on values other than latitude and longitude. These coordinate systems have their own points, lines, and planes of reference, units of measure, and additional associated parameters (such as the projection transformation).

The simplest spatial data item consists of a single coordinate pair that defines the position of a single geographic location. A more extensive spatial data item consists of several coordinates that define a linear path that a road or river might form. A third kind consists of coordinates that define the boundary of an area; for example, the boundary of a land parcel or flood plain.

Each spatial data item is an instance of a spatial data type. The data type for coordinates that mark a single location is ST_Point; the data type for coordinates that define a linear path is ST_LineString; and the data type for coordinates that define the boundary of an area is ST_Polygon. These types, together with the other spatial data types, are structured types that belong to a single hierarchy.