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Small grammatical changes in the geodjango wiki

TOC(GeoDjangoDatabaseAPI)?

Database API

Note: All GeoDjango queries are performed via the GeoQuerySet, a modified version of Django's QuerySet that enables the construction of spatial SQL.

GeoDjango's lookup types can only be used on geographic fields with filter(). Filters on 'normal' fields (e.g. CharField) may be chained with those on geographic fields. Thus, geographic queries take the following form (assuming the Zip model used in the GeoDjango Model API docs):

>>> qs = Zip.objects.filter(<field>__<lookup_type>=<parameter>)
>>> qs = Zip.objects.exclude(...)

For example:

>>> qs = Zip.objects.filter(poly__contains=pnt)

In this case, poly is the geographic field, contains is the lookup type, and pnt is the parameter (which may be a GEOSGeometry object, a string of WKT, or a string of HEXEWKB).

Creating and Saving Geographic Models

Here is an example of how to create a geometry object (assuming the Zip model):

>>> from zipcode.models import Zip
>>> z = Zip(code=77096, poly='POLYGON(( 10 10, 10 20, 20 20, 20 15, 10 10))')
>>> z.save()

GEOSGeometry objects may also be used to save geometric models:

>>> from django.contrib.gis.geos import GEOSGeometry
>>> z = Zip(code=77096, poly=GEOSGeometry('POLYGON(( 10 10, 10 20, 20 20, 20 15, 10 10))'))
>>> z.save()

Moreover, if the GEOSGeometry is in a different coordinate system (has a different SRID value) than that of the field, then it will be implicitly transformed into the SRID of the model's field, using the spatial database's transform procedure:

>>> poly_3084 = GEOSGeometry('POLYGON(( 10 10, 10 20, 20 20, 20 15, 10 10))', srid=3084) # SRID 3084 is 'NAD83(HARN) / Texas Centric Lambert Conformal'
>>> z = Zip(code=78212, poly=poly_3084)
>>> z.save()
>>> from django.db import connection
>>> print connection.queries[-1]['sql'] # printing the last SQL statement executed
INSERT INTO "geoapp_zip" ("code", "poly") VALUES (78212, ST_Transform(ST_GeomFromWKB('\\001 ... ', 3084), 4326))

Thus, geometry parameters may be passed in using the GEOSGeometry object, WKT (Well Known Text) or HEXEWKB (PostGIS specific, essentially a WKB geometry in hexadecimal). Essentially, if the input is not a GEOSGeometry object, it will attempt to instantiate a GEOSGeometry object from the input.

Below are some examples and references for GEOS Geometry objects, WKT, and HEXEWKB.

  • GEOS Geometry:
    >>> from django.contrib.gis.geos import *
    >>> pnt  = Point(5, 23)
    >>> ls   = LineString((0, 0), (5, 23))
    >>> poly = GEOSGeometry('POLYGON (( 10 10, 10 20, 20 20, 20 15, 10 10))')
    
  • WKT Polygon: 'POLYGON(( 10 10, 10 20, 20 20, 20 15, 10 10))'
  • HEXEWKB Polygon: '0103000000010000000 ... 00000000000002440'

Distance Lookups

Availability: PostGIS, Oracle

Distance lookups take a tuple parameter comprising:

  1. A geometry to base calculations from; and
  2. A number or Distance object containing the distance.

If a Distance object is used, it may be expressed in any units (the SQL generated will use units converted to those of the field); otherwise, numeric parameters will be assumed to be in the units of the field.

Note: Distance lookups for geodetic coordinate systems was added in r7104 (fixing ticket #6414). For those using Postgresql, the PostGIS routine ST_distance_spheroid is used -- which is limited to calculating distances only to/from points. Thus, geodetic distance lookups on PostGIS are only allowed with PointFields using points as the geographic parameter.

