Source

djangofrdoc / ref / contrib / gis / gdal.txt

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.. _ref-gdal:

========
GDAL API
========

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

`GDAL`__ stands for **G**\ eospatial **D**\ ata **A**\ bstraction **L**\ ibrary,
and is a veritable "swiss army knife" of GIS data functionality.  A subset
of GDAL is the `OGR`__ Simple Features Library, which specializes
in reading and writing vector geographic data in a variety of standard
formats.

GeoDjango provides a high-level Python interface for some of the 
capabilities of OGR, including the reading and coordinate transformation
of vector spatial data.

.. note::

     Although the module is named ``gdal``, GeoDjango only supports
     some of the capabilities of OGR.  Thus, none of GDAL's features
     with respect to raster (image) data are supported at this time.
   
__ http://www.gdal.org/
__ http://www.gdal.org/ogr/

Overview
========

Sample Data
-----------

The GDAL/OGR tools described here are designed to help you read in
your geospatial data, in order for most of them to be useful you have
to have some data to work with.  If you're starting out and don't yet
have any data of your own to use, GeoDjango comes with a number of
simple data sets that you can use for testing.  This snippet will
determine where these sample files are installed on your computer::

    >>> import os
    >>> import django.contrib.gis
    >>> GIS_PATH = os.path.dirname(django.contrib.gis.__file__)
    >>> CITIES_PATH = os.path.join(GIS_PATH, 'tests/data/cities/cities.shp')

Vector Data Source Objects
==========================

``DataSource``
--------------

:class:`DataSource` is a wrapper for the OGR data source object that
supports reading data from a variety of OGR-supported geospatial file
formats and data sources using a simple, consistent interface.  Each
data source is represented by a :class:`DataSource` object which contains
one or more layers of data.  Each layer, represented by a :class:`Layer`
object, contains some number of geographic features (:class:`Feature`),
information about the type of features contained in that layer (e.g.
points, polygons, etc.), as well as the names and types of any
additional fields (:class:`Field`) of data that may be associated with
each feature in that layer.

.. class:: DataSource(ds_input)

   The constructor for ``DataSource`` just a single parameter: the path of
   the file you want to read.  However, OGR
   also supports a variety of more complex data sources, including
   databases, that may be accessed by passing a special name string instead
   of a path.  For more information, see the `OGR Vector Formats`__
   documentation.  The :attr:`name` property of a ``DataSource`` 
   instance gives the OGR name of the underlying data source that it is
   using.

   Once you've created your ``DataSource``, you can find out how many 
   layers of data it contains by accessing the :attr:`layer_count` property, 
   or (equivalently) by using the ``len()`` function.  For information on 
   accessing the layers of data themselves, see the next section::

       >>> from django.contrib.gis.gdal import DataSource
       >>> ds = DataSource(CITIES_PATH)
       >>> ds.name                         # The exact filename may be different on your computer
       '/usr/local/lib/python2.6/site-packages/django/contrib/gis/tests/data/cities/cities.shp'
       >>> ds.layer_count                  # This file only contains one layer
       1

   .. attribute:: layer_count

   Returns the number of layers in the data source.

   .. attribute:: name

   Returns the name of the data source.

__ http://www.gdal.org/ogr/ogr_formats.html

``Layer``
---------

.. class:: Layer

   ``Layer`` is a wrapper for a layer of data in a ``DataSource`` object.
   You never create a ``Layer`` object directly.  Instead, you retrieve
   them from a :class:`DataSource` object, which is essentially a standard
   Python container of ``Layer`` objects.  For example, you can access a
   specific layer by its index (e.g. ``ds[0]`` to access the first
   layer), or you can iterate over all the layers in the container in a
   ``for`` loop.  The ``Layer`` itself acts as a container for geometric 
   features.

   Typically, all the features in a given layer have the same geometry type.
   The :attr:`geom_type` property of a layer is an :class:`OGRGeomType`
   that identifies the feature type.  We can use it to print out some basic
   information about each layer in a :class:`DataSource`::

       >>> for layer in ds:
       ...     print 'Layer "%s": %i %ss' % (layer.name, len(layer), layer.geom_type.name)
       ...
       Layer "cities": 3 Points

   The example output is from the cities data source, loaded above, which
   evidently contains one layer, called ``"cities"``, which contains three
   point features.  For simplicity, the examples below assume that you've 
   stored that layer in the variable ``layer``::

       >>> layer = ds[0]

   .. attribute:: name

   Returns the name of this layer in the data source.

