django/docs/ref/contrib/gis/geos.txt

.. _ref-geos:

========
GEOS API
========

.. module:: django.contrib.gis.geos
   :synopsis: GeoDjango's high-level interface to the GEOS library.

Background
==========

What is GEOS?
-------------

`GEOS`__ stands for **G**\ eometry **E**\ ngine - **O**\ pen **S**\ ource,
and is a C++ library, ported from the  `Java Topology Suite`__.  GEOS
implements the OpenGIS `Simple Features for SQL`__ spatial predicate functions
and spatial operators. GEOS, now an OSGeo project, was initially developed and
maintained by `Refractions Research`__ of Victoria, Canada.

__ http://trac.osgeo.org/geos/
__ http://sourceforge.net/projects/jts-topo-suite/
__ http://www.opengeospatial.org/standards/sfs
__ http://www.refractions.net/

Features
--------

GeoDjango implements a high-level Python wrapper for the GEOS library, its
features include:

* A BSD-licensed interface to the GEOS geometry routines, implemented purely
  in Python using ``ctypes``.
* Loosely-coupled to GeoDjango.  For example, :class:`GEOSGeometry` objects
  may be used outside of a django project/application.  In other words, 
  no need to have ``DJANGO_SETTINGS_MODULE`` set or use a database, etc.
* Mutability: :class:`GEOSGeometry` objects may be modified.
* Cross-platform and tested; compatible with Windows, Linux, Solaris, and Mac 
  OS X platforms.

.. _geos-tutorial:

Tutorial
========

This section contains a brief introduction and tutorial to using 
:class:`GEOSGeometry` objects.

Creating a Geometry
-------------------

:class:`GEOSGeometry` objects may be created in a few ways.  The first is
to simply instantiate the object on some spatial input -- the following
are examples of creating the same geometry from WKT, HEX, WKB, and GeoJSON::

    >>> from django.contrib.gis.geos import GEOSGeometry
    >>> pnt = GEOSGeometry('POINT(5 23)') # WKT
    >>> pnt = GEOSGeometry('010100000000000000000014400000000000003740') # HEX
    >>> pnt = GEOSGeometry(buffer('\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x14@\x00\x00\x00\x00\x00\x007@'))
    >>> pnt = GEOSGeometry('{ "type": "Point", "coordinates": [ 5.000000, 23.000000 ] }') # GeoJSON

Another option is to use the constructor for the specific geometry type
that you wish to create.  For example, a :class:`Point` object may be
created by passing in the X and Y coordinates into its constructor::

    >>> from django.contrib.gis.geos import Point
    >>> pnt = Point(5, 23)

Finally, there are :func:`fromstr` and :func:`fromfile` factory methods, which
return a :class:`GEOSGeometry` object from an input string or a file::

    >>> from django.contrib.gis.geos import fromstr, fromfile
    >>> pnt = fromstr('POINT(5 23)')
    >>> pnt = fromfile('/path/to/pnt.wkt')
    >>> pnt = fromfile(open('/path/to/pnt.wkt'))

Geometries are Pythonic 
-----------------------
:class:`GEOSGeometry` objects are 'Pythonic', in other words components may
be accessed, modified, and iterated over using standard Python conventions.
For example, you can iterate over the coordinates in a :class:`Point`::

    >>> pnt = Point(5, 23)
    >>> [coord for coord in pnt]
    [5.0, 23.0]

With any geometry object, the :attr:`GEOSGeometry.coords` property
may be used to get the geometry coordinates as a Python tuple::

    >>> pnt.coords
    (5.0, 23.0)

You can get/set geometry components using standard Python indexing
techniques.  However, what is returned depends on the geometry type
of the object.  For example, indexing on a :class:`LineString`
returns a coordinate tuple::

    >>> from django.contrib.gis.geos import LineString
    >>> line = LineString((0, 0), (0, 50), (50, 50), (50, 0), (0, 0))
    >>> line[0]
    (0.0, 0.0)
    >>> line[-2]
    (50.0, 0.0)

Whereas indexing on a :class:`Polygon` will return the ring
(a :class:`LinearRing` object) corresponding to the index::

    >>> from django.contrib.gis.geos import Polygon
    >>> poly = Polygon( ((0.0, 0.0), (0.0, 50.0), (50.0, 50.0), (50.0, 0.0), (0.0, 0.0)) )
    >>> poly[0]
    <LinearRing object at 0x1044395b0>
    >>> poly[0][-2] # second-to-last coordinate of external ring
    (50.0, 0.0)

