GDAL stands for Geospatial Data Abstraction Library, and is a veritable “swiss army knife” of GIS data functionality. A subset of GDAL is the OGR Simple Features Library, which specializes in reading and writing vector geographic data in a variety of standard formats.
GeoDjango provides a high-level Python interface for some of the capabilities of OGR, including the reading and coordinate transformation of vector spatial data.
Note
Although the module is named gdal, GeoDjango only supports some of the capabilities of OGR. Thus, none of GDAL’s features with respect to raster (image) data are supported at this time.
The GDAL/OGR tools described here are designed to help you read in your geospatial data, in order for most of them to be useful you have to have some data to work with. If you’re starting out and don’t yet have any data of your own to use, GeoDjango comes with a number of simple data sets that you can use for testing. This snippet will determine where these sample files are installed on your computer:
>>> import os
>>> import django.contrib.gis
>>> GIS_PATH = os.path.dirname(django.contrib.gis.__file__)
>>> CITIES_PATH = os.path.join(GIS_PATH, 'tests/data/cities/cities.shp')
DataSource is a wrapper for the OGR data source object that supports reading data from a variety of OGR-supported geospatial file formats and data sources using a simple, consistent interface. Each data source is represented by a DataSource object which contains one or more layers of data. Each layer, represented by a Layer object, contains some number of geographic features (Feature), information about the type of features contained in that layer (e.g. points, polygons, etc.), as well as the names and types of any additional fields (Field) of data that may be associated with each feature in that layer.
The constructor for DataSource just a single parameter: the path of the file you want to read. However, OGR also supports a variety of more complex data sources, including databases, that may be accessed by passing a special name string instead of a path. For more information, see the OGR Vector Formats documentation. The name property of a DataSource instance gives the OGR name of the underlying data source that it is using.
Once you’ve created your DataSource, you can find out how many layers of data it contains by accessing the layer_count property, or (equivalently) by using the len() function. For information on accessing the layers of data themselves, see the next section:
>>> from django.contrib.gis.gdal import DataSource
>>> ds = DataSource(CITIES_PATH)
>>> ds.name # The exact filename may be different on your computer
'/usr/local/lib/python2.6/site-packages/django/contrib/gis/tests/data/cities/cities.shp'
>>> ds.layer_count # This file only contains one layer
1
Returns the number of layers in the data source.
Returns the name of the data source.
Layer is a wrapper for a layer of data in a DataSource object. You never create a Layer object directly. Instead, you retrieve them from a DataSource object, which is essentially a standard Python container of Layer objects. For example, you can access a specific layer by its index (e.g. ds[0] to access the first layer), or you can iterate over all the layers in the container in a for loop. The Layer itself acts as a container for geometric features.
Typically, all the features in a given layer have the same geometry type. The geom_type property of a layer is an OGRGeomType that identifies the feature type. We can use it to print out some basic information about each layer in a DataSource:
>>> for layer in ds:
... print('Layer "%s": %i %ss' % (layer.name, len(layer), layer.geom_type.name))
...
Layer "cities": 3 Points
The example output is from the cities data source, loaded above, which evidently contains one layer, called "cities", which contains three point features. For simplicity, the examples below assume that you’ve stored that layer in the variable layer:
>>> layer = ds[0]
Returns the name of this layer in the data source.