The following distance lookups are available:

  • distance_lt
  • distance_lte
  • distance_gt
  • distance_gte

For example, let's say we have a SouthTexasCity model (from [browser:django/branches/gis/django/contrib/gis/tests/distapp/models.py GeoDjango distance tests]) on a projected coordinate system valid for cities in southern Texas:

from django.contrib.gis.db import models

class SouthTexasCity(models.Model):
    name = models.CharField(max_length=30)
    point = models.PointField(srid=32140) # A projected coordinate system (only valid for South Texas!) is used, units are in meters.
    objects = models.GeoManager()

Then distance queries may be performed as follows:

>>> from django.contrib.gis.geos import *
>>> from django.contrib.gis.measure import D # `D` is a shortcut for `Distance`
>>> from geoapp import SouthTexasCity 
>>> pnt = fromstr('POINT(-96.876369 29.905320)', srid=4326) # Distances will be calculated from this point, which does _not_ have to be projected.
>>> qs = SouthTexasCity.objects.filter(point__distance_lte=(pnt, 7000)) # If numeric parameter, units of field (meters in this case) are assumed.
>>> qs = SouthTexasCity.objects.filter(point__distance_lte=(pnt, D(km=7))) # Find all Cities w/in 7km of pnt
>>> qs = SouthTexasCity.objects.filter(point__distance_gte=(pnt, D(mi=20))) # Find all Cities > 20 miles away from pnt.
>>> qs = SouthTexasCity.objects.filter(point__distance_gte=(pnt, D(chain=100))) # More obscure units, such as chains, are supported.

PostGIS

PostGIS Operator Field Lookup Types

For more information, see generally, "Operators", PostGIS Documentation at Ch. 6.2.2

  • overlaps_left
    • Returns true if A's bounding box overlaps or is to the left of B's bounding box.
    • PostGIS equivalent "&<"
  • overlaps_right
    • Returns true if A's bounding box overlaps or is to the right of B's bounding box.
    • PostGIS equivalent "&>"
  • left
    • Returns true if A's bounding box is strictly to the left of B's bounding box.
    • PostGIS equivalent "<<"
  • right
    • Returns true if A's bounding box is strictly to the right of B's bounding box.
    • PostGIS equivalent ">>"
  • overlaps_below
    • Returns true if A's bounding box overlaps or is below B's bounding box.
    • PostGIS equivalent "&<|"
  • overlaps_above
    • Returns true if A's bounding box overlaps or is above B's bounding box.
    • PostGIS equivalent "|&>"
  • strictly_below
    • Returns true if A's bounding box is strictly below B's bounding box.
    • PostGIS equivalent "<<|"
  • strictly_above
    • Returns true if A's bounding box is strictly above B's bounding box.
    • PostGIS equivalent "|>>"
  • same_as or exact
    • The "same as" operator. It tests actual geometric equality of two features. So if A and B are the same feature, vertex-by-vertex, the operator returns true.
    • PostGIS equivalent "~="
  • contained
    • Returns true if A's bounding box is completely contained by B's bounding box.
    • PostGIS equivalent "@"
  • bbcontains
    • Returns true if A's bounding box completely contains B's bounding box.
    • PostGIS equivalent "~"
  • bboverlaps
    • Returns true if A's bounding box overlaps B's bounding box.
    • PostGIS equivalent "&&"

PostGIS GEOS Function Field Lookup Types

For more information, see generally Geometry Relationship Functions, PostGIS Documentation at Ch. 6.1.2.

Please note that when using PostGIS 1.3.1 and above, index support is automatically "inlined" -- in other words, the bounding box equivalent is automatically evaluated prior to calling these, more computationally expensive, functions.

  • equals
    • Returns 1 (TRUE) if the given Geometries are "spatially equal".
    • Use this for a 'better' answer than '='. equals('LINESTRING(0 0, 10 10)','LINESTRING(0 0, 5 5, 10 10)') is true.
    • PostGIS equivalent Equals(geometry, geometry), OGC SPEC s2.1.1.2
  • disjoint
    • Returns 1 (TRUE) if the Geometries are "spatially disjoint".
    • PostGIS equivalent Disjoint(geometry, geometry)
  • touches
    • Returns 1 (TRUE) if the Geometries "spatially touch".
    • PostGIS equivalent Touches(geometry, geometry)
  • crosses
    • Returns 1 (TRUE) if the Geometries "spatially cross".
    • PostGIS equivalent Crosses(geometry, geometry)
  • within
    • Returns 1 (TRUE) if Geometry A is "spatially within" Geometry B.
    • PostGIS equivalent Within(geometry, geometry)
  • overlaps
    • Returns 1 (TRUE) if the Geometries "spatially overlap".
    • PostGIS equivalent Overlaps(geometry, geometry)
  • contains
    • Returns 1 (TRUE) if Geometry A "spatially contains" Geometry B.
    • PostGIS equivalent Contains(geometry, geometry)
  • relate
    • Returns the DE-9IM (dimensionally extended nine-intersection matrix) between the two geometries.
    • Tuple parameter (geom, pattern) required for lookup type, where pattern is an intersection pattern -- a string comprising nine characters, where each character is one of T, F, or *.).
    • PostGIS equivelent Relate(geometry, geometry, intersectionPatternMatrix)

