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439 lines
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439 lines
15 KiB
Plaintext
=============================
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Geographic Database Functions
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=============================
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.. module:: django.contrib.gis.db.models.functions
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:synopsis: Geographic Database Functions
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.. versionadded:: 1.9
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The functions documented on this page allow users to access geographic database
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functions to be used in annotations, aggregations, or filters in Django.
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Example::
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>>> from django.contrib.gis.db.models.functions import Length
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>>> Track.objects.annotate(length=Length('line')).filter(length__gt=100)
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Not all backends support all functions, so refer to the documentation of each
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function to see if your database backend supports the function you want to use.
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If you call a geographic function on a backend that doesn't support it, you'll
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get a ``NotImplementedError`` exception.
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Function's summary:
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================== ======================= ====================== =================== ================== =====================
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Measurement Relationships Operations Editors Output format Miscellaneous
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================== ======================= ====================== =================== ================== =====================
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:class:`Area` :class:`BoundingCircle` :class:`Difference` :class:`ForceRHR` :class:`AsGeoJSON` :class:`MemSize`
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:class:`Distance` :class:`Centroid` :class:`Intersection` :class:`Reverse` :class:`AsGML` :class:`NumGeometries`
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:class:`Length` :class:`Envelope` :class:`SymDifference` :class:`Scale` :class:`AsKML` :class:`NumPoints`
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:class:`Perimeter` :class:`PointOnSurface` :class:`Union` :class:`SnapToGrid` :class:`AsSVG`
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:class:`Transform` :class:`GeoHash`
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:class:`Translate`
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================== ======================= ====================== =================== ================== =====================
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Area
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----
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.. class:: Area(expression, **extra)
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*Availability*: MySQL, Oracle, PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns the area of the
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field as an :class:`~django.contrib.gis.measure.Area` measure. On MySQL, a raw
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float value is returned, as it's not possible to automatically determine the
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unit of the field.
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AsGeoJSON
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---------
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.. class:: AsGeoJSON(expression, bbox=False, crs=False, precision=8, **extra)
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*Availability*: PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns a `GeoJSON
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<http://geojson.org/>`_ representation of the geometry.
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Example::
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>>> City.objects.annotate(json=AsGeoJSON('point')).get(name='Chicago').json
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{"type":"Point","coordinates":[-87.65018,41.85039]}
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===================== =====================================================
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Keyword Argument Description
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===================== =====================================================
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``bbox`` Set this to ``True`` if you want the bounding box
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to be included in the returned GeoJSON.
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``crs`` Set this to ``True`` if you want the coordinate
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reference system to be included in the returned
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GeoJSON.
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``precision`` It may be used to specify the number of significant
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digits for the coordinates in the GeoJSON
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representation -- the default value is 8.
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===================== =====================================================
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AsGML
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-----
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.. class:: AsGML(expression, version=2, precision=8, **extra)
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*Availability*: Oracle, PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns a `Geographic Markup
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Language (GML)`__ representation of the geometry.
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Example::
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>>> qs = Zipcode.objects.annotate(gml=AsGML('poly'))
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>>> print(qs[0].gml)
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<gml:Polygon srsName="EPSG:4326"><gml:OuterBoundaryIs>-147.78711,70.245363 ...
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-147.78711,70.245363</gml:OuterBoundaryIs></gml:Polygon>
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===================== =====================================================
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Keyword Argument Description
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===================== =====================================================
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``precision`` Not used on Oracle. It may be used to specify the number
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of significant digits for the coordinates in the GML
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representation -- the default value is 8.
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``version`` Not used on Oracle. It may be used to specify the GML
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version used, and may only be values of 2 or 3. The
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default value is 2.
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===================== =====================================================
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__ https://en.wikipedia.org/wiki/Geography_Markup_Language
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AsKML
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-----
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.. class:: AsKML(expression, precision=8, **extra)
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*Availability*: PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns a `Keyhole Markup
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Language (KML)`__ representation of the geometry.
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Example::
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>>> qs = Zipcode.objects.annotate(kml=AsKML('poly'))
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>>> print(qs[0].kml)
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<Polygon><outerBoundaryIs><LinearRing><coordinates>-103.04135,36.217596,0 ...