       >>> layer.name
       'cities'

   .. attribute:: num_feat

   Returns the number of features in the layer.  Same as ``len(layer)``::

       >>> layer.num_feat
       3

   .. attribute:: geom_type

   Returns the geometry type of the layer, as an :class:`OGRGeomType`
   object::

       >>> layer.geom_type.name
       'Point'

   .. attribute:: num_fields

   Returns the number of fields in the layer, i.e the number of fields of
   data associated with each feature in the layer::

       >>> layer.num_fields
       4

   .. attribute:: fields

   Returns a list of the names of each of the fields in this layer::

       >>> layer.fields
       ['Name', 'Population', 'Density', 'Created']

   .. attribute field_types

   Returns a list of the data types of each of the fields in this layer.
   These are subclasses of ``Field``, discussed below::

       >>> [ft.__name__ for ft in layer.field_types]
       ['OFTString', 'OFTReal', 'OFTReal', 'OFTDate']
 
   .. attribute:: field_widths

   Returns a list of the maximum field widths for each of the fields in
   this layer::

      >>> layer.field_widths
      [80, 11, 24, 10]

   .. attribute:: field_precisions

   Returns a list of the numeric precisions for each of the fields in
   this layer.  This is meaningless (and set to zero) for non-numeric 
   fields::

       >>> layer.field_precisions
       [0, 0, 15, 0]

   .. attribute:: extent

   Returns the spatial extent of this layer, as an :class:`Envelope` 
   object::

      >>> layer.extent.tuple
      (-104.609252, 29.763374, -95.23506, 38.971823)

   .. attribute:: srs

   Property that returns the :class:`SpatialReference` associated
   with this layer::

       >>> print layer.srs
       GEOGCS["GCS_WGS_1984",
           DATUM["WGS_1984",
               SPHEROID["WGS_1984",6378137,298.257223563]],
           PRIMEM["Greenwich",0],
           UNIT["Degree",0.017453292519943295]]

   If the :class:`Layer` has no spatial reference information associated
   with it, ``None`` is returned.

   .. attribute:: spatial_filter

   .. versionadded:: 1.2

   Property that may be used to retrieve or set a spatial filter for this
   layer.  A spatial filter can only be set with an :class:`OGRGeometry`
   instance, a 4-tuple extent, or ``None``.  When set with something 
   other than ``None``, only features that intersect the filter will be
   returned when iterating over the layer::

       >>> print layer.spatial_filter
       None
       >>> print len(layer)
       3
       >>> [feat.get('Name') for feat in layer]
       ['Pueblo', 'Lawrence', 'Houston']
       >>> ks_extent = (-102.051, 36.99, -94.59, 40.00) # Extent for state of Kansas
       >>> layer.spatial_filter = ks_extent
       >>> len(layer)
       1
       >>> [feat.get('Name') for feat in layer]
       ['Lawrence']
       >>> layer.spatial_filter = None
       >>> len(layer)
       3

   .. method:: get_fields()

   A method that returns a list of the values of a given field for each
   feature in the layer::

      >>> layer.get_fields('Name')
      ['Pueblo', 'Lawrence', 'Houston']

   .. method:: get_geoms([geos=False])

   A method that returns a list containing the geometry of each feature
   in the layer.  If the optional argument ``geos`` is set to ``True``
   then the geometries are converted to :class:`~django.contrib.gis.geos.GEOSGeometry`
   objects. Otherwise, they are returned as :class:`OGRGeometry` objects::

       >>> [pt.tuple for pt in layer.get_geoms()]
       [(-104.609252, 38.255001), (-95.23506, 38.971823), (-95.363151, 29.763374)]

   .. method:: test_capability(capability)

   Returns a boolean indicating whether this layer supports the
   given capability (a string).  Examples of valid capability strings
   include: ``'RandomRead'``, ``'SequentialWrite'``, ``'RandomWrite'``,
   ``'FastSpatialFilter'``, ``'FastFeatureCount'``, ``'FastGetExtent'``,
   ``'CreateField'``, ``'Transactions'``, ``'DeleteFeature'``, and 
   ``'FastSetNextByIndex'``.
   