The following lookup types are only available in PostGIS versions 1.3.1 and above:

  • dwithin
    • Returns true if geometries are within the specified distance of one another. Uses indexes if available.
    • Tuple parameter (geom, distance) required for lookup type.
  • coveredby
    • Returns 1 (TRUE) if no point in Geometry A is outside Geometry B
    • Refer to this resource for an explaination of the need of this function.
  • covers
    • Returns 1 (TRUE) if no point in Geometry B is outside Geometry A
    • See link in coveredby documentation above for more information.

Oracle

For more information, see generally, Spatial Operators, Oracle Spatial User's Guide and Manual, at Ch. 11.

  • contains
    • Oracle equivalent SDO_CONTAINS(geometry1, geometry2)
  • coveredby
    • Oracle equivalent SDO_COVEREDBY(geometry1, geometry2)
  • covers
    • Oracle equivalent SDO_COVERS(geometry1, geometry2)
  • disjoint
    • Oracle equivalent SDO_GEOM.RELATE(geometry1, 'DISJOINT', geometry2, 0.05)
  • dwithin
    • Oracle equivalent SDO_WITHIN_DISTANCE(geometry1, geometry2, 'distance=<param>')
    • Tuple parameter (geom, distance) required for lookup type.
  • equals, exact, same_as
    • Oracle equivalent, SDO_EQUALS(geometry1, geometry2)
  • intersects
    • Oracle equivalent SDO_OVERLAPBDYINTERSECT(geometry1, geometry2)
  • overlaps
    • Oracle equivalent SDO_OVERLAPS(geometry1, geometry2)
  • relate
    • Oracle equivalent SDO_RELATE(geometry1, geometry2, <param>)
    • Tuple parameter (geom, mask), where the mask component is at least one of the nine-intersection patterns: TOUCH, OVERLAPBDYDISJOINT, OVERLAPBDYINTERSECT, EQUAL, INSIDE, COVEREDBY, CONTAINS, COVERS, ANYINTERACT, ON. Multiple masks may be combined with the logical Boolean operator OR, for example, 'inside+touch'. The mask relation strings are case-insensitive.
  • touches
    • Oracle equivalent SDO_TOUCH(geometry1, geometry2)
  • within
    • Oracle equivalent SDO_INSIDE(geometry1, geometry2)

MySQL

For more information, see generally, Relations on Geometry Minimal Bounding Rectangles (MBRs), MySQL 5.0 Reference Manual, at Ch. 17.5.5.

  • bbcontains, contains
    • MySQL equivalent MBRContains(g1, g2)
  • contained, within
    • MySQL equivalent MBRWithin(g1, g2)
  • disjoint
    • MySQL equivalent MBRDisjoint(g1, g2)
  • equals, exact, same_as
    • MySQL equivalent MBREqual(g1, g2)
  • intersects
    • MySQL equivalent MBRIntersects(g1, g2)
  • overlaps
    • MySQL equivalent MBROverlaps(g1, g2)
  • touches
    • MySQL equivalent MBRTouches(g1, g2)

GeoQuerySet Methods

Note: All of these methods may take an optional field_name keyword parameter that will specify the field on the model to perform the method on. If no field name is specified, then the first geographic field encountered in the model will be assumed.

distance

Availability: PostGIS, Oracle

The distance method takes a geometry as a parameter, and will attach a distance attribute to every model in the returned queryset that contains the distance (in units of the field) to the given geometry. If a geodetic coordinate system is used (like WGS84), then the distance attribute will be in meters.

