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Simple GML Geometries

Before we look at the Simple Geometry profile of GML, a short discussion of geometry is required. In GML, objects have characteristics some of which may be geometric. We can talk about the extent of a lake, the position of a vehicle, the edge or centerline of a road, etc. These geometric characteristics are described by a property of the object, which is a relationship between the object and some geometric object.

In the Simple Geometry profile of GML there are only three kinds of geometry objects, namely:

  • Point
  • LineString
  • Polygon

Point

This one is pretty self explanatory. A point has “coordinates” that determine its location relative to some coordinate system (e.g. (latitude, longitude).

LineString

Think of a LineString as a curve, line or edge. If you look really close you see that it is made up of a bunch of line segments strung together – so is a string of lines or LineString. See the …

Polygon

A polygon is just a bit more complex. A Polygon is a region or area.

Think of the state of California (all of it) as having a Polygon extent. The boundary of the State of California is the black line around the outside.

So, Polygons have a boundary as shown in the figure. Now Simple Geometry ONLY allows the boundary of the Polygon to made of a single piece called a LinearRing. This is a LineString that is closed on itself like the ring on your finger.

In full GML, the boundary of a polygon may be composed of separate LineStrings much as the boundary of the State of California is composed of parts of the border of Arizona, Oregon, Nevada, and the Pacific Ocean.

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