Geometry properties
There are many properties that characterize geometry. You use accessor functions to return the properties of a geometry. Some of these geometry properties are described in this topic.
Dimensionality
The dimensions of a geometry are the minimum coordinates (none, x, y) required to define the spatial extent of the geometry.
A geometry can have a dimension of 0, 1, or 2.
The dimensions are as follows:
- 0—Has neither length nor area
- 1—Has a length (x or y)
- 2—Contains area (x and y)
Point features have a dimension of 0, lines a dimension of 1, polygons a dimension of 2.
Dimension is important not only as a property of the subtype but also in determining the spatial relationship of two features. The dimension of the resulting feature or features determines whether or not the operation was successful. The dimensions of the features are examined to determine how they should be compared.
The coordinates of a geometry also have dimensions. If a geometry has only x- and y-coordinates, the coordinate dimension is 2. If a geometry has x-, y-, and z-coordinates, the coordinate dimension is 3. If a geometry has x-, y-, z-, and m-coordinates, the coordinate dimension is 4.
Z-coordinates
Some geometries have an associated altitude or depth—a third dimension. Each of the points that form the geometry of a feature can include an optional z-coordinate that represents an altitude or depth relative to the earth's surface.
Measures
Measures are values assigned to each coordinate. They are used for linear referencing and dynamic segmentation applications. For example, milepost locations along a highway can contain measures that indicate their position. The value represents anything that can be stored as a double-precision number.
Geometry type
The geometry type refers to type of geometric entity. These include the following:
- Points and multipoints
- Lines and multilines
- Polygons and multipolygons
In multipart geometries, such as multipoints, multilines, and multipolygons, one feature is made up of multiple simple geometries (points, lines, or polygons).
Interior, boundary, exterior
All geometries occupy a position in space defined by their interiors, boundaries, and exteriors. The exterior of a geometry is all the space not occupied by the geometry. The interior is the space occupied by the geometry. The boundary of a geometry is the location between its interior and exterior. The subtype inherits the interior and exterior properties directly; however, the boundary property differs for each.
Empty or not empty
A geometry is empty if it does not have any points. An empty geometry has a null envelope, boundary, interior, and exterior. An empty geometry is always simple. Empty linestrings and multilinestrings have a 0 length. Empty polygons and multipolygons have a 0 area.
Envelope
Every geometry has an envelope. The envelope of a geometry is the bounding geometry formed by the minimum and maximum x,y coordinates. For point geometries, since the minimum and maximum x,y coordinates are the same, a rectangle, or envelope, is created around these coordinates. For line geometries, the endpoints of the line represent two sides of the envelope and the other two sides are created just above and just below the line.
Spatial reference system
The spatial reference system identifies the coordinate transformation matrix for each geometry. It is made up of a coordinate system, resolution, and tolerance.