In addition, coordinates/components of the geometry may added or modified,
just like a Python list::

    >>> line[0] = (1.0, 1.0)
    >>> line.pop()
    (0.0, 0.0)
    >>> line.append((1.0, 1.0))
    >>> line.coords
    ((1.0, 1.0), (0.0, 50.0), (50.0, 50.0), (50.0, 0.0), (1.0, 1.0))

Geometry Objects
================

``GEOSGeometry``
----------------

.. class:: GEOSGeometry(geo_input[, srid=None])

  :param geo_input: Geometry input value
  :type geo_input: string or buffer
  :param srid: spatial reference identifier
  :type srid: integer

This is the base class for all GEOS geometry objects.  It initializes on the
given ``geo_input`` argument, and then assumes the proper geometry subclass
(e.g., ``GEOSGeometry('POINT(1 1)')`` will create a :class:`Point` object).

The following input formats, along with their corresponding Python types,
are accepted:

=============  ======================
Format         Input Type
=============  ======================
WKT / EWKT     ``str`` or ``unicode`` 
HEX / HEXEWKB  ``str`` or ``unicode``
WKB / EWKB     ``buffer``
GeoJSON        ``str`` or ``unicode``
=============  ======================

Properties
~~~~~~~~~~

.. attribute:: GEOSGeometry.coords

Returns the coordinates of the geometry as a tuple.

.. attribute:: GEOSGeometry.empty

Returns whether or not the set of points in the geometry is empty. 

.. attribute:: GEOSGeometry.geom_type

Returns a string corresponding to the type of geometry.  For example::

    >>> pnt = GEOSGeometry('POINT(5 23)')
    >>> pnt.geom_type
    'Point'

.. attribute:: GEOSGeometry.geom_typeid

Returns the GEOS geometry type identification number.  The following table
shows the value for each geometry type:

===========================  ========
Geometry                     ID
===========================  ========
:class:`Point`               0
:class:`LineString`          1
:class:`LinearRing`          2
:class:`Polygon`             3
:class:`MultiPoint`          4
:class:`MultiLineString`     5
:class:`MultiPolygon`        6
:class:`GeometryCollection`  7
===========================  ========

.. attribute:: GEOSGeometry.num_coords

Returns the number of coordinates in the geometry.

.. attribute:: GEOSGeometry.num_geom

Returns the number of geometries in this geometry.  In other words, will
return 1 on anything but geometry collections.

.. attribute:: GEOSGeometry.hasz

Returns a boolean indicating whether the geometry is three-dimensional.

.. attribute:: GEOSGeometry.ring

Returns a boolean indicating whether the geometry is a ``LinearRing``.

.. attribute:: GEOSGeometry.simple

Returns a boolean indicating whether the geometry is 'simple'. A geometry
is simple if and only if it does not intersect itself (except at boundary
points).  For example, a :class:`LineString` object is not simple if it
intersects itself. Thus, :class:`LinearRing` and :class`Polygon` objects
are always simple because they do cannot intersect themselves, by
definition.

.. attribute:: GEOSGeometry.valid

Returns a boolean indicating whether the geometry is valid.

.. attribute:: GEOSGeometry.srid

Property that may be used to retrieve or set the SRID associated with the
geometry.  For example::

    >>> pnt = Point(5, 23)
    >>> print pnt.srid
    None
    >>> pnt.srid = 4326
    >>> pnt.srid
    4326

Output Properties
~~~~~~~~~~~~~~~~~

The properties in this section export the :class:`GEOSGeometry` object into
a different.  This output may be in the form of a string, buffer, or even
another object.

.. attribute:: GEOSGeometry.ewkt

Returns the "extended" Well-Known Text of the geometry.  This representation
is specific to PostGIS and is a super set of the OGC WKT standard. [#fnogc]_
Essentially the SRID is prepended to the WKT representation, for example 
``SRID=4326;POINT(5 23)``. 

.. note::

   The output from this property does not include the 3dm, 3dz, and 4d 
   information that PostGIS supports in its EWKT representations.

