GEOS API

Background

What is GEOS?

GEOS stands for Geometry Engine - Open Source, and is a C++ library, ported from the Java Topology Suite. GEOS implements the OpenGIS Simple Features for SQL spatial predicate functions and spatial operators. GEOS, now an OSGeo project, was initially developed and maintained by Refractions Research of Victoria, Canada.

Features

GeoDjango implements a high-level Python wrapper for the GEOS library, its features include:

  • A BSD-licensed interface to the GEOS geometry routines, implemented purely in Python using ctypes.
  • Loosely-coupled to GeoDjango. For example, GEOSGeometry objects may be used outside of a django project/application. In other words, no need to have DJANGO_SETTINGS_MODULE set or use a database, etc.
  • Mutability: GEOSGeometry objects may be modified.
  • Cross-platform and tested; compatible with Windows, Linux, Solaris, and Mac OS X platforms.

Tutorial

This section contains a brief introduction and tutorial to using GEOSGeometry objects.

Creating a Geometry

GEOSGeometry objects may be created in a few ways. The first is to simply instantiate the object on some spatial input – the following are examples of creating the same geometry from WKT, HEX, WKB, and GeoJSON:

>>> from django.contrib.gis.geos import GEOSGeometry
>>> pnt = GEOSGeometry('POINT(5 23)') # WKT
>>> pnt = GEOSGeometry('010100000000000000000014400000000000003740') # HEX
>>> pnt = GEOSGeometry(buffer('\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x14@\x00\x00\x00\x00\x00\x007@'))
>>> pnt = GEOSGeometry('{ "type": "Point", "coordinates": [ 5.000000, 23.000000 ] }') # GeoJSON

Another option is to use the constructor for the specific geometry type that you wish to create. For example, a Point object may be created by passing in the X and Y coordinates into its constructor:

>>> from django.contrib.gis.geos import Point
>>> pnt = Point(5, 23)

Finally, there are fromstr() and fromfile() factory methods, which return a GEOSGeometry object from an input string or a file:

>>> from django.contrib.gis.geos import fromstr, fromfile
>>> pnt = fromstr('POINT(5 23)')
>>> pnt = fromfile('/path/to/pnt.wkt')
>>> pnt = fromfile(open('/path/to/pnt.wkt'))

Geometries are Pythonic

GEOSGeometry objects are 'Pythonic', in other words components may be accessed, modified, and iterated over using standard Python conventions. For example, you can iterate over the coordinates in a Point:

>>> pnt = Point(5, 23)
>>> [coord for coord in pnt]
[5.0, 23.0]

With any geometry object, the GEOSGeometry.coords property may be used to get the geometry coordinates as a Python tuple:

>>> pnt.coords
(5.0, 23.0)

You can get/set geometry components using standard Python indexing techniques. However, what is returned depends on the geometry type of the object. For example, indexing on a LineString returns a coordinate tuple:

>>> from django.contrib.gis.geos import LineString
>>> line = LineString((0, 0), (0, 50), (50, 50), (50, 0), (0, 0))
>>> line[0]
(0.0, 0.0)
>>> line[-2]
(50.0, 0.0)

Whereas indexing on a Polygon will return the ring (a LinearRing object) corresponding to the index:

>>> from django.contrib.gis.geos import Polygon
>>> poly = Polygon( ((0.0, 0.0), (0.0, 50.0), (50.0, 50.0), (50.0, 0.0), (0.0, 0.0)) )
>>> poly[0]
<LinearRing object at 0x1044395b0>
>>> poly[0][-2] # second-to-last coordinate of external ring
(50.0, 0.0)

In addition, coordinates/components of the geometry may added or modified, just like a Python list:

>>> line[0] = (1.0, 1.0)
>>> line.pop()
(0.0, 0.0)
>>> line.append((1.0, 1.0))
>>> line.coords
((1.0, 1.0), (0.0, 50.0), (50.0, 50.0), (50.0, 0.0), (1.0, 1.0))

Geometry Objects

GEOSGeometry

class GEOSGeometry(geo_input[srid=None])
Parameters:
  • geo_input (string or buffer) -- Geometry input value
  • srid (integer) -- spatial reference identifier

This is the base class for all GEOS geometry objects. It initializes on the given geo_input argument, and then assumes the proper geometry subclass (e.g., GEOSGeometry('POINT(1 1)') will create a Point object).

