## 11.4.2.6 Surface Class 

A Surface  is a two-dimensional geometry. It is a noninstantiable class. Its only instantiable subclass is Polygon . 

## Surface  Properties 

•A Surface  is defined as a two-dimensional geometry. 

•The OpenGIS specification defines a simple Surface  as a geometry that consists of a single “patch” that is associated with a single exterior boundary and zero or more interior boundaries. 

•The boundary of a simple Surface  is the set of closed curves corresponding to its exterior and interior boundaries. 

## 11.4.2.7 Polygon Class 

A Polygon  is a planar Surface  representing a multisided geometry. It is defined by a single exterior boundary and zero or more interior boundaries, where each interior boundary defines a hole in the Polygon . 

## Polygon  Examples 

•On a region map, Polygon  objects could represent forests, districts, and so on. 

## Polygon  Assertions 

•The boundary of a Polygon  consists of a set of LinearRing  objects (that is, LineString  objects that are both simple and closed) that make up its exterior and interior boundaries. 

•A Polygon  has no rings that cross. The rings in the boundary of a Polygon  may intersect at a Point , but only as a tangent. 

•A Polygon  has no lines, spikes, or punctures. 

•A Polygon  has an interior that is a connected point set. 

•A Polygon  may have holes. The exterior of a Polygon  with holes is not connected. Each hole defines a connected component of the exterior. 

## 11.4.2.8 GeometryCollection Class 

The preceding assertions make a Polygon  a simple geometry. 

A GeomCollection  is a geometry that is a collection of zero or more geometries of any class. 

GeomCollection  and GeometryCollection  are synonymous, with GeomCollection  the preferred type name. 

All the elements in a geometry collection must be in the same spatial reference system (that is, in the same coordinate system). There are no other constraints on the elements of a geometry collection, although the subclasses of GeomCollection  described in the following sections may restrict membership. Restrictions may be based on: 

•Element type (for example, a MultiPoint  may contain only Point  elements) 

•Dimension 