``Feature``
-----------

.. class:: Feature


   ``Feature`` wraps an OGR feature.  You never create a ``Feature``
   object directly.  Instead, you retrieve them from a :class:`Layer` object.
   Each feature consists of a geometry and a set of fields containing
   additional properties.  The geometry of a field is accessible via its
   ``geom`` property, which returns an :class:`OGRGeometry` object.  A ``Feature``
   behaves like a standard Python container for its fields, which it returns as
   :class:`Field` objects: you can access a field directly by its index or name,
   or you can iterate over a feature's fields, e.g. in a ``for`` loop.

   .. attribute:: geom

   Returns the geometry for this feature, as an ``OGRGeometry`` object::

       >>> city.geom.tuple
       (-104.609252, 38.255001)

   .. attribute:: get

   A method that returns the value of the given field (specified by name)
   for this feature, **not** a ``Field`` wrapper object::

       >>> city.get('Population')
       102121

   .. attribute:: geom_type

   Returns the type of geometry for this feature, as an :class:`OGRGeomType`
   object.  This will be the same for all features in a given layer, and
   is equivalent to the :attr:`Layer.geom_type` property of the 
   :class:`Layer`` object the feature came from.

   .. attribute:: num_fields

   Returns the number of fields of data associated with the feature.
   This will be the same for all features in a given layer, and is
   equivalent to the :attr:`Layer.num_fields` property of the 
   :class:`Layer` object the feature came from.

   .. attribute:: fields

   Returns a list of the names of the fields of data associated with the
   feature.  This will be the same for all features in a given layer, and
   is equivalent to the :attr:`Layer.fields` property of the :class:`Layer`
   object the feature came from.

   .. attribute:: fid

   Returns the feature identifier within the layer::

       >>> city.fid
       0

   .. attribute:: layer_name

   Returns the name of the :class:`Layer` that the feature came from.
   This will be the same for all features in a given layer::

       >>> city.layer_name
       'cities'

   .. attribute:: index

   A method that returns the index of the given field name.  This will be
   the same for all features in a given layer::

       >>> city.index('Population')
       1

``Field``
---------

.. class:: Field

   .. attribute:: name

   Returns the name of this field::

       >>> city['Name'].name
       'Name'

   .. attribute:: type

   Returns the OGR type of this field, as an integer.  The
   ``FIELD_CLASSES`` dictionary maps these values onto 
   subclasses of ``Field``::

       >>> city['Density'].type
       2

   .. attribute:: type_name

   Returns a string with the name of the data type of this field::

       >>> city['Name'].type_name
       'String'

   .. attribute:: value

   Returns the value of this field.  The ``Field`` class itself 
   returns the value as a string, but each subclass returns the 
   value in the most appropriate form::

       >>> city['Population'].value
       102121

   .. attribute:: width

   Returns the width of this field::

       >>> city['Name'].width
       80

   .. attribute:: precision

   Returns the numeric precision of this field.  This is meaningless (and
   set to zero) for non-numeric fields::

       >>> city['Density'].precision
       15

   .. method:: as_double()

   Returns the value of the field as a double (float)::

       >>> city['Density'].as_double()
       874.7

   .. method:: as_int()

   Returns the value of the field as an integer::

       >>> city['Population'].as_int()
       102121

   .. method:: as_string()

   Returns the value of the field as a string::

       >>> city['Name'].as_string()
       'Pueblo'

   .. method:: as_datetime()

   Returns the value of the field as a tuple of date and time components::

       >>> city['Created'].as_datetime()
       (c_long(1999), c_long(5), c_long(23), c_long(0), c_long(0), c_long(0), c_long(0))

``Driver``
----------

.. class:: Driver(dr_input)

   The ``Driver`` class is used internally to wrap an OGR :class:`DataSource` driver.

   .. attribute:: driver_count

   Returns the number of OGR vector drivers currently registered.


