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.. _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
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may be used outside of a django project/application. In other words,
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no need to have ``DJANGO_SETTINGS_MODULE`` set or use a database, etc.
* Mutability: :class:`GEOSGeometry` objects may be modified.
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* Cross-platform and tested; compatible with Windows, Linux, Solaris, and Mac
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OS X platforms.
.. _geos-tutorial:
Tutorial
========
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This section contains a brief introduction and tutorial to using
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: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'))
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.. _geos-exceptions-in-logfile:
.. admonition:: My logs are filled with GEOS-related errors
You find many ``TypeError`` or ``AttributeError`` exceptions filling your
Web server's log files. This generally means that you are creating GEOS
objects at the top level of some of your Python modules. Then, due to a race
condition in the garbage collector, your module is garbage collected before
the GEOS object. To prevent this, create :class:`GEOSGeometry` objects
inside the local scope of your functions/methods.
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Geometries are Pythonic
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-----------------------
: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])
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:param geo_input: Geometry input value (string or buffer)
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:param srid: spatial reference identifier
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:type srid: int
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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
============= ======================
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WKT / EWKT ``str`` or ``unicode``
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HEX / HEXEWKB ``str`` or ``unicode``
WKB / EWKB ``buffer``
GeoJSON ``str`` or ``unicode``
============= ======================
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.. note::
The new 3D/4D WKT notation with an intermediary Z or M (like
``POINT Z (3, 4, 5)``) is only supported with GEOS 3.3.0 or later.
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Properties
~~~~~~~~~~
.. attribute:: GEOSGeometry.coords
Returns the coordinates of the geometry as a tuple.
.. attribute:: GEOSGeometry.empty
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Returns whether or not the set of points in the geometry is empty.
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.. 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.
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.. attribute:: GEOSGeometry.valid_reason
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Returns a string describing the reason why a geometry is invalid.
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.. attribute:: GEOSGeometry.srid
Property that may be used to retrieve or set the SRID associated with the
geometry. For example::
>>> pnt = Point(5, 23)
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>>> print(pnt.srid)
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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]_
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Essentially the SRID is prepended to the WKT representation, for example
``SRID=4326;POINT(5 23)``.
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.. note::
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The output from this property does not include the 3dm, 3dz, and 4d
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information that PostGIS supports in its EWKT representations.
.. attribute:: GEOSGeometry.hex
Returns the WKB of this Geometry in hexadecimal form. Please note
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that the SRID value is not included in this representation
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because it is not a part of the OGC specification (use the
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:attr:`GEOSGeometry.hexewkb` property instead).
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.. versionchanged:: 1.5
Prior to Django 1.5, the Z value of the geometry was dropped.
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.. attribute:: GEOSGeometry.hexewkb
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Returns the EWKB of this Geometry in hexadecimal form. This is an
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extension of the WKB specification that includes the SRID value
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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
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geometry. This should only be used for geometries with an SRID of
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4326 (WGS84), but this restriction is not enforced.
.. attribute:: GEOSGeometry.ogr
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Returns an :class:`~django.contrib.gis.gdal.OGRGeometry` object
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correspondg to the GEOS geometry.
.. note::
Requires GDAL.
.. _wkb:
.. attribute:: GEOSGeometry.wkb
Returns the WKB (Well-Known Binary) representation of this Geometry
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as a Python buffer. SRID value is not included, use the
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:attr:`GEOSGeometry.ewkb` property instead.
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.. versionchanged:: 1.5
Prior to Django 1.5, the Z value of the geometry was dropped.
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.. _ewkb:
.. attribute:: GEOSGeometry.ewkb
Return the EWKB representation of this Geometry as a Python buffer.
This is an extension of the WKB specification that includes any SRID
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value that are a part of this geometry.
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.. 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).
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__ https://developers.google.com/kml/documentation/
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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
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is ``T*T******`` (for a point and a curve,a point and an area or a line
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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)
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Returns ``True`` if the DE-9IM intersection matrix for the two geometries
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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)
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Returns ``True`` if the elements in the DE-9IM intersection matrix
for this geometry and the other matches the given ``pattern`` --
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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
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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
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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.