The following input formats, along with their corresponding Python types, are accepted:

Format Input Type
WKT / EWKT str or unicode
HEX / HEXEWKB str or unicode
WKB / EWKB buffer
GeoJSON str or unicode

Properties

GEOSGeometry.coords

Returns the coordinates of the geometry as a tuple.

GEOSGeometry.empty

Returns whether or not the set of points in the geometry is empty.

GEOSGeometry.geom_type

Returns a string corresponding to the type of geometry. For example:

>>> pnt = GEOSGeometry('POINT(5 23)')
>>> pnt.geom_type
'Point'
GEOSGeometry.geom_typeid

Returns the GEOS geometry type identification number. The following table shows the value for each geometry type:

Geometry ID
Point 0
LineString 1
LinearRing 2
Polygon 3
MultiPoint 4
MultiLineString 5
MultiPolygon 6
GeometryCollection 7
GEOSGeometry.num_coords

Returns the number of coordinates in the geometry.

GEOSGeometry.num_geom

Returns the number of geometries in this geometry. In other words, will return 1 on anything but geometry collections.

GEOSGeometry.hasz

Returns a boolean indicating whether the geometry is three-dimensional.

GEOSGeometry.ring

Returns a boolean indicating whether the geometry is a LinearRing.

GEOSGeometry.simple

Returns a boolean indicating whether the geometry is 'simple'. A geometry is simple if and only if it does not intersect itself (except at boundary points). For example, a LineString object is not simple if it intersects itself. Thus, LinearRing and :class`Polygon` objects are always simple because they do cannot intersect themselves, by definition.

GEOSGeometry.valid

Returns a boolean indicating whether the geometry is valid.

GEOSGeometry.valid_reason
New in Django 1.3: Please, see the release notes

Returns a string describing the reason why a geometry is invalid.

GEOSGeometry.srid

Property that may be used to retrieve or set the SRID associated with the geometry. For example:

>>> pnt = Point(5, 23)
>>> print pnt.srid
None
>>> pnt.srid = 4326
>>> pnt.srid
4326

Output Properties

The properties in this section export the GEOSGeometry object into a different. This output may be in the form of a string, buffer, or even another object.

GEOSGeometry.ewkt

Returns the "extended" Well-Known Text of the geometry. This representation is specific to PostGIS and is a super set of the OGC WKT standard. [1] Essentially the SRID is prepended to the WKT representation, for example SRID=4326;POINT(5 23).

Note

The output from this property does not include the 3dm, 3dz, and 4d information that PostGIS supports in its EWKT representations.

GEOSGeometry.hex

Returns the WKB of this Geometry in hexadecimal form. Please note that the SRID and Z values are not included in this representation because it is not a part of the OGC specification (use the GEOSGeometry.hexewkb property instead).

GEOSGeometry.hexewkb
New in Django 1.2: Please, see the release notes

Returns the EWKB of this Geometry in hexadecimal form. This is an extension of the WKB specification that includes SRID and Z values that are a part of this geometry.

Note

GEOS 3.1 is required if you want valid 3D HEXEWKB.

GEOSGeometry.json

Returns the GeoJSON representation of the geometry.

Note

Requires GDAL.

GEOSGeometry.geojson

Alias for GEOSGeometry.json.

GEOSGeometry.kml

Returns a KML (Keyhole Markup Language) representation of the geometry. This should only be used for geometries with an SRID of 4326 (WGS84), but this restriction is not enforced.

GEOSGeometry.ogr

Returns an OGRGeometry object correspondg to the GEOS geometry.

Note

Requires GDAL.

GEOSGeometry.wkb

Returns the WKB (Well-Known Binary) representation of this Geometry as a Python buffer. SRID and Z values are not included, use the GEOSGeometry.ewkb property instead.