OGR Geometries
==============

``OGRGeometry``
---------------

:class:`OGRGeometry` objects share similar functionality with 
:class:`~django.contrib.gis.geos.GEOSGeometry` objects, and are thin
wrappers around OGR's internal geometry representation.  Thus, 
they allow for more efficient access to data when using :class:`DataSource`. 
Unlike its GEOS counterpart, :class:`OGRGeometry` supports spatial reference
systems and coordinate transformation::

    >>> from django.contrib.gis.gdal import OGRGeometry
    >>> polygon = OGRGeometry('POLYGON((0 0, 5 0, 5 5, 0 5))')

.. class:: OGRGeometry(geom_input[, srs=None])

   This object is a wrapper for the `OGR Geometry`__ class.
   These objects are instantiated directly from the given ``geom_input`` 
   parameter, which may be a string containing WKT or HEX, a ``buffer``
   containing WKB data, or an :class:`OGRGeomType` object. These objects
   are also returned from the :class:`Feature.geom` attribute, when 
   reading vector data from :class:`Layer` (which is in turn a part of
   a :class:`DataSource`).

   __ http://www.gdal.org/ogr/classOGRGeometry.html

   .. classmethod:: from_bbox(bbox)

   Constructs a :class:`Polygon` from the given bounding-box (a 4-tuple).

   .. method:: __len__

   Returns the number of points in a :class:`LineString`, the
   number of rings in a :class:`Polygon`, or the number of geometries in a
   :class:`GeometryCollection`. Not applicable to other geometry types.

   .. method:: __iter__

   Iterates over the points in a :class:`LineString`, the rings in a
   :class:`Polygon`, or the geometries in a :class:`GeometryCollection`.
   Not applicable to other geometry types.

   .. method:: __getitem__

   Returns the point at the specified index for a :class:`LineString`, the
   interior ring at the specified index for a :class:`Polygon`, or the geometry
   at the specified index in a :class:`GeometryCollection`.  Not applicable to
   other geometry types.

   .. attribute:: dimension

   Returns the number of coordinated dimensions of the geometry, i.e. 0
   for points, 1 for lines, and so forth::

       >> polygon.dimension
       2

   .. attribute:: coord_dim

   .. versionchanged:: 1.2

   Returns or sets the coordinate dimension of this geometry.  For
   example, the value would be 2 for two-dimensional geometries.

   .. note::

      Setting this property is only available in versions 1.2 and above.

   .. attribute:: geom_count

   Returns the number of elements in this geometry::

       >>> polygon.geom_count
       1

   .. attribute:: point_count

   Returns the number of points used to describe this geometry::

      >>> polygon.point_count
      4

   .. attribute:: num_points

   Alias for :attr:`point_count`.

   .. attribute:: num_coords

   Alias for :attr:`point_count`.

   .. attribute:: geom_type

   Returns the type of this geometry, as an :class:`OGRGeomType` object.

   .. attribute:: geom_name

   Returns the name of the type of this geometry::

       >>> polygon.geom_name
       'POLYGON'

   .. attribute:: area

   Returns the area of this geometry, or 0 for geometries that do not
   contain an area::

       >>> polygon.area
       25.0

   .. attribute:: envelope

   Returns the envelope of this geometry, as an :class:`Envelope` object.

   .. attribute:: extent

   Returns the envelope of this geometry as a 4-tuple, instead of as an
   :class:`Envelope` object::

       >>> point.extent
       (0.0, 0.0, 5.0, 5.0)

   .. attribute:: srs

   This property controls the spatial reference for this geometry, or
   ``None`` if no spatial reference system has been assigned to it.
   If assigned, accessing this property returns a :class:`SpatialReference`
   object.  It may be set with another :class:`SpatialReference` object,
   or any input that :class:`SpatialReference` accepts. Example::

       >>> city.geom.srs.name
       'GCS_WGS_1984'

   .. attribute:: srid

   Returns or sets the spatial reference identifier corresponding to 
   :class:`SpatialReference` of this geometry.  Returns ``None`` if
   there is no spatial reference information associated with this
   geometry, or if an SRID cannot be determined.

   .. attribute:: geos

   Returns a :class:`~django.contrib.gis.geos.GEOSGeometry` object 
   corresponding to this geometry.

   .. attribute:: gml

   Returns a string representation of this geometry in GML format::

       >>> OGRGeometry('POINT(1 2)').gml
       '<gml:Point><gml:coordinates>1,2</gml:coordinates></gml:Point>'

   .. attribute:: hex

   Returns a string representation of this geometry in HEX WKB format::

       >>> OGRGeometry('POINT(1 2)').hex
       '0101000000000000000000F03F0000000000000040'

   .. attribute:: json

   Returns a string representation of this geometry in JSON format::

       >>> OGRGeometry('POINT(1 2)').json
       '{ "type": "Point", "coordinates": [ 1.000000, 2.000000 ] }'


   .. attribute:: kml

   Returns a string representation of this geometry in KML format.