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-103.04135,36.217596,0</coordinates></LinearRing></outerBoundaryIs></Polygon>
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===================== =====================================================
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Keyword Argument Description
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===================== =====================================================
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``precision`` This keyword may be used to specify the number of
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significant digits for the coordinates in the KML
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representation -- the default value is 8.
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===================== =====================================================
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__ https://developers.google.com/kml/documentation/
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AsSVG
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-----
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.. class:: AsSVG(expression, relative=False, precision=8, **extra)
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*Availability*: PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns a `Scalable Vector
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Graphics (SVG)`__ representation of the geometry.
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===================== =====================================================
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Keyword Argument Description
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===================== =====================================================
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``relative`` If set to ``True``, the path data will be implemented
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in terms of relative moves. Defaults to ``False``,
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meaning that absolute moves are used instead.
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``precision`` This keyword may be used to specify the number of
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significant digits for the coordinates in the SVG
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representation -- the default value is 8.
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===================== =====================================================
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__ http://www.w3.org/Graphics/SVG/
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BoundingCircle
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--------------
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.. class:: BoundingCircle(expression, num_seg=48, **extra)
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*Availability*: `PostGIS <http://postgis.net/docs/ST_MinimumBoundingCircle.html>`__
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Accepts a single geographic field or expression and returns the smallest circle
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polygon that can fully contain the geometry.
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Centroid
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--------
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.. class:: Centroid(expression, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts a single geographic field or expression and returns the ``centroid``
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value of the geometry.
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Difference
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----------
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.. class:: Difference(expr1, expr2, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts two geographic fields or expressions and returns the geometric
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difference, that is the part of geometry A that does not intersect with
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geometry B.
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Distance
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--------
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.. class:: Distance(expr1, expr2, spheroid=None, **extra)
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*Availability*: MySQL, PostGIS, Oracle, SpatiaLite
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Accepts two geographic fields or expressions and returns the distance between
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them, as a :class:`~django.contrib.gis.measure.Distance` object. On MySQL, a raw
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float value is returned, as it's not possible to automatically determine the
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unit of the field.
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On backends that support distance calculation on geodetic coordinates, the
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proper backend function is automatically chosen depending on the SRID value of
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the geometries (e.g. ``ST_Distance_Sphere`` on PostGIS).
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When distances are calculated with geodetic (angular) coordinates, as is the
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case with the default WGS84 (4326) SRID, you can set the ``spheroid`` keyword
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argument to decide if the calculation should be based on a simple sphere (less
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accurate, less resource-intensive) or on a spheroid (more accurate, more
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resource-intensive).
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In the following example, the distance from the city of Hobart to every other
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:class:`~django.contrib.gis.db.models.PointField` in the ``AustraliaCity``
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queryset is calculated::
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>>> from django.contrib.gis.db.models.functions import Distance
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>>> pnt = AustraliaCity.objects.get(name='Hobart').point
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>>> for city in AustraliaCity.objects.annotate(distance=Distance('point', pnt)):
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... print(city.name, city.distance)
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Wollongong 990071.220408 m
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Shellharbour 972804.613941 m
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Thirroul 1002334.36351 m
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...
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.. note::
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Because the ``distance`` attribute is a
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:class:`~django.contrib.gis.measure.Distance` object, you can easily express
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the value in the units of your choice. For example, ``city.distance.mi`` is
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the distance value in miles and ``city.distance.km`` is the distance value
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in kilometers. See :doc:`measure` for usage details and the list of
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:ref:`supported_units`.
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Envelope
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--------
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.. class:: Envelope(expression, **extra)
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*Availability*: PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns the geometry
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representing the bounding box of the geometry.
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ForceRHR
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--------
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.. class:: ForceRHR(expression, **extra)
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*Availability*: `PostGIS <http://postgis.net/docs/ST_ForceRHR.html>`__
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Accepts a single geographic field or expression and returns a modified version
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of the polygon/multipolygon in which all of the vertices follow the
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right-hand rule.
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GeoHash
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-------
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.. class:: GeoHash(expression, **extra)
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*Availability*: PostGIS
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Accepts a single geographic field or expression and returns a `GeoHash`__
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representation of the geometry.
