Geometries are used for specification of location shape and orientation of of a real world object in geographic and projected spatial systems. In Redbex geometries are used to define the LSO of features observations and are also used as property values for properties of type Geometry.

The basic idea behind geometries is that usually it is not enough to represent the spatial information of a real world phenomenon as simple point (in either geodetic or Cartesian coordinates) and therefore mightier data structures are needed to store more spatial information.

Geometries are a very common concept that is also used within nearly all Geographic Information Systems (GIS) available. The implementation of geometries as provided by the Application Server conforms to the Open Geospatial Consortium (OGC) Simple Features For SQL Specification [2]

To be fully specified a geometry must always map to a specific reference system. This combination of geometry and reference system is called a geometric object. Geometric objects are the basis for geometric calculations and spatial query operators like Overlaps, within, Contains, Disjoint etc.

Both the geographic and the projected or Cartesian spatial system types use Geometries, but there are some key differences in how the data is stored and manipulated.  

The defining data for Line-String and Polygon types are vertices only. The connecting edge between two vertices in a geometry type is a straight line. However, the connecting edge between two vertices in a geographic system is a short great elliptic arc between the two vertices. A great ellipse is the intersection of the ellipsoid with a plane through its center and a great elliptic arc is an arc segment on the great ellipse.

In a projected system or in a Cartesian system, measurements of distances and areas are given in the same unit of measurement as coordinates. The distance between (2, 2) and (5, 6) is 5 units, regardless of the units used. With geographic spatial data, coordinates are given in degrees of latitude and longitude. However, lengths and areas are measured in the unit defined by the horizontal geographic coordinate system.

In the projected system or the Cartesian system, the ring orientation of a polygon is not an important factor. For example, a polygon described by ((0, 0), (10, 0), (0, 20), (0, 0)) is the same as a polygon described by ((0, 0), (0, 20), (10, 0), (0, 0)). The OGC Simple Features for SQL Specification does not dictate a ring ordering. In an geographic system, a polygon has no meaning, or is ambiguous, without an orientation. For example, does a ring around the equator describe the northern or southern hemisphere? If we use the geography data type to store the spatial instance, we must specify the orientation of the ring and accurately describe the location of the instance. The interior of the polygon in an ellipsoidal system is defined by the left-hand rule.

Different GIS systems support different types of geometries the following chapters describe the geometries supported by the Application Server.

The feature type defines which geometries can be used to describe its location shape and orientation.