.. attribute:: GEOSGeometry.hex

Returns the WKB of this Geometry in hexadecimal form.  Please note
that the SRID and Z values are not included in this representation
because it is not a part of the OGC specification (use the 
:attr:`GEOSGeometry.hexewkb` property instead).

.. attribute:: GEOSGeometry.hexewkb

.. versionadded:: 1.2

Returns the EWKB of this Geometry in hexadecimal form.  This is an 
extension of the WKB specification that includes SRID and Z values 
that are a part of this geometry.

.. note::

   GEOS 3.1 is *required* if you want valid 3D HEXEWKB.

.. attribute:: GEOSGeometry.json

Returns the GeoJSON representation of the geometry.

.. note::

    Requires GDAL.

.. attribute:: GEOSGeometry.geojson

Alias for :attr:`GEOSGeometry.json`.

.. attribute:: GEOSGeometry.kml

Returns a `KML`__ (Keyhole Markup Language) representation of the
geometry.  This should only be used for geometries with an SRID of 
4326 (WGS84), but this restriction is not enforced.

.. attribute:: GEOSGeometry.ogr

Returns an :class:`~django.contrib.gis.gdal.OGRGeometry` object 
correspondg to the GEOS geometry.

.. note::

    Requires GDAL.

.. _wkb:

.. attribute:: GEOSGeometry.wkb

Returns the WKB (Well-Known Binary) representation of this Geometry
as a Python buffer.  SRID and Z values are not included, use the
:attr:`GEOSGeometry.ewkb` property instead.

.. _ewkb:

.. attribute:: GEOSGeometry.ewkb

.. versionadded:: 1.2

Return the EWKB representation of this Geometry as a Python buffer.
This is an extension of the WKB specification that includes any SRID
and Z values that are a part of this geometry.

.. note::

   GEOS 3.1 is *required* if you want valid 3D EWKB.

.. attribute:: GEOSGeometry.wkt

Returns the Well-Known Text of the geometry (an OGC standard).

__ http://code.google.com/apis/kml/documentation/

Spatial Predicate Methods
~~~~~~~~~~~~~~~~~~~~~~~~~

All of the following spatial predicate methods take another
:class:`GEOSGeometry` instance (``other``) as a parameter, and
return a boolean.

.. method:: GEOSGeometry.contains(other)

Returns ``True`` if :meth:`GEOSGeometry.within` is ``False``.

.. method:: GEOSGeometry.crosses(other)

Returns ``True`` if the DE-9IM intersection matrix for the two Geometries
is ``T*T******`` (for a point and a curve,a point and an area or a line 
and an area) ``0********`` (for two curves).

.. method:: GEOSGeometry.disjoint(other)

Returns ``True`` if the DE-9IM intersection matrix for the two geometries
is ``FF*FF****``.

.. method:: GEOSGeometry.equals(other)

Returns ``True`` if the DE-9IM intersection matrix for the two geometries 
is ``T*F**FFF*``.

.. method:: GEOSGeometry.equals_exact(other, tolerance=0)

Returns true if the two geometries are exactly equal, up to a
specified tolerance.  The ``tolerance`` value should be a floating
point number representing the error tolerance in the comparison, e.g.,
``poly1.equals_exact(poly2, 0.001)`` will compare equality to within
one thousandth of a unit.

.. method:: GEOSGeometry.intersects(other)

Returns ``True`` if :meth:`GEOSGeometry.disjoint` is ``False``.

.. method:: GEOSGeometry.overlaps(other)

Returns true if the DE-9IM intersection matrix for the two geometries
is ``T*T***T**`` (for two points or two surfaces) ``1*T***T**``
(for two curves).

.. method:: GEOSGeometry.relate_pattern(other, pattern)

Returns ``True`` if the elements in the DE-9IM intersection matrix 
for this geometry and the other matches the given ``pattern`` -- 
a string of nine characters from the alphabet: {``T``, ``F``, ``*``, ``0``}.

.. method:: GEOSGeometry.touches(other)

Returns ``True`` if the DE-9IM intersection matrix for the two geometries
is ``FT*******``, ``F**T*****`` or ``F***T****``.

.. method:: GEOSGeometry.within(other)

Returns ``True`` if the DE-9IM intersection matrix for the two geometries
is ``T*F**F***``.

Topological Methods
~~~~~~~~~~~~~~~~~~~

.. method:: GEOSGeometry.buffer(width, quadsegs=8)

Returns a :class:`GEOSGeometry` that represents all points whose distance
from this geometry is less than or equal to the given ``width``. The optional 
``quadsegs`` keyword sets the number of segments used to approximate a 
quarter circle (defaults is 8).

