mirror of
https://github.com/django/django.git
synced 2024-12-25 10:35:48 +00:00
260 lines
9.8 KiB
Plaintext
260 lines
9.8 KiB
Plaintext
.. _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://web.archive.org/web/20080302095452/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``
|
|
-------
|
|
|
|
.. 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-dimensional support.
|
|
|
|
.. note::
|
|
|
|
At this time 3D support is limited to the PostGIS spatial backend.
|
|
|
|
``geography``
|
|
-------------
|
|
|
|
.. 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
|
|
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
|
|
|
|
class Address(models.Model):
|
|
num = models.IntegerField()
|
|
street = models.CharField(max_length=100)
|
|
city = models.CharField(max_length=100)
|
|
state = models.CharField(max_length=2)
|
|
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.opengeospatial.org/standards/sfs>`_.
|
|
.. [#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.
|