>>> layer.name
'cities'
Returns the number of features in the layer. Same as len(layer):
>>> layer.num_feat
3
Returns the geometry type of the layer, as an OGRGeomType object:
>>> layer.geom_type.name
'Point'
Returns the number of fields in the layer, i.e the number of fields of data associated with each feature in the layer:
>>> layer.num_fields
4
Returns a list of the names of each of the fields in this layer:
>>> layer.fields
['Name', 'Population', 'Density', 'Created']
Returns a list of the data types of each of the fields in this layer. These are subclasses of Field, discussed below:
>>> [ft.__name__ for ft in layer.field_types]
['OFTString', 'OFTReal', 'OFTReal', 'OFTDate']
Returns a list of the maximum field widths for each of the fields in this layer:
>>> layer.field_widths
[80, 11, 24, 10]
Returns a list of the numeric precisions for each of the fields in this layer. This is meaningless (and set to zero) for non-numeric fields:
>>> layer.field_precisions
[0, 0, 15, 0]
Returns the spatial extent of this layer, as an Envelope object:
>>> layer.extent.tuple
(-104.609252, 29.763374, -95.23506, 38.971823)
Property that returns the SpatialReference associated with this layer:
>>> print(layer.srs)
GEOGCS["GCS_WGS_1984",
DATUM["WGS_1984",
SPHEROID["WGS_1984",6378137,298.257223563]],
PRIMEM["Greenwich",0],
UNIT["Degree",0.017453292519943295]]
If the Layer has no spatial reference information associated with it, None is returned.
Property that may be used to retrieve or set a spatial filter for this layer. A spatial filter can only be set with an OGRGeometry instance, a 4-tuple extent, or None. When set with something other than None, only features that intersect the filter will be returned when iterating over the layer:
>>> print(layer.spatial_filter)
None
>>> print(len(layer))
3
>>> [feat.get('Name') for feat in layer]
['Pueblo', 'Lawrence', 'Houston']
>>> ks_extent = (-102.051, 36.99, -94.59, 40.00) # Extent for state of Kansas
>>> layer.spatial_filter = ks_extent
>>> len(layer)
1
>>> [feat.get('Name') for feat in layer]
['Lawrence']
>>> layer.spatial_filter = None
>>> len(layer)
3
A method that returns a list of the values of a given field for each feature in the layer:
>>> layer.get_fields('Name')
['Pueblo', 'Lawrence', 'Houston']
A method that returns a list containing the geometry of each feature in the layer. If the optional argument geos is set to True then the geometries are converted to GEOSGeometry objects. Otherwise, they are returned as OGRGeometry objects:
>>> [pt.tuple for pt in layer.get_geoms()]
[(-104.609252, 38.255001), (-95.23506, 38.971823), (-95.363151, 29.763374)]
Returns a boolean indicating whether this layer supports the given capability (a string). Examples of valid capability strings include: 'RandomRead', 'SequentialWrite', 'RandomWrite', 'FastSpatialFilter', 'FastFeatureCount', 'FastGetExtent', 'CreateField', 'Transactions', 'DeleteFeature', and 'FastSetNextByIndex'.
Feature wraps an OGR feature. You never create a Feature object directly. Instead, you retrieve them from a Layer object. Each feature consists of a geometry and a set of fields containing additional properties. The geometry of a field is accessible via its geom property, which returns an OGRGeometry object. A Feature behaves like a standard Python container for its fields, which it returns as Field objects: you can access a field directly by its index or name, or you can iterate over a feature’s fields, e.g. in a for loop.
Returns the geometry for this feature, as an OGRGeometry object:
>>> city.geom.tuple
(-104.609252, 38.255001)
A method that returns the value of the given field (specified by name) for this feature, not a Field wrapper object:
>>> city.get('Population')
102121
Returns the type of geometry for this feature, as an OGRGeomType object. This will be the same for all features in a given layer, and is equivalent to the Layer.geom_type property of the Layer` object the feature came from.
Returns the number of fields of data associated with the feature. This will be the same for all features in a given layer, and is equivalent to the Layer.num_fields property of the Layer object the feature came from.
Returns a list of the names of the fields of data associated with the feature. This will be the same for all features in a given layer, and is equivalent to the Layer.fields property of the Layer object the feature came from.