GEOSGeometry.ewkb
New in Django 1.2: Please, see the release notes

Return the EWKB representation of this Geometry as a Python buffer. This is an extension of the WKB specification that includes any SRID and Z values that are a part of this geometry.

Note

GEOS 3.1 is required if you want valid 3D EWKB.

GEOSGeometry.wkt

Returns the Well-Known Text of the geometry (an OGC standard).

Spatial Predicate Methods

All of the following spatial predicate methods take another GEOSGeometry instance (other) as a parameter, and return a boolean.

GEOSGeometry.contains(other)

Returns True if GEOSGeometry.within() is False.

GEOSGeometry.crosses(other)

Returns True if the DE-9IM intersection matrix for the two Geometries is T*T****** (for a point and a curve,a point and an area or a line and an area) 0******** (for two curves).

GEOSGeometry.disjoint(other)

Returns True if the DE-9IM intersection matrix for the two geometries is FF*FF****.

GEOSGeometry.equals(other)

Returns True if the DE-9IM intersection matrix for the two geometries is T*F**FFF*.

GEOSGeometry.equals_exact(othertolerance=0)

Returns true if the two geometries are exactly equal, up to a specified tolerance. The tolerance value should be a floating point number representing the error tolerance in the comparison, e.g., poly1.equals_exact(poly2, 0.001) will compare equality to within one thousandth of a unit.

GEOSGeometry.intersects(other)

Returns True if GEOSGeometry.disjoint() is False.

GEOSGeometry.overlaps(other)

Returns true if the DE-9IM intersection matrix for the two geometries is T*T***T** (for two points or two surfaces) 1*T***T** (for two curves).

GEOSGeometry.relate_pattern(otherpattern)

Returns True if the elements in the DE-9IM intersection matrix for this geometry and the other matches the given pattern -- a string of nine characters from the alphabet: {T, F, *, 0}.

GEOSGeometry.touches(other)

Returns True if the DE-9IM intersection matrix for the two geometries is FT*******, F**T***** or F***T****.

GEOSGeometry.within(other)

Returns True if the DE-9IM intersection matrix for the two geometries is T*F**F***.

Topological Methods

GEOSGeometry.buffer(widthquadsegs=8)

Returns a GEOSGeometry that represents all points whose distance from this geometry is less than or equal to the given width. The optional quadsegs keyword sets the number of segments used to approximate a quarter circle (defaults is 8).

GEOSGeometry.difference(other)

Returns a GEOSGeometry representing the points making up this geometry that do not make up other.

GEOSGeometry:intersection(other)

Returns a GEOSGeometry representing the points shared by this geometry and other.

GEOSGeometry.relate(other)

Returns the DE-9IM intersection matrix (a string) representing the topological relationship between this geometry and the other.

GEOSGeometry.simplify(tolerance=0.0preserve_topology=False)

Returns a new GEOSGeometry, simplified using the Douglas-Peucker algorithm to the specified tolerance. A higher tolerance value implies less points in the output. If no tolerance is tolerance provided, it defaults to 0.

By default, this function does not preserve topology - e.g., Polygon objects can be split, collapsed into lines or disappear. Polygon holes can be created or disappear, and lines can cross. By specifying preserve_topology=True, the result will have the same dimension and number of components as the input, however, this is significantly slower.

GEOSGeometry.sym_difference(other)

Returns a GEOSGeometry combining the points in this geometry not in other, and the points in other not in this geometry.

GEOSGeometry.union(other)

Returns a GEOSGeometry representing all the points in this geometry and the other.

Topological Properties

GEOSGeometry.boundary

Returns the boundary as a newly allocated Geometry object.

GEOSGeometry.centroid

Returns a Point object representing the geometric center of the geometry. The point is not guaranteed to be on the interior of the geometry.

GEOSGeometry.convex_hull

Returns the smallest Polygon that contains all the points in the geometry.

GEOSGeometry.envelope

Returns a Polygon that represents the bounding envelope of this geometry.

GEOSGeometry.point_on_surface

Computes and returns a Point guaranteed to be on the interior of this geometry.