.. method:: GEOSGeometry.difference(other)

Returns a :class:`GEOSGeometry` representing the points making up this
geometry that do not make up other.

.. method:: GEOSGeometry:intersection(other)

Returns a :class:`GEOSGeometry` representing the points shared by this
geometry and other.

.. method:: GEOSGeometry.relate(other)

Returns the DE-9IM intersection matrix (a string) representing the
topological relationship between this geometry and the other.

.. method:: GEOSGeometry.simplify(tolerance=0.0, preserve_topology=False)

Returns a new :class:`GEOSGeometry`, simplified using the Douglas-Peucker
algorithm to the specified tolerance.  A higher tolerance value implies
less points in the output.  If no tolerance is tolerance provided,
it defaults to 0.

By default, this function does not preserve topology - e.g., 
:class:`Polygon` objects can be split, collapsed into lines or disappear.
:class:`Polygon` holes can be created or disappear, and lines can cross.
By specifying ``preserve_topology=True``, the result will have the same
dimension and number of components as the input, however, this is 
significantly slower.   

.. method:: GEOSGeometry.sym_difference(other)

Returns a :class:`GEOSGeometry` combining the points in this geometry 
not in other, and the points in other not in this geometry.

.. method:: GEOSGeometry.union(other)

Returns a :class:`GEOSGeometry` representing all the points in this 
geometry and the other.

Topological Properties
~~~~~~~~~~~~~~~~~~~~~~

.. attribute:: GEOSGeometry.boundary

Returns the boundary as a newly allocated Geometry object.

.. attribute:: GEOSGeometry.centroid

Returns a :class:`Point` object representing the geometric center of
the geometry.  The point is not guaranteed to be on the interior
of the geometry.

.. attribute:: GEOSGeometry.convex_hull

Returns the smallest :class:`Polygon` that contains all the points in
the geometry.

.. attribute:: GEOSGeometry.envelope

Returns a :class:`Polygon` that represents the bounding envelope of
this geometry.

.. attribute:: GEOSGeometry.point_on_surface

Computes and returns a :class:`Point` guaranteed to be on the interior
of this geometry.

Other Properties & Methods
~~~~~~~~~~~~~~~~~~~~~~~~~~

.. attribute:: GEOSGeometry.area

This property returns the area of the Geometry.

.. attribute:: GEOSGeometry.extent

This property returns the extent of this geometry as a 4-tuple, 
consisting of (xmin, ymin, xmax, ymax).

.. method:: GEOSGeometry.clone()

This method returns a :class:`GEOSGeometry` that is a clone of the original.

.. method:: GEOSGeometry.distance(geom)

Returns the distance between the closest points on this geometry and the given
``geom`` (another :class:`GEOSGeometry` object).

.. note::

    GEOS distance calculations are  linear -- in other words, GEOS does not
    perform a spherical calculation even if the SRID specifies a geographic 
    coordinate system.

.. attribute:: GEOSGeometry.length

Returns the length of this geometry (e.g., 0 for a :class:`Point`, 
the length of a :class:`LineString`, or the circumference of 
a :class:`Polygon`).

.. attribute:: GEOSGeometry.prepared

.. versionadded:: 1.1

.. note::

    Support for prepared geometries requires GEOS 3.1.

Returns a GEOS ``PreparedGeometry`` for the contents of this geometry.  
``PreparedGeometry`` objects are optimized for the contains, intersects,
and covers operations.  Refer to the :ref:`prepared-geometries` documentation
for more information.

.. attribute:: GEOSGeometry.srs

Returns a :class:`~django.contrib.gis.gdal.SpatialReference` object
corresponding to the SRID of the geometry or ``None``.

.. note::

    Requires GDAL.

.. method:: transform(ct, clone=False)

Transforms the geometry according to the given coordinate transformation paramter
(``ct``), which may be an integer SRID, spatial reference WKT string,
a PROJ.4 string, a :class:`~django.contrib.gis.gdal.SpatialReference` object, or a 
:class:`~django.contrib.gis.gdal.CoordTransform` object. By default, the geometry
is transformed in-place and nothing is returned. However if the ``clone`` keyword
is set, then the geometry is not modified and a transformed clone of the geometry
is returned instead.

