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djangofrdoc / ref / contrib / gis / model-api.txt

.. _ref-gis-model-api:

===================
GeoDjango Model API
===================

.. module:: django.contrib.gis.db.models
   :synopsis: GeoDjango model and field API.

This document explores the details of the GeoDjango Model API.  Throughout this
section, we'll be using the following geographic model of a `ZIP code`__ as our
example::

    from django.contrib.gis.db import models

    class Zipcode(models.Model):
        code = models.CharField(max_length=5)
        poly = models.PolygonField()
        objects = models.GeoManager()

__ http://en.wikipedia.org/wiki/ZIP_code

Geometry Field Types
====================

Each of the following geometry field types correspond with the
OpenGIS Simple Features specification [#fnogc]_.

``GeometryField``
-----------------

.. class:: GeometryField

``PointField``
--------------

.. class:: PointField

``LineStringField``
-------------------

.. class:: LineStringField

``PolygonField``
----------------

.. class:: PolygonField

``MultiPointField``
-------------------

.. class:: MultiPointField

``MultiLineStringField``
------------------------

.. class:: MultiLineStringField

``MultiPolygonField``
---------------------

.. class:: MultiPolygonField

``GeometryCollectionField``
---------------------------

.. class:: GeometryCollectionField

.. _geometry-field-options:

Geometry Field Options
======================

In addition to the regular :ref:`common-model-field-options` available for
Django model fields, geometry fields have the following additional options.
All are optional.

``srid``
--------

.. attribute:: GeometryField.srid

Sets the SRID [#fnogcsrid]_ (Spatial Reference System Identity) of the geometry field to
the given value. Defaults to 4326 (also known as `WGS84`__, units are in degrees
of longitude and latitude).

__ http://en.wikipedia.org/wiki/WGS84

.. _selecting-an-srid:

Selecting an SRID
^^^^^^^^^^^^^^^^^

Choosing an appropriate SRID for your model is an important decision that the
developer should consider carefully.  The SRID is an integer specifier that
corresponds to the projection system that will be used to interpret the data
in the spatial database. [#fnsrid]_  Projection systems give the context to the
coordinates that specify a location.  Although the details of `geodesy`__ are
beyond the scope of this documentation, the general problem is that the earth
is spherical and representations of the earth (e.g., paper maps, Web maps)
are not.

Most people are familiar with using latitude and longitude to reference a
location on the earth's surface.  However, latitude and longitude are angles,
not distances. [#fnharvard]_  In other words, while the shortest path between two points on
a flat surface is a straight line, the shortest path between two points on a curved
surface (such as the earth) is an *arc* of a `great circle`__. [#fnthematic]_  Thus,
additional computation is required to obtain distances in planar units (e.g.,
kilometers and miles).  Using a geographic coordinate system may introduce
complications for the developer later on.  For example, PostGIS versions 1.4
and below do not have the capability to perform distance calculations between
non-point geometries using geographic coordinate systems, e.g., constructing a
query to  find all points within 5 miles of a county boundary stored as WGS84.
[#fndist]_

Portions of the earth's surface may projected onto a two-dimensional, or
Cartesian, plane.  Projected coordinate systems are especially convenient
for region-specific applications, e.g., if you know that your database will
only cover geometries in `North Kansas`__, then you may consider using projection
system specific to that region.  Moreover, projected coordinate systems are
defined in Cartesian units (such as meters or feet), easing distance
calculations.

.. note::

    If you wish to peform arbitrary distance queries using non-point
    geometries in WGS84, consider upgrading to PostGIS 1.5. For
    better performance, enable the :attr:`GeometryField.geography`
    keyword so that :ref:`geography database type <geography-type>`
    is used instead.

Additional Resources:

* `spatialreference.org`__: A Django-powered database of spatial reference
  systems.
* `The State Plane Coordinate System`__: A Web site covering the various
  projection systems used in the United States.  Much of the U.S. spatial
  data encountered will be in one of these coordinate systems rather than
  in a geographic coordinate system such as WGS84.

__ http://en.wikipedia.org/wiki/Geodesy
__ http://en.wikipedia.org/wiki/Great_circle
__ http://www.spatialreference.org/ref/epsg/2796/
__ http://spatialreference.org/
__ http://welcome.warnercnr.colostate.edu/class_info/nr502/lg3/datums_coordinates/spcs.html

``spatial_index``
-----------------

.. attribute:: GeometryField.spatial_index

Defaults to ``True``.  Creates a spatial index for the given geometry
field.

.. note::

    This is different from the ``db_index`` field option because spatial
    indexes are created in a different manner than regular database
    indexes.  Specifically, spatial indexes are typically created using
    a variant of the R-Tree, while regular database indexes typically
    use B-Trees.