Returns the feature identifier within the layer:
>>> city.fid
0
Returns the name of the Layer that the feature came from. This will be the same for all features in a given layer:
>>> city.layer_name
'cities'
A method that returns the index of the given field name. This will be the same for all features in a given layer:
>>> city.index('Population')
1
Returns the name of this field:
>>> city['Name'].name
'Name'
Returns the OGR type of this field, as an integer. The FIELD_CLASSES dictionary maps these values onto subclasses of Field:
>>> city['Density'].type
2
Returns a string with the name of the data type of this field:
>>> city['Name'].type_name
'String'
Returns the value of this field. The Field class itself returns the value as a string, but each subclass returns the value in the most appropriate form:
>>> city['Population'].value
102121
Returns the width of this field:
>>> city['Name'].width
80
Returns the numeric precision of this field. This is meaningless (and set to zero) for non-numeric fields:
>>> city['Density'].precision
15
Returns the value of the field as a double (float):
>>> city['Density'].as_double()
874.7
Returns the value of the field as an integer:
>>> city['Population'].as_int()
102121
Returns the value of the field as a string:
>>> city['Name'].as_string()
'Pueblo'
Returns the value of the field as a tuple of date and time components:
>>> city['Created'].as_datetime()
(c_long(1999), c_long(5), c_long(23), c_long(0), c_long(0), c_long(0), c_long(0))
The Driver class is used internally to wrap an OGR DataSource driver.
Returns the number of OGR vector drivers currently registered.
OGRGeometry objects share similar functionality with GEOSGeometry objects, and are thin wrappers around OGR’s internal geometry representation. Thus, they allow for more efficient access to data when using DataSource. Unlike its GEOS counterpart, OGRGeometry supports spatial reference systems and coordinate transformation:
>>> from django.contrib.gis.gdal import OGRGeometry
>>> polygon = OGRGeometry('POLYGON((0 0, 5 0, 5 5, 0 5))')
This object is a wrapper for the OGR Geometry class. These objects are instantiated directly from the given geom_input parameter, which may be a string containing WKT, HEX, GeoJSON, a buffer containing WKB data, or an OGRGeomType object. These objects are also returned from the Feature.geom attribute, when reading vector data from Layer (which is in turn a part of a DataSource).
Constructs a Polygon from the given bounding-box (a 4-tuple).
Returns the number of points in a LineString, the number of rings in a Polygon, or the number of geometries in a GeometryCollection. Not applicable to other geometry types.
Iterates over the points in a LineString, the rings in a Polygon, or the geometries in a GeometryCollection. Not applicable to other geometry types.
Returns the point at the specified index for a LineString, the interior ring at the specified index for a Polygon, or the geometry at the specified index in a GeometryCollection. Not applicable to other geometry types.
Returns the number of coordinated dimensions of the geometry, i.e. 0 for points, 1 for lines, and so forth:
>> polygon.dimension
2
Returns or sets the coordinate dimension of this geometry. For example, the value would be 2 for two-dimensional geometries.
Returns the number of elements in this geometry:
>>> polygon.geom_count
1
Returns the number of points used to describe this geometry:
>>> polygon.point_count
4
Alias for point_count.
Alias for point_count.
Returns the type of this geometry, as an OGRGeomType object.
Returns the name of the type of this geometry:
>>> polygon.geom_name
'POLYGON'
Returns the area of this geometry, or 0 for geometries that do not contain an area:
>>> polygon.area
25.0
Returns the envelope of this geometry, as an Envelope object.
Returns the envelope of this geometry as a 4-tuple, instead of as an Envelope object:
>>> point.extent
(0.0, 0.0, 5.0, 5.0)
This property controls the spatial reference for this geometry, or None if no spatial reference system has been assigned to it. If assigned, accessing this property returns a SpatialReference object. It may be set with another SpatialReference object, or any input that SpatialReference accepts. Example:
>>> city.geom.srs.name
'GCS_WGS_1984'
Returns or sets the spatial reference identifier corresponding to SpatialReference of this geometry. Returns None if there is no spatial reference information associated with this geometry, or if an SRID cannot be determined.