Other Properties & Methods

GEOSGeometry.area

This property returns the area of the Geometry.

GEOSGeometry.extent

This property returns the extent of this geometry as a 4-tuple, consisting of (xmin, ymin, xmax, ymax).

GEOSGeometry.clone()

This method returns a GEOSGeometry that is a clone of the original.

GEOSGeometry.distance(geom)

Returns the distance between the closest points on this geometry and the given geom (another GEOSGeometry object).

Note

GEOS distance calculations are linear -- in other words, GEOS does not perform a spherical calculation even if the SRID specifies a geographic coordinate system.

GEOSGeometry.length

Returns the length of this geometry (e.g., 0 for a Point, the length of a LineString, or the circumference of a Polygon).

GEOSGeometry.prepared

Note

Support for prepared geometries requires GEOS 3.1.

Returns a GEOS PreparedGeometry for the contents of this geometry. PreparedGeometry objects are optimized for the contains, intersects, and covers operations. Refer to the Prepared Geometries documentation for more information.

GEOSGeometry.srs

Returns a SpatialReference object corresponding to the SRID of the geometry or None.

Note

Requires GDAL.

GEOSGeometry.transform(ctclone=False)
Changed in Django 1.3: Please, see the release notes

Transforms the geometry according to the given coordinate transformation paramter (ct), which may be an integer SRID, spatial reference WKT string, a PROJ.4 string, a SpatialReference object, or a CoordTransform object. By default, the geometry is transformed in-place and nothing is returned. However if the clone keyword is set, then the geometry is not modified and a transformed clone of the geometry is returned instead.

Note

Requires GDAL.

Note

Prior to 1.3, this method would silently no-op if GDAL was not available. Now, a GEOSException is raised as application code relying on this behavior is in error. In addition, use of this method when the SRID is None or less than 0 now generates a warning because a GEOSException will be raised instead in version 1.5.

Point

class Point(xyz=Nonesrid=None)

Point objects are instantiated using arguments that represent the component coordinates of the point or with a single sequence coordinates. For example, the following are equivalent:

>>> pnt = Point(5, 23)
>>> pnt = Point([5, 23])

LineString

class LineString(*args**kwargs)

LineString objects are instantiated using arguments that are either a sequence of coordinates or Point objects. For example, the following are equivalent:

>>> ls = LineString((0, 0), (1, 1))
>>> ls = LineString(Point(0, 0), Point(1, 1))

In addition, LineString objects may also be created by passing in a single sequence of coordinate or Point objects:

>>> ls = LineString( ((0, 0), (1, 1)) )
>>> ls = LineString( [Point(0, 0), Point(1, 1)] )

LinearRing

class LinearRing(*args**kwargs)

LinearRing objects are constructed in the exact same way as LineString objects, however the coordinates must be closed, in other words, the first coordinates must be the same as the last coordinates. For example:

>>> ls = LinearRing((0, 0), (0, 1), (1, 1), (0, 0))

Notice that (0, 0) is the first and last coordinate -- if they were not equal, an error would be raised.

Polygon

class Polygon(*args**kwargs)

Polygon objects may be instantiated by passing in one or more parameters that represent the rings of the polygon. The parameters must either be LinearRing instances, or a sequence that may be used to construct a LinearRing:

>>> ext_coords = ((0, 0), (0, 1), (1, 1), (1, 0), (0, 0))
>>> int_coords = ((0.4, 0.4), (0.4, 0.6), (0.6, 0.6), (0.6, 0.4), (0.4, 0.4))
>>> poly = Polygon(ext_coords, int_coords)
>>> poly = Polygon(LinearRing(ext_coords), LinearRing(int_coords))
classmethod from_bbox(bbox)

Returns a polygon object from the given bounding-box, a 4-tuple comprising (xmin, ymin, xmax, ymax).

num_interior_rings

Returns the number of interior rings in this geometry.