   .. attribute:: wkb_size

   Returns the size of the WKB buffer needed to hold a WKB representation
   of this geometry::

       >>> OGRGeometry('POINT(1 2)').wkb_size
       21

   .. attribute:: wkb

   Returns a ``buffer`` containing a WKB representation of this geometry.

   .. attribute:: wkt

   Returns a string representation of this geometry in WKT format.

   .. attribute:: ewkt

   .. versionadded:: 1.2

   Returns the EWKT representation of this geometry.

   .. method:: clone()

   Returns a new :class:`OGRGeometry` clone of this geometry object.

   .. method:: close_rings()

   If there are any rings within this geometry that have not been closed,
   this routine will do so by adding the starting point to the end::

       >>> triangle = OGRGeometry('LINEARRING (0 0,0 1,1 0)')
       >>> triangle.close_rings()
       >>> triangle.wkt
       'LINEARRING (0 0,0 1,1 0,0 0)'

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

   Transforms this geometry to a different spatial reference system.  May
   take a :class:`CoordTransform` object, a :class:`SpatialReference` object,
   or any other input accepted by :class:`SpatialReference` (including
   spatial reference WKT and PROJ.4 strings, or an integer SRID).
   By default nothing is returned and the geometry is transformed in-place.
   However, if the `clone` keyword is set to ``True`` then a transformed clone
   of this geometry is returned instead.

   .. method:: intersects(other)

   Returns ``True`` if this geometry intersects the other, otherwise returns
   ``False``.

   .. method:: equals(other)

   Returns ``True`` if this geometry is equivalent to the other, otherwise returns
   ``False``.

   .. method:: disjoint(other)

   Returns ``True`` if this geometry is spatially disjoint to (i.e. does
   not intersect) the other, otherwise returns ``False``.

   .. method:: touches(other)

   Returns ``True`` if this geometry touches the other, otherwise returns
   ``False``.

   .. method:: crosses(other)

   Returns ``True`` if this geometry crosses the other, otherwise returns
   ``False``.

   .. method:: within(other)

   Returns ``True`` if this geometry is contained within the other, otherwise returns
   ``False``.

   .. method:: contains(other)

   Returns ``True`` if this geometry contains the other, otherwise returns
   ``False``.

   .. method:: overlaps(other)

   Returns ``True`` if this geometry overlaps the other, otherwise returns
   ``False``.

   .. method:: boundary

   The boundary of this geometry, as a new :class:`OGRGeometry` object.

   .. attribute:: convex_hull

   The smallest convex polygon that contains this geometry, as a new
   :class:`OGRGeometry` object.

   .. method:: difference

   Returns the region consisting of the difference of this geometry and
   the other, as a new :class:`OGRGeometry` object.

   .. method:: intersection

   Returns the region consisting of the intersection of this geometry and
   the other, as a new :class:`OGRGeometry` object.

   .. method:: sym_difference

   Returns the region consisting of the symmetric difference of this
   geometry and the other, as a new :class:`OGRGeometry` object.

   .. method:: union

   Returns the region consisting of the union of this geometry and
   the other, as a new :class:`OGRGeometry` object.

   .. attribute:: tuple

   Returns the coordinates of a point geometry as a tuple, the
   coordinates of a line geometry as a tuple of tuples, and so forth::

       >>> OGRGeometry('POINT (1 2)').tuple
       (1.0, 2.0)
       >>> OGRGeometry('LINESTRING (1 2,3 4)').tuple
       ((1.0, 2.0), (3.0, 4.0))

   .. attribute:: coords

   An alias for :attr:`tuple`.