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__ https://en.wikipedia.org/wiki/Geohash
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Intersection
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------------
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.. class:: Intersection(expr1, expr2, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts two geographic fields or expressions and returns the geometric
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intersection between them.
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Length
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------
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.. class:: Length(expression, spheroid=True, **extra)
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*Availability*: MySQL, Oracle, PostGIS, SpatiaLite
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Accepts a single geographic linestring or multilinestring field or expression
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and returns its length as an :class:`~django.contrib.gis.measure.Distance`
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measure. On MySQL, a raw float value is returned, as it's not possible to
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automatically determine the unit of the field.
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On PostGIS and SpatiaLite, when the coordinates are geodetic (angular), you can
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specify if the calculation should be based on a simple sphere (less
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accurate, less resource-intensive) or on a spheroid (more accurate, more
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resource-intensive) with the ``spheroid`` keyword argument.
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MemSize
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-------
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.. class:: MemSize(expression, **extra)
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*Availability*: PostGIS
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Accepts a single geographic field or expression and returns the memory size
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(number of bytes) that the geometry field takes.
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NumGeometries
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-------------
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.. class:: NumGeometries(expression, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts a single geographic field or expression and returns the number of
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geometries if the geometry field is a collection (e.g., a ``GEOMETRYCOLLECTION``
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or ``MULTI*`` field); otherwise returns ``None``.
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NumPoints
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---------
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.. class:: NumPoints(expression, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts a single geographic field or expression and returns the number of points
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in the first linestring in the geometry field; otherwise returns ``None``.
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Perimeter
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---------
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.. class:: Perimeter(expression, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts a single geographic field or expression and returns the perimeter of the
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geometry field as a :class:`~django.contrib.gis.measure.Distance` object. On
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MySQL, a raw float value is returned, as it's not possible to automatically
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determine the unit of the field.
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PointOnSurface
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--------------
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.. class:: PointOnSurface(expression, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts a single geographic field or expression and returns a ``Point`` geometry
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guaranteed to lie on the surface of the field; otherwise returns ``None``.
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Reverse
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-------
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.. class:: Reverse(expression, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite (≥ 4.0)
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Accepts a single geographic field or expression and returns a geometry with
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reversed coordinates.
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Scale
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-----
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.. class:: Scale(expression, x, y, z=0.0, **extra)
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*Availability*: PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns a geometry with
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scaled coordinates by multiplying them with the ``x``, ``y``, and optionally
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``z`` parameters.
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SnapToGrid
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----------
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.. class:: SnapToGrid(expression, *args, **extra)
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*Availability*: PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns a geometry with all
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points snapped to the given grid. How the geometry is snapped to the grid
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depends on how many numeric (either float, integer, or long) arguments are
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given.
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=================== =====================================================
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Number of Arguments Description
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=================== =====================================================
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1 A single size to snap both the X and Y grids to.
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2 X and Y sizes to snap the grid to.
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4 X, Y sizes and the corresponding X, Y origins.
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=================== =====================================================
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SymDifference
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-------------
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.. class:: SymDifference(expr1, expr2, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts two geographic fields or expressions and returns the geometric
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symmetric difference (union without the intersection) between the given
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parameters.
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Transform
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---------
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.. class:: Transform(expression, srid, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts a geographic field or expression and a SRID integer code, and returns
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the transformed geometry to the spatial reference system specified by the
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``srid`` parameter.
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.. note::
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What spatial reference system an integer SRID corresponds to may depend on
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the spatial database used. In other words, the SRID numbers used for Oracle
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are not necessarily the same as those used by PostGIS.
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Translate
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---------
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.. class:: Translate(expression, x, y, z=0.0, **extra)
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*Availability*: PostGIS, SpatiaLite
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Accepts a single geographic field or expression and returns a geometry with
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its coordinates offset by the ``x``, ``y``, and optionally ``z`` numeric
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parameters.
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Union
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-----
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.. class:: Union(expr1, expr2, **extra)
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*Availability*: PostGIS, Oracle, SpatiaLite
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Accepts two geographic fields or expressions and returns the union of both
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geometries.
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