Geometry Collections

MultiPoint

class MultiPoint(*args**kwargs)

MultiPoint objects may be instantiated by passing in one or more Point objects as arguments, or a single sequence of Point objects:

>>> mp = MultiPoint(Point(0, 0), Point(1, 1))
>>> mp = MultiPoint( (Point(0, 0), Point(1, 1)) )

MultiLineString

class MultiLineString(*args**kwargs)

MultiLineString objects may be instantiated by passing in one or more LineString objects as arguments, or a single sequence of LineString objects:

>>> ls1 = LineString((0, 0), (1, 1))
>>> ls2 = LineString((2, 2), (3, 3))
>>> mls = MultiLineString(ls1, ls2)
>>> mls = MultiLineString([ls1, ls2])
merged

Returns a LineString representing the line merge of all the components in this MultiLineString.

MultiPolygon

class MultiPolygon(*args**kwargs)

MultiPolygon objects may be instantiated by passing one or more Polygon objects as arguments, or a single sequence of Polygon objects:

>>> p1 = Polygon( ((0, 0), (0, 1), (1, 1), (0, 0)) )
>>> p2 = Polygon( ((1, 1), (1, 2), (2, 2), (1, 1)) )
>>> mp = MultiPolygon(p1, p2)
>>> mp = MultiPolygon([p1, p2])
cascaded_union

Returns a Polygon that is the union of all of the component polygons in this collection. The algorithm employed is significantly more efficient (faster) than trying to union the geometries together individually. [2]

Note

GEOS 3.1 is required to peform cascaded unions.

GeometryCollection

class GeometryCollection(*args**kwargs)

GeometryCollection objects may be instantiated by passing in one or more other GEOSGeometry as arguments, or a single sequence of GEOSGeometry objects:

>>> poly = Polygon( ((0, 0), (0, 1), (1, 1), (0, 0)) )
>>> gc = GeometryCollection(Point(0, 0), MultiPoint(Point(0, 0), Point(1, 1)), poly)
>>> gc = GeometryCollection((Point(0, 0), MultiPoint(Point(0, 0), Point(1, 1)), poly))

Prepared Geometries

In order to obtain a prepared geometry, just access the GEOSGeometry.prepared property. Once you have a PreparedGeometry instance its spatial predicate methods, listed below, may be used with other GEOSGeometry objects. An operation with a prepared geometry can be orders of magnitude faster -- the more complex the geometry that is prepared, the larger the speedup in the operation. For more information, please consult the GEOS wiki page on prepared geometries.

Note

GEOS 3.1 is required in order to use prepared geometries.

For example:

>>> from django.contrib.gis.geos import Point, Polygon
>>> poly = Polygon.from_bbox((0, 0, 5, 5))
>>> prep_poly = poly.prepared
>>> prep_poly.contains(Point(2.5, 2.5))
True

PreparedGeometry

class PreparedGeometry

All methods on PreparedGeometry take an other argument, which must be a GEOSGeometry instance.

contains(other)
contains_properly(other)
covers(other)
intersects(other)

Geometry Factories

fromfile(file_h)
Parameters:file_h (a Python file object or a string path to the file) -- input file that contains spatial data
Return type:a GEOSGeometry corresponding to the spatial data in the file

Example:

>>> from django.contrib.gis.geos import fromfile
>>> g = fromfile('/home/bob/geom.wkt')
fromstr(string[srid=None])
Parameters:
  • string (string) -- string that contains spatial data
  • srid (integer) -- spatial reference identifier
Return type:

a GEOSGeometry corresponding to the spatial data in the string

Example:

>>> from django.contrib.gis.geos import fromstr
>>> pnt = fromstr('POINT(-90.5 29.5)', srid=4326)

I/O Objects

Reader Objects

The reader I/O classes simply return a GEOSGeometry instance from the WKB and/or WKT input given to their read(geom) method.

class WKBReader

Example:

>>> from django.contrib.gis.geos import WKBReader
>>> wkb_r = WKBReader()
>>> wkb_r.read('0101000000000000000000F03F000000000000F03F')
<Point object at 0x103a88910>
class WKTReader

Example:

>>> from django.contrib.gis.geos import WKTReader
>>> wkt_r = WKTReader()
>>> wkt_r.read('POINT(1 1)')
<Point object at 0x103a88b50>

Writer Objects

All writer objects have a write(geom) method that returns either the WKB or WKT of the given geometry. In addition, WKBWriter objects also have properties that may be used to change the byte order, and or include the SRID and 3D values (in other words, EWKB).

class WKBWriter

WKBWriter provides the most control over its output. By default it returns OGC-compliant WKB when it's write method is called. However, it has properties that allow for the creation of EWKB, a superset of the WKB standard that includes additional information.

