5 Storing Geography within a GIS
Image 13: The ring is defined by a linear sequence of points. The order of the points in the sequence matters. If the start and end point in the sequence are the same then the line is said to form a ring. Here is the sequence of points that define the ring which in turn defines the polygon of our Output Area:
Image 14: Point: Each point within the ring consists of a pair of coordinates. These coordinates describe the location of the point relative to some origin within a spatial reference system such as the British National Grid. Here is a zoomed in view of some of the points that make up our point sequence:
Image 16: Here we see that the order of the points that define the ring goes clockwise. The interior of the polygon is the area shaded in pink. Below we see the start and end points present in the feature.
A point describes the offset from the origin of a coordinate system. A sequence of points can be built up to form a line. Where the start and end points in a line are the same the line forms a ring. A ring can be used to form a polygon and a polygon is used to represent the extent of a geographical region. Finally within a GIS a polygon may be aggregated with a geographic identifier and other information about that region to form a geographic feature such as a 2001 census Output Area.