What is a G-Polygon? What is a G-Ring? What is the difference between an "outer" and "inner" G-Ring?

The terminology of G-Polygon and G-Ring is taken from the Spatial Data Transfer Standard. In simple terms, a G-Polygon is a closed, connected (contiguous) area. A G-Ring is a set of coordinates that defines a boundary of the area. The first and last points in the set of coordinates must be the same. The "outer" G-Ring describes the outside edge of the G-Polygon. "Inner" G-Rings describe any "holes" that might occur in the G-Polygon.

What is the difference between the Bounding Coordinates and the Data Set G-Polygon?

The Bounding Coordinates are the west-, east-, north-, and south-most extents of the data set.

The g-rings that describe the Data Set G-Polygon form the outline of the data set.

Why do the standards allow both Bounding Coordinates and Data Set G-Polygons? Why are Bounding Coordinates mandatory, and the Data Set G-Polygons optional?

The purpose of the Spatial Domain element is to describe the "footprint" of the data set. This footprint can be used for spatial searches in data catalogs and other purposes.

Comments received during the development of the standards recommended requiring the Bounding Coordinates instead of the more demanding (but more exact) Data Set G-Polygons. To provide a common means of conducting spatial searches on all metadata, Bounding Coordinates were made mandatory. Many users indicated a desire to provide the Data Set G-Polygons to allow the results of spatial searches to be more exact, and so the Data Set G-Polygon elements were added as an option.

Why are the g-rings composed of four or more points? A triangle can be represented by three points.

The ring must be closed (that is, the first and last points must be the same). So the minimum number of points to describe a triangle is four (point 1, point 2, point 3, point 1).

Return to: Spatial Domain Button Spatial Domain