.. note::

    Requires GDAL.

``Point``
---------

.. class:: Point(x, y, z=None, srid=None)

   ``Point`` objects are instantiated using arguments that represent
   the component coordinates of the point or with a single sequence
   coordinates.  For example, the following are equivalent::

       >>> pnt = Point(5, 23)
       >>> pnt = Point([5, 23])

``LineString``
--------------

.. class:: LineString(*args, **kwargs)

   ``LineString`` objects are instantiated using arguments that are
   either a sequence of coordinates or :class:`Point` objects.
   For example, the following are equivalent::

       >>> ls = LineString((0, 0), (1, 1))
       >>> ls = LineString(Point(0, 0), Point(1, 1))

   In addition, ``LineString`` objects may also be created by passing
   in a single sequence of coordinate or :class:`Point` objects::

       >>> ls = LineString( ((0, 0), (1, 1)) )
       >>> ls = LineString( [Point(0, 0), Point(1, 1)] )

``LinearRing``
--------------

.. class:: LinearRing(*args, **kwargs)

   ``LinearRing`` objects are constructed in the exact same way as
   :class:`LineString` objects, however the coordinates must be
   *closed*, in other words, the first coordinates must be the
   same as the last coordinates.  For example::

       >>> ls = LinearRing((0, 0), (0, 1), (1, 1), (0, 0))

   Notice that ``(0, 0)`` is the first and last coordinate -- if
   they were not equal, an error would be raised.

``Polygon``
-----------

.. class:: Polygon(*args, **kwargs)

   ``Polygon`` objects may be instantiated by passing in one or
   more parameters that represent the rings of the polygon.  The
   parameters must either be :class:`LinearRing` instances, or
   a sequence that may be used to construct a :class:`LinearRing`::

       >>> ext_coords = ((0, 0), (0, 1), (1, 1), (1, 0), (0, 0))
       >>> int_coords = ((0.4, 0.4), (0.4, 0.6), (0.6, 0.6), (0.6, 0.4), (0.4, 0.4))
       >>> poly = Polygon(ext_coords, int_coords)
       >>> poly = Polygon(LinearRing(ext_coords), LinearRing(int_coords))

   .. classmethod:: from_bbox(bbox)

   .. versionadded:: 1.1

   Returns a polygon object from the given bounding-box, a 4-tuple
   comprising (xmin, ymin, xmax, ymax).

   .. attribute:: num_interior_rings

   Returns the number of interior rings in this geometry.

Geometry Collections
====================

``MultiPoint``
--------------

.. class:: MultiPoint(*args, **kwargs)

   ``MultiPoint`` objects may be instantiated by passing in one
   or more :class:`Point` objects as arguments, or a single
   sequence of :class:`Point` objects::

       >>> mp = MultiPoint(Point(0, 0), Point(1, 1))
       >>> mp = MultiPoint( (Point(0, 0), Point(1, 1)) )

``MultiLineString``
-------------------

.. class:: MultiLineString(*args, **kwargs)

   ``MultiLineString`` objects may be instantiated by passing in one
   or more :class:`LineString` objects as arguments, or a single
   sequence of :class:`LineString` objects::

       >>> ls1 = LineString((0, 0), (1, 1))
       >>> ls2 = LineString((2, 2), (3, 3))
       >>> mls = MultiLineString(ls1, ls2)
       >>> mls = MultiLineString([ls1, ls2])

   .. attribute:: merged

   .. versionadded:: 1.1

   Returns a :class:`LineString` representing the line merge of
   all the components in this ``MultiLineString``.
       

``MultiPolygon``
----------------

.. class:: MultiPolygon(*args, **kwargs)

   ``MultiPolygon`` objects may be instantiated by passing one or
   more :class:`Polygon` objects as arguments, or a single sequence
   of :class:`Polygon` objects::

       >>> p1 = Polygon( ((0, 0), (0, 1), (1, 1), (0, 0)) )
       >>> p2 = Polygon( ((1, 1), (1, 2), (2, 2), (1, 1)) )
       >>> mp = MultiPolygon(p1, p2)
       >>> mp = MultiPolygon([p1, p2])

   .. attribute:: cascaded_union

   .. versionadded:: 1.1

   Returns a :class:`Polygon` that is the union of all of the component
   polygons in this collection.  The algorithm employed is significantly
   more efficient (faster) than trying to union the geometries together
   individually. [#fncascadedunion]_

   .. note::

       GEOS 3.1 is *required* to peform cascaded unions.