WKBWriter.write(geom)

Returns the WKB of the given geometry as a Python buffer object. Example:

>>> from django.contrib.gis.geos import Point, WKBWriter
>>> pnt = Point(1, 1)
>>> wkb_w = WKBWriter()
>>> wkb_w.write(pnt)
<read-only buffer for 0x103a898f0, size -1, offset 0 at 0x103a89930>
WKBWriter.write_hex(geom)

Returns WKB of the geometry in hexadecimal. Example:

>>> from django.contrib.gis.geos import Point, WKBWriter
>>> pnt = Point(1, 1)
>>> wkb_w = WKBWriter()
>>> wkb_w.write_hex(pnt)
'0101000000000000000000F03F000000000000F03F'
WKBWriter.byteorder

This property may be be set to change the byte-order of the geometry representation.

Byteorder Value Description
0 Big Endian (e.g., compatible with RISC systems)
1 Little Endian (e.g., compatible with x86 systems)

Example:

>>> from django.contrib.gis.geos import Point, WKBWriter
>>> wkb_w = WKBWriter()
>>> pnt = Point(1, 1)
>>> wkb_w.write_hex(pnt)
'0101000000000000000000F03F000000000000F03F'
>>> wkb_w.byteorder = 0
'00000000013FF00000000000003FF0000000000000'
WKBWriter.outdim

This property may be set to change the output dimension of the geometry representation. In other words, if you have a 3D geometry then set to 3 so that the Z value is included in the WKB.

Outdim Value Description
2 The default, output 2D WKB.
3 Output 3D EWKB.

Example:

>>> from django.contrib.gis.geos import Point, WKBWriter
>>> wkb_w = WKBWriter()
>>> wkb_w.outdim
2
>>> pnt = Point(1, 1, 1)
>>> wkb_w.write_hex(pnt) # By default, no Z value included:
'0101000000000000000000F03F000000000000F03F'
>>> wkb_w.outdim = 3 # Tell writer to include Z values
>>> wkb_w.write_hex(pnt)
'0101000080000000000000F03F000000000000F03F000000000000F03F'
WKBWriter.srid

Set this property with a boolean to indicate whether the SRID of the geometry should be included with the WKB representation. Example:

>>> from django.contrib.gis.geos import Point, WKBWriter
>>> wkb_w = WKBWriter()
>>> pnt = Point(1, 1, srid=4326)
>>> wkb_w.write_hex(pnt) # By default, no SRID included:
'0101000000000000000000F03F000000000000F03F'
>>> wkb_w.srid = True # Tell writer to include SRID
>>> wkb_w.write_hex(pnt)
'0101000020E6100000000000000000F03F000000000000F03F'
class WKTWriter
WKTWriter.write(geom)

Returns the WKT of the given geometry. Example:

>>> from django.contrib.gis.geos import Point, WKTWriter
>>> pnt = Point(1, 1)
>>> wkt_w = WKTWriter()
>>> wkt_w.write(pnt)
'POINT (1.0000000000000000 1.0000000000000000)'

Footnotes

[1]See PostGIS EWKB, EWKT and Canonical Forms, PostGIS documentation at Ch. 4.1.2.
[2]For more information, read Paul Ramsey's blog post about (Much) Faster Unions in PostGIS 1.4 and Martin Davis' blog post on Fast polygon merging in JTS using Cascaded Union.

Settings

GEOS_LIBRARY_PATH

A string specifying the location of the GEOS C library. Typically, this setting is only used if the GEOS C library is in a non-standard location (e.g., /home/bob/lib/libgeos_c.so).

Note

The setting must be the full path to the C shared library; in other words you want to use libgeos_c.so, not libgeos.so.