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.. method:: GEOSGeometry.interpolate(distance)
.. method:: GEOSGeometry.interpolate_normalized(distance)
.. versionadded:: 1.5
Given a distance (float), returns the point (or closest point) within the
geometry (:class:`LineString` or :class:`MultiLineString`) at that distance.
The normalized version takes the distance as a float between 0 (origin) and 1
(endpoint).
Reverse of :meth:`GEOSGeometry.project`.
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.. method:: GEOSGeometry:intersection(other)
Returns a :class:`GEOSGeometry` representing the points shared by this
geometry and other.
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.. method:: GEOSGeometry.project(point)
.. method:: GEOSGeometry.project_normalized(point)
.. versionadded:: 1.5
Returns the distance (float) from the origin of the geometry
(:class:`LineString` or :class:`MultiLineString`) to the point projected on the
geometry (that is to a point of the line the closest to the given point).
The normalized version returns the distance as a float between 0 (origin) and 1
(endpoint).
Reverse of :meth:`GEOSGeometry.interpolate`.
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.. 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.
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By default, this function does not preserve topology - e.g.,
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: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
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dimension and number of components as the input, however, this is
significantly slower.
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.. method:: GEOSGeometry.sym_difference(other)
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Returns a :class:`GEOSGeometry` combining the points in this geometry
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not in other, and the points in other not in this geometry.
.. method:: GEOSGeometry.union(other)
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Returns a :class:`GEOSGeometry` representing all the points in this
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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
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This property returns the extent of this geometry as a 4-tuple,
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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
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perform a spherical calculation even if the SRID specifies a geographic
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coordinate system.
.. attribute:: GEOSGeometry.length
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Returns the length of this geometry (e.g., 0 for a :class:`Point`,
the length of a :class:`LineString`, or the circumference of
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a :class:`Polygon`).
.. attribute:: GEOSGeometry.prepared
.. note::
Support for prepared geometries requires GEOS 3.1.
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Returns a GEOS ``PreparedGeometry`` for the contents of this geometry.
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``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.
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.. method:: GEOSGeometry.transform(ct, clone=False)
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Transforms the geometry according to the given coordinate transformation paramter
(``ct``), which may be an integer SRID, spatial reference WKT string,
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a PROJ.4 string, a :class:`~django.contrib.gis.gdal.SpatialReference` object, or a
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: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.
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.. note::
Prior to 1.3, this method would silently no-op if GDAL was not available.
Now, a :class:`~django.contrib.gis.geos.GEOSException` is raised as
application code relying on this behavior is in error. In addition,
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use of this method when the SRID is ``None`` or less than 0 now also generates
a :class:`~django.contrib.gis.geos.GEOSException`.
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``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)
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.
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.. admonition:: Comparing Polygons
Note that it is possible to compare ``Polygon`` objects directly with ``<``
or ``>``, but as the comparison is made through Polygon's
:class:`LineString`, it does not mean much (but is consistent and quick).
You can always force the comparison with the :attr:`~GEOSGeometry.area`
property::
>>> if poly_1.area > poly_2.area:
>>> pass
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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
Returns a :class:`LineString` representing the line merge of
all the components in this ``MultiLineString``.
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``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
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
===================
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
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geometry can be orders of magnitude faster -- the more complex the geometry
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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
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:type srid: int
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: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
===========
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
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include the SRID value (in other words, EWKB).
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.. class:: WKBWriter
``WKBWriter`` provides the most control over its output. By default it
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returns OGC-compliant WKB when its ``write`` method is called. However,
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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.
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3 Output 3D WKB.
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============ ===========================
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
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.. [#fnogc] *See* `PostGIS EWKB, EWKT and Canonical Forms <http://postgis.refractions.net/docs/using_postgis_dbmanagement.html#EWKB_EWKT>`_, PostGIS documentation at Ch. 4.1.2.
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.. [#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::
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The setting must be the *full* path to the **C** shared library; in
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other words you want to use ``libgeos_c.so``, not ``libgeos.so``.
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Exceptions
==========
.. exception:: GEOSException
The base GEOS exception, indicates a GEOS-related error.