``GeometryCollection``
----------------------

.. class:: GeometryCollection(*args, **kwargs)

   ``GeometryCollection`` objects may be instantiated by passing in
   one or more other :class:`GEOSGeometry` as arguments, or a single
   sequence of :class:`GEOSGeometry` objects::

       >>> poly = Polygon( ((0, 0), (0, 1), (1, 1), (0, 0)) )
       >>> gc = GeometryCollection(Point(0, 0), MultiPoint(Point(0, 0), Point(1, 1)), poly)
       >>> gc = GeometryCollection((Point(0, 0), MultiPoint(Point(0, 0), Point(1, 1)), poly))

.. _prepared-geometries:

Prepared Geometries
===================

.. versionadded: 1.1

In order to obtain a prepared geometry, just access the
:attr:`GEOSGeometry.prepared` property.  Once you have a
``PreparedGeometry`` instance its spatial predicate methods, listed below,
may be used with other ``GEOSGeometry`` objects.  An operation with a prepared
geometry can be orders of magnitude faster -- the more complex the geometry 
that is prepared, the larger the speedup in the operation.  For more information,
please consult the `GEOS wiki page on prepared geometries <http://trac.osgeo.org/geos/wiki/PreparedGeometry>`_.

.. note::

   GEOS 3.1 is *required* in order to use prepared geometries.

For example::

    >>> from django.contrib.gis.geos import Point, Polygon
    >>> poly = Polygon.from_bbox((0, 0, 5, 5))
    >>> prep_poly = poly.prepared
    >>> prep_poly.contains(Point(2.5, 2.5))
    True

``PreparedGeometry``
--------------------

.. class:: PreparedGeometry

  All methods on ``PreparedGeometry`` take an ``other`` argument, which
  must be a :class:`GEOSGeometry` instance.

  .. method:: contains(other)

  .. method:: contains_properly(other)

  .. method:: covers(other)

  .. method:: intersects(other)

Geometry Factories
==================

.. function:: fromfile(file_h)

   :param file_h: input file that contains spatial data
   :type file_h: a Python ``file`` object or a string path to the file
   :rtype: a :class:`GEOSGeometry` corresponding to the spatial data in the file

Example::

    >>> from django.contrib.gis.geos import fromfile
    >>> g = fromfile('/home/bob/geom.wkt')

.. function:: fromstr(string, [,srid=None])

   :param string: string that contains spatial data
   :type string: string
   :param srid: spatial reference identifier
   :type srid: integer
   :rtype: a :class:`GEOSGeometry` corresponding to the spatial data in the string

Example::

    >>> from django.contrib.gis.geos import fromstr
    >>> pnt = fromstr('POINT(-90.5 29.5)', srid=4326)

I/O Objects
===========

.. versionadded: 1.1

Reader Objects
--------------

The reader I/O classes simply return a :class:`GEOSGeometry` instance from the
WKB and/or WKT input given to their ``read(geom)`` method.

.. class:: WKBReader

Example::

    >>> from django.contrib.gis.geos import WKBReader
    >>> wkb_r = WKBReader()
    >>> wkb_r.read('0101000000000000000000F03F000000000000F03F')
    <Point object at 0x103a88910>

.. class:: WKTReader

Example::

    >>> from django.contrib.gis.geos import WKTReader
    >>> wkt_r = WKTReader()
    >>> wkt_r.read('POINT(1 1)')
    <Point object at 0x103a88b50>

Writer Objects
--------------

All writer objects have a ``write(geom)`` method that returns either the
WKB or WKT of the given geometry.  In addition, :class:`WKBWriter` objects
also have properties that may be used to change the byte order, and or
include the SRID and 3D values (in other words, EWKB).

.. class:: WKBWriter

``WKBWriter`` provides the most control over its output.  By default it
returns OGC-compliant WKB when it's ``write`` method is called.  However, 
it has properties that allow for the creation of EWKB, a superset of the
WKB standard that includes additional information.