.. class:: Point

   .. attribute:: x

   Returns the X coordinate of this point::

       >>> OGRGeometry('POINT (1 2)').x
       1.0

   .. attribute:: y

   Returns the Y coordinate of this point::

       >>> OGRGeometry('POINT (1 2)').y
       2.0

   .. attribute:: z

   Returns the Z coordinate of this point, or ``None`` if the
   the point does not have a Z coordinate::

       >>> OGRGeometry('POINT (1 2 3)').z
       3.0

.. class:: LineString

   .. attribute:: x

   Returns a list of X coordinates in this line::

       >>> OGRGeometry('LINESTRING (1 2,3 4)').x
       [1.0, 3.0]

   .. attribute:: y

   Returns a list of Y coordinates in this line::

       >>> OGRGeometry('LINESTRING (1 2,3 4)').y
       [2.0, 4.0]

   .. attribute:: z

   Returns a list of Z coordinates in this line, or ``None`` if the 
   line does not have Z coordinates::
 
       >>> OGRGeometry('LINESTRING (1 2 3,4 5 6)').z
       [3.0, 6.0]


.. class:: Polygon

   .. attribute:: shell

   Returns the shell or exterior ring of this polygon, as a ``LinearRing``
   geometry.

   .. attribute:: exterior_ring

   An alias for :attr:`shell`.

   .. attribute:: centroid

   Returns a :class:`Point` representing the centroid of this polygon.

.. class:: GeometryCollection

   .. method:: add(geom)

   Adds a geometry to this geometry collection.  Not applicable to other
   geometry types.


``OGRGeomType``
---------------

.. class:: OGRGeomType(type_input)

   This class allows for the representation of an OGR geometry type
   in any of several ways::

       >>> from django.contrib.gis.gdal import OGRGeomType
       >>> gt1 = OGRGeomType(3)             # Using an integer for the type
       >>> gt2 = OGRGeomType('Polygon')     # Using a string
       >>> gt3 = OGRGeomType('POLYGON')     # It's case-insensitive
       >>> print gt1 == 3, gt1 == 'Polygon' # Equivalence works w/non-OGRGeomType objects
       True True

   .. attribute:: name

   Returns a short-hand string form of the OGR Geometry type::

       >>> gt1.name
       'Polygon'

   .. attribute:: num

   Returns the number corresponding to the OGR geometry type::

       >>> gt1.num
       3

   .. attribute:: django

   Returns the Django field type (a subclass of GeometryField) to use for
   storing this OGR type, or ``None`` if there is no appropriate Django
   type::

       >>> gt1.django
       'PolygonField'

``Envelope``
------------

.. class:: Envelope(*args)

   Represents an OGR Envelope structure that contains the
   minimum and maximum X, Y coordinates for a rectangle bounding box.
   The naming of the variables is compatible with the OGR Envelope
   C structure.

   .. attribute:: min_x

   The value of the minimum X coordinate.

   .. attribute:: min_y

   The value of the maximum X coordinate.

   .. attribute:: max_x

   The value of the minimum Y coordinate.

   .. attribute:: max_y

   The value of the maximum Y coordinate.

   .. attribute:: ur

   The upper-right coordinate, as a tuple.

   .. attribute:: ll

   The lower-left coordinate, as a tuple.

   .. attribute:: tuple

   A tuple representing the envelope.

   .. attribute:: wkt

   A string representing this envelope as a polygon in WKT format.


   .. method:: expand_to_include(self, *args)

Coordinate System Objects
=========================

``SpatialReference``
--------------------

.. class:: SpatialReference(srs_input)

   Spatial reference objects are initialized on the given ``srs_input``,
   which may be one of the following:
 
   * OGC Well Known Text (WKT) (a string)
   * EPSG code (integer or string)
   * PROJ.4 string
   * A shorthand string for well-known standards (``'WGS84'``, ``'WGS72'``, ``'NAD27'``, ``'NAD83'``)

   Example::

       >>> wgs84 = SpatialReference('WGS84') # shorthand string
       >>> wgs84 = SpatialReference(4326) # EPSG code
       >>> wgs84 = SpatialReference('EPSG:4326') # EPSG string
       >>> proj4 = '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs '
       >>> wgs84 = SpatialReference(proj4) # PROJ.4 string
       >>> wgs84 = SpatialReference("""GEOGCS["WGS 84",
       DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.01745329251994328,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4326"]]""") # OGC WKT

   .. method:: __getitem__(target)

   Returns the value of the given string attribute node, ``None`` if the node
   doesn't exist.  Can also take a tuple as a parameter, (target, child), 
   where child is the index of the attribute in the WKT.  For example::