In the following example (from [browser:django/branches/gis/django/contrib/gis/tests/distapp GeoDjango distance tests]), the distance from the Tasmanian city of Hobart to every other PointField in the AustraliaCity queryset will be calculated. The distance attribute is in meters because the AustraliaCity [browser:django/branches/gis/django/contrib/gis/tests/distapp/models.py model] uses WGS84 (the default when no SRID is specified via the srid keyword).

>>> pnt = AustraliaCity.objects.get(name='Hobart').point # See django.contrib.gis.tests.distapp
>>> for cty in AustraliaCity.objects.distance(pnt): print cty.name, cty.distance
Wollongong 988817.397733
Shellharbour 971590.470258
Thirroul 1001051.96023
Mittagong 974374.007691
Batemans Bay 940359.188397
Canberra 597578.655272
Melbourne 574635.996781
Sydney 1055625.32642
Hobart 0.0
Adelaide 1162133.07421

extent

Availability: PostGIS

Returns the extent (aggregate) of the features in the GeoQuerySet. The extent will be returned as a 4-tuple, consisting of (xmin, ymin, xmax, ymax).

>>> qs = City.objects.filter(name__in=('Houston', 'Dallas'))
>>> print qs.extent()
(-96.8016128540039, 29.7633724212646, -95.3631439208984, 32.782058715820)

gml

Availability: PostGIS, Oracle

The gml method attaches a gml attribute to every model in the queryset that contains the Geographic Markup Language (GML) representation of the geometry.

>>> qs = Zip.objects.all().gml()
>>> print qs[0].gml
<gml:Polygon srsName="EPSG:4326"><gml:OuterBoundaryIs>-147.78711,70.245363 ...  -147.78711,70.245363</gml:OuterBoundaryIs></gml:Polygon>

Keyword arguments:

precision

This keyword may be used to specify the number of significant digits for the coordinates in the GML representation -- the default value is 8. This keyword may not be used on Oracle.

kml

Availability: PostGIS 1.2.1+

The kml method attaches a kml attribute to every model in the queryset that contains the Keyhole Markup Language (KML) representation of the geometry. It should be noted that the contents of the KML are in WGS84, and will be transformed if necessary -- the geometry field attribute itself is not affected.

>>> qs = Zip.objects.all().kml()
>>> print qs[0].kml
<Polygon><outerBoundaryIs><LinearRing><coordinates>-103.04135,36.217596,0 ... -103.04135,36.217596,0</coordinates></LinearRing></outerBoundaryIs></Polygon>

Keyword arguments:

precision

This keyword may be used to specify the number of significant digits for the coordinates in the KML representation -- the default value is 8.

transform

Availability: PostGIS, Oracle

The transform method transforms the geometries in a model to a different spatial reference system. If the srid parameter is not specified, WGS84 is used by default.

>>> qs = Zip.objects.all().transform() # Transforms to WGS84
>>> qs = Zip.objects.all().transform(32140) # Transforming to "NAD83 / Texas South Central"
>>> print qs[0].poly.srid
32140
>>> print qs[0].poly
POLYGON ((234055.1698884720099159 4937796.9232223574072123 ...

Keyword arguments:

srid

The srid keyword may be used to specify the spatial reference system identifier. Please note that this may depend on your spatial database backend, e.g., those used for Oracle are not the same as those used by PostGIS.

union

Availability: PostGIS, Oracle

This union method returns a GEOSGeometry object comprising the union of every geometry in the queryset. Please note that use of union is processor intensive and may take a significant amount of time on large querysets.

>>> u = Zip.objects.union() # This may take a LONG time, but returns a geometry representing the union of all Zip code polygons.
>>> u = Zip.objects.filter(poly__within=bbox).union() # A more sensible approach.

Keyword arguments:

tolerance

This keyword is only available on Oracle platforms, and is for the tolerance value for SDOAGGRTYPE. See the Oracle documentation for more details.

Extra Instance Methods

Update: All of the extra instance methods have been deprecated as of r6467 because lazy geometry support includes all of their functionality (including OGR geometries and OSR spatial references with the ogr and srs properties, respectively). In other words, these properties may be directly accessed as attributes from the geometry field.

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