Geometries¶

Points¶

Different types of points

A spatial point corresponds to a single location, defined by a single coordinate (including either 2-, 3- or 4-dimensions).

SELECT ST_AsText(geom)
  FROM geometries
  WHERE name = 'Point';

POINT(0 0)

Points are used to represent objects when the exact details, such as shape and size, are not important at the target scale. For example, cities on a map of the world are best depicted as points, while a map of a single state might represent cities as polygons.

Some of the specific spatial functions for working with points include:

• ST_X(geometry)—Returns the X ordinate
• ST_Y(geometry)—Returns the Y ordinate

To return the ordinate values of a point, execute the following SQL query:

SELECT ST_X(geom), ST_Y(geom)
  FROM geometries
  WHERE name = 'Point';

0     0

To return the geometry of a point (stored in the ST_AsText column), execute:

SELECT name, ST_AsText(geom)
  FROM geometries
  LIMIT 1;

Point  Point (0 0 )

Linestrings¶

Different types of lines

A linestring, in the form of an ordered series of two or more points, represents a path between locations. Most linear features, such as roads and rivers, are typically represented as linestrings. A linestring is said to be closed if it starts and ends on the same point. It is said to be simple if it does not cross or touch itself (except at its endpoints if it is closed). A linestring can be both closed and simple. A single real world street may consist of many linestrings, each representing a segment of road with different attributes.

The following SQL query will return the geometry associated with one linestring (again, stored in the ST_AsText column).

SELECT ST_AsText(geom)
  FROM geometries
  WHERE name = 'Linestring';

LINESTRING(0 0, 1 1, 2 1, 2 2)

Some of the specific spatial functions for working with linestrings include:

• ST_Length(geometry)—Returns the length of the linestring
• ST_StartPoint(geometry)—Returns the first coordinate as a point
• ST_EndPoint(geometry)—Returns the last coordinate as a point
• ST_NPoints(geometry)—Returns the number of coordinates in the linestring

To determine the length of a linestring, execute the following:

SELECT ST_Length(geom)
  FROM geometries
  WHERE name = 'Linestring';

3.41421356237309

Polygons¶

Different kinds of polygons

A polygon is a representation of an area. The outer boundary of the polygon is defined by a ring—a linestring that is both closed and simple. Holes within the polygon are also defined by rings.

Polygons generally represent objects whose size and shape are important. City limits, parks, building footprints or bodies of water are all commonly depicted as polygons when the scale is sufficiently large to see their area. Roads and rivers can sometimes be represented as polygons.

The following SQL query will return the geometry associated with one polygon (in the ST_AsText column).

SELECT ST_AsText(geom)
  FROM geometries
  WHERE name LIKE 'Polygon%';

POLYGON((0 0, 1 0, 1 1, 0 1, 0 0))
POLYGON((0 0, 10 0, 10 10, 0 10, 0 0),(1 1, 1 2, 2 2, 2 1, 1 1))

The first polygon has only one ring. The second one has an interior “hole”. Although many graphics applications can work with polygons, GIS systems are relatively unique in allowing polygons to explicitly have holes.

Two polygons, one with a hole

Some of the specific spatial functions for working with polygons include:

• ST_Area(geometry)—Returns the area of the polygons
• ST_NRings(geometry)—returns the number of rings (usually 1, more if there are holes)
• ST_ExteriorRing(geometry)—Returns the outer ring as a linestring
• ST_InteriorRingN(geometry,n)—Returns a specified interior ring as a linestring
• ST_Perimeter(geometry)—Returns the length of all rings

To calculate the area of a polygon, use the ST_Area() function:

SELECT name, ST_Area(geom)
  FROM geometries
  WHERE name LIKE 'Polygon%';

Polygon            1
PolygonWithHole    99

Collections¶

Collections group multiple simple geometries into sets and provide a framework for modeling real world objects as spatial objects.

There are four collection types:

• MultiPoint—Collection of points
• MultiLineString—Collection of linestrings
• MultiPolygon—Collection of polygons
• GeometryCollection—Heterogeneous collection of any geometry (including other collections)

Collections provide a solution for modeling situations such as a parcel of land that is split by a right-of-way. In this case, a MultiPolygon, with a section of the land parcel on either side of the right-of-way, would be required.

Polygon collection

The example collection in the geometries table contains a polygon and a point:

SELECT name, ST_AsText(geom)
  FROM geometries
  WHERE name = 'Collection';

GEOMETRYCOLLECTION(POINT(2 0),POLYGON((0 0, 1 0, 1 1, 0 1, 0 0)))

Geometry collection

Some of the specific spatial functions for working with collections include:

• ST_NumGeometries(geometry)—Returns the number of parts in the collection
• ST_GeometryN(geometry,n)—Returns the specified part
• ST_Area(geometry)—Returns the total area of all polygonal parts
• ST_Length(geometry)—Returns the total length of all linear parts