.. method:: WKBWriter.write(geom)

Returns the WKB of the given geometry as a Python ``buffer`` object.
Example::

    >>> from django.contrib.gis.geos import Point, WKBWriter
    >>> pnt = Point(1, 1)
    >>> wkb_w = WKBWriter()
    >>> wkb_w.write(pnt)
    <read-only buffer for 0x103a898f0, size -1, offset 0 at 0x103a89930>

.. method:: WKBWriter.write_hex(geom)

Returns WKB of the geometry in hexadecimal.  Example::

    >>> from django.contrib.gis.geos import Point, WKBWriter
    >>> pnt = Point(1, 1)
    >>> wkb_w = WKBWriter()
    >>> wkb_w.write_hex(pnt)
    '0101000000000000000000F03F000000000000F03F'

.. attribute:: WKBWriter.byteorder

This property may be be set to change the byte-order of the geometry
representation.

=============== =================================================
Byteorder Value Description
=============== =================================================
0               Big Endian (e.g., compatible with RISC systems)
1               Little Endian (e.g., compatible with x86 systems)
=============== =================================================

Example::

    >>> from django.contrib.gis.geos import Point, WKBWriter
    >>> wkb_w = WKBWriter()
    >>> pnt = Point(1, 1)
    >>> wkb_w.write_hex(pnt)
    '0101000000000000000000F03F000000000000F03F'
    >>> wkb_w.byteorder = 0
    '00000000013FF00000000000003FF0000000000000'

.. attribute:: WKBWriter.outdim

This property may be set to change the output dimension of the geometry
representation.  In other words, if you have a 3D geometry then set to 3
so that the Z value is included in the WKB.

============ ===========================
Outdim Value Description
============ ===========================
2            The default, output 2D WKB.
3            Output 3D EWKB.
============ ===========================

Example::

    >>> from django.contrib.gis.geos import Point, WKBWriter
    >>> wkb_w = WKBWriter()
    >>> wkb_w.outdim
    2
    >>> pnt = Point(1, 1, 1)
    >>> wkb_w.write_hex(pnt) # By default, no Z value included:
    '0101000000000000000000F03F000000000000F03F'
    >>> wkb_w.outdim = 3 # Tell writer to include Z values
    >>> wkb_w.write_hex(pnt)
    '0101000080000000000000F03F000000000000F03F000000000000F03F'

.. attribute:: WKBWriter.srid

Set this property with a boolean to indicate whether the SRID of the
geometry should be included with the WKB representation.  Example::

    >>> from django.contrib.gis.geos import Point, WKBWriter
    >>> wkb_w = WKBWriter()
    >>> pnt = Point(1, 1, srid=4326)
    >>> wkb_w.write_hex(pnt) # By default, no SRID included:
    '0101000000000000000000F03F000000000000F03F'
    >>> wkb_w.srid = True # Tell writer to include SRID
    >>> wkb_w.write_hex(pnt)
    '0101000020E6100000000000000000F03F000000000000F03F'

.. class:: WKTWriter

.. method:: WKTWriter.write(geom)

Returns the WKT of the given geometry. Example::

    >>> from django.contrib.gis.geos import Point, WKTWriter
    >>> pnt = Point(1, 1)
    >>> wkt_w = WKTWriter()
    >>> wkt_w.write(pnt)
    'POINT (1.0000000000000000 1.0000000000000000)'


.. rubric:: Footnotes
.. [#fnogc] *See* `PostGIS EWKB, EWKT and Canonical Forms <http://postgis.refractions.net/docs/ch04.html#id2591381>`_, PostGIS documentation at Ch. 4.1.2.
.. [#fncascadedunion] For more information, read Paul Ramsey's blog post about `(Much) Faster Unions in PostGIS 1.4 <http://blog.cleverelephant.ca/2009/01/must-faster-unions-in-postgis-14.html>`_ and Martin Davis' blog post on `Fast polygon merging in JTS using Cascaded Union <http://lin-ear-th-inking.blogspot.com/2007/11/fast-polygon-merging-in-jts-using.html>`_.

Settings
========

.. setting:: GEOS_LIBRARY_PATH

GEOS_LIBRARY_PATH
-----------------

A string specifying the location of the GEOS C library.  Typically,
this setting is only used if the GEOS C library is in a non-standard
location (e.g., ``/home/bob/lib/libgeos_c.so``).

.. note::

    The setting must be the *full* path to the **C** shared library; in 
    other words you want to use ``libgeos_c.so``, not ``libgeos.so``.