       >>> wkt = 'GEOGCS["WGS 84", DATUM["WGS_1984, ... AUTHORITY["EPSG","4326"]]')
       >>> srs = SpatialReference(wkt) # could also use 'WGS84', or 4326
       >>> print srs['GEOGCS']
       WGS 84
       >>> print srs['DATUM']
       WGS_1984
       >>> print srs['AUTHORITY']
       EPSG
       >>> print srs['AUTHORITY', 1] # The authority value
       4326
       >>> print srs['TOWGS84', 4] # the fourth value in this wkt
       0
       >>> print srs['UNIT|AUTHORITY'] # For the units authority, have to use the pipe symbole.
       EPSG
       >>> print srs['UNIT|AUTHORITY', 1] # The authority value for the untis
       9122

   .. method:: attr_value(target, index=0)

   The attribute value for the given target node (e.g. ``'PROJCS'``).
   The index keyword specifies an index of the child node to return.

   .. method:: auth_name(target)

   Returns the authority name for the given string target node.

   .. method:: auth_code(target)

   Returns the authority code for the given string target node.

   .. method:: clone()

   Returns a clone of this spatial reference object.

   .. method:: identify_epsg()

   This method inspects the WKT of this SpatialReference, and will
   add EPSG authority nodes where an EPSG identifier is applicable.

   .. method:: from_esri()

   Morphs this SpatialReference from ESRI's format to EPSG

   .. method:: to_esri()

   Morphs this SpatialReference to ESRI's format.

   .. method:: validate()

   Checks to see if the given spatial reference is valid, if not
   an exception will be raised.

   .. method:: import_epsg(epsg)

   Import spatial reference from EPSG code.

   .. method:: import_proj(proj)

   Import spatial reference from PROJ.4 string.

   .. method:: import_user_input(user_input)

   .. method:: import_wkt(wkt)

   Import spatial reference from WKT.

   .. method:: import_xml(xml)

   Import spatial reference from XML.

   .. attribute:: name

   Returns the name of this Spatial Reference.

   .. attribute:: srid

   Returns the SRID of top-level authority, or ``None`` if undefined.

   .. attribute:: linear_name

   Returns the name of the linear units.

   .. attribute:: linear_units

   Returns the value of the linear units.

   .. attribute:: angular_name

   Returns the name of the angular units."

   .. attribute:: angular_units

   Returns the value of the angular units.

   .. attribute:: units

   Returns a 2-tuple of the units value and the units name, 
   and will automatically determines whether to return the linear
   or angular units.

   .. attribute:: ellisoid

   Returns a tuple of the ellipsoid parameters for this spatial
   reference: (semimajor axis, semiminor axis, and inverse flattening)

   .. attribute:: semi_major

   Returns the semi major axis of the ellipsoid for this spatial reference.

   .. attribute:: semi_minor

   Returns the semi minor axis of the ellipsoid for this spatial reference.

   .. attribute:: inverse_flattening

   Returns the inverse flattening of the ellipsoid for this spatial reference.

   .. attribute:: geographic

   Returns ``True`` if this spatial reference is geographic
   (root node is ``GEOGCS``).

   .. attribute:: local

   Returns ``True`` if this spatial reference is local
   (root node is ``LOCAL_CS``).

   .. attribute:: projected

   Returns ``True`` if this spatial reference is a projected coordinate
   system (root node is ``PROJCS``).

   .. attribute:: wkt

   Returns the WKT representation of this spatial reference.

   .. attribute:: pretty_wkt

   Returns the 'pretty' representation of the WKT.

   .. attribute:: proj

   Returns the PROJ.4 representation for this spatial reference.

   .. attribute:: proj4

   Alias for :attr:`SpatialReference.proj`.

   .. attribute:: xml

   Returns the XML representation of this spatial reference.


``CoordTransform``
------------------

.. class:: CoordTransform(source, target)

Represents a coordinate system transform.  It is initialized with two 
:class:`SpatialReference`, representing the source and target coordinate
systems, respectively.  These objects should be used when performing
the same coordinate transformation repeatedly on different geometries::

    >>> ct = CoordTransform(SpatialReference('WGS84'), SpatialReference('NAD83'))
    >>> for feat in layer:
    ...     geom = feat.geom # getting clone of feature geometry
    ...     geom.transform(ct) # transforming

Settings
========

.. setting:: GDAL_LIBRARY_PATH

GDAL_LIBRARY_PATH
-----------------

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