``dim``
-------

.. versionadded:: 1.2

.. attribute:: GeometryField.dim

This option may be used for customizing the coordinate dimension of the
geometry field.  By default, it is set to 2, for representing two-dimensional
geometries.  For spatial backends that support it, it may be set to 3 for
three-dimensonal support.

.. note::

    At this time 3D support requires that GEOS 3.1 be installed, and is
    limited only to the PostGIS spatial backend.

``geography``
-------------

.. versionadded:: 1.2

.. attribute:: GeometryField.geography

If set to ``True``, this option will create a database column of
type geography, rather than geometry.  Please refer to the
:ref:`geography type <geography-type>` section below for more
details.

.. note::

    Geography support is limited only to PostGIS 1.5+, and will
    force the SRID to be 4326.

.. _geography-type:

Geography Type
^^^^^^^^^^^^^^

In PostGIS 1.5, the geography type was introduced -- it provides
provides native support for spatial features represented with geographic
coordinates (e.g., WGS84 longitude/latitude). [#fngeography]_
Unlike the plane used by a geometry type, the geography type uses a spherical
representation of its data.  Distance and measurement operations
performed on a geography column automatically employ great circle arc
calculations and return linear units.  In other words, when ``ST_Distance``
is called on two geographies, a value in meters is returned (as opposed
to degrees if called on a geometry column in WGS84).

Because geography calculations involve more mathematics, only a subset of the
PostGIS spatial lookups are available for the geography type. Practically,
this means that in addition to the :ref:`distance lookups <distance-lookups>`
only the following additional :ref:`spatial lookups <spatial-lookups>` are
available for geography columns:

* :lookup:`bboverlaps`
* :lookup:`coveredby`
* :lookup:`covers`
* :lookup:`intersects`

For more information, the PostGIS documentation contains a helpful section on
determining `when to use geography data type over geometry data type
<http://postgis.refractions.net/documentation/manual-1.5/ch04.html#PostGIS_GeographyVSGeometry>`_.

``GeoManager``
==============

.. currentmodule:: django.contrib.gis.db.models
.. class:: GeoManager

In order to conduct geographic queries, each geographic model requires
a ``GeoManager`` model manager.  This manager allows for the proper SQL
construction for geographic queries; thus, without it, all geographic filters
will fail.  It should also be noted that ``GeoManager`` is required even if the
model does not have a geographic field itself, e.g., in the case of a
``ForeignKey`` relation to a model with a geographic field.  For example,
if we had an ``Address`` model with a ``ForeignKey`` to our ``Zipcode``
model::

    from django.contrib.gis.db import models
    from django.contrib.localflavor.us.models import USStateField

    class Address(models.Model):
        num = models.IntegerField()
        street = models.CharField(max_length=100)
        city = models.CharField(max_length=100)
        state = USStateField()
        zipcode = models.ForeignKey(Zipcode)
        objects = models.GeoManager()

The geographic manager is needed to do spatial queries on related ``Zipcode`` objects,
for example::

    qs = Address.objects.filter(zipcode__poly__contains='POINT(-104.590948 38.319914)')

.. rubric:: Footnotes
.. [#fnogc] OpenGIS Consortium, Inc., `Simple Feature Specification For SQL <http://www.opengis.org/docs/99-049.pdf>`_, Document 99-049 (May 5, 1999).
.. [#fnogcsrid] *See id.* at Ch. 2.3.8, p. 39 (Geometry Values and Spatial Reference Systems).
.. [#fnsrid] Typically, SRID integer corresponds to an EPSG (`European Petroleum Survey Group <http://www.epsg.org>`_) identifier.  However, it may also be associated with custom projections defined in spatial database's spatial reference systems table.
.. [#fnharvard] Harvard Graduate School of Design, `An Overview of Geodesy and Geographic Referencing Systems <http://www.gsd.harvard.edu/gis/manual/projections/fundamentals/>`_.  This is an excellent resource for an overview of principles relating to geographic and Cartesian coordinate systems.
.. [#fnthematic] Terry A. Slocum, Robert B. McMaster, Fritz C. Kessler, & Hugh H. Howard, *Thematic Cartography and Geographic Visualization* (Prentice Hall, 2nd edition), at Ch. 7.1.3.
.. [#fndist] This limitation does not apply to PostGIS 1.5.  It should be noted that even in previous versions of PostGIS, this isn't impossible using GeoDjango; you could for example, take a known point in a projected coordinate system, buffer it to the appropriate radius, and then perform an intersection operation with the buffer transformed to the geographic coordinate system.
.. [#fngeography] Please refer to the `PostGIS Geography Type <http://postgis.refractions.net/documentation/manual-1.5/ch04.html#PostGIS_Geography>`_ documentation for more details.