Returns a GEOSGeometry object corresponding to this geometry.
Returns a string representation of this geometry in GML format:
>>> OGRGeometry('POINT(1 2)').gml
'<gml:Point><gml:coordinates>1,2</gml:coordinates></gml:Point>'
Returns a string representation of this geometry in HEX WKB format:
>>> OGRGeometry('POINT(1 2)').hex
'0101000000000000000000F03F0000000000000040'
Returns a string representation of this geometry in JSON format:
>>> OGRGeometry('POINT(1 2)').json
'{ "type": "Point", "coordinates": [ 1.000000, 2.000000 ] }'
Returns a string representation of this geometry in KML format.
Returns the size of the WKB buffer needed to hold a WKB representation of this geometry:
>>> OGRGeometry('POINT(1 2)').wkb_size
21
Returns a buffer containing a WKB representation of this geometry.
Returns a string representation of this geometry in WKT format.
Returns the EWKT representation of this geometry.
Returns a new OGRGeometry clone of this geometry object.
If there are any rings within this geometry that have not been closed, this routine will do so by adding the starting point to the end:
>>> triangle = OGRGeometry('LINEARRING (0 0,0 1,1 0)')
>>> triangle.close_rings()
>>> triangle.wkt
'LINEARRING (0 0,0 1,1 0,0 0)'
Transforms this geometry to a different spatial reference system. May take a CoordTransform object, a SpatialReference object, or any other input accepted by SpatialReference (including spatial reference WKT and PROJ.4 strings, or an integer SRID). By default nothing is returned and the geometry is transformed in-place. However, if the clone keyword is set to True then a transformed clone of this geometry is returned instead.
Returns True if this geometry intersects the other, otherwise returns False.
Returns True if this geometry is equivalent to the other, otherwise returns False.
Returns True if this geometry is spatially disjoint to (i.e. does not intersect) the other, otherwise returns False.
Returns True if this geometry touches the other, otherwise returns False.
Returns True if this geometry crosses the other, otherwise returns False.
Returns True if this geometry is contained within the other, otherwise returns False.
Returns True if this geometry contains the other, otherwise returns False.
Returns True if this geometry overlaps the other, otherwise returns False.
The boundary of this geometry, as a new OGRGeometry object.
The smallest convex polygon that contains this geometry, as a new OGRGeometry object.
Returns the region consisting of the difference of this geometry and the other, as a new OGRGeometry object.
Returns the region consisting of the intersection of this geometry and the other, as a new OGRGeometry object.
Returns the region consisting of the symmetric difference of this geometry and the other, as a new OGRGeometry object.
Returns the region consisting of the union of this geometry and the other, as a new OGRGeometry object.
Returns the coordinates of a point geometry as a tuple, the coordinates of a line geometry as a tuple of tuples, and so forth:
>>> OGRGeometry('POINT (1 2)').tuple
(1.0, 2.0)
>>> OGRGeometry('LINESTRING (1 2,3 4)').tuple
((1.0, 2.0), (3.0, 4.0))
An alias for tuple.
Returns the X coordinate of this point:
>>> OGRGeometry('POINT (1 2)').x
1.0
Returns the Y coordinate of this point:
>>> OGRGeometry('POINT (1 2)').y
2.0
Returns the Z coordinate of this point, or None if the the point does not have a Z coordinate:
>>> OGRGeometry('POINT (1 2 3)').z
3.0
Returns a list of X coordinates in this line:
>>> OGRGeometry('LINESTRING (1 2,3 4)').x
[1.0, 3.0]
Returns a list of Y coordinates in this line:
>>> OGRGeometry('LINESTRING (1 2,3 4)').y
[2.0, 4.0]
Returns a list of Z coordinates in this line, or None if the line does not have Z coordinates:
>>> OGRGeometry('LINESTRING (1 2 3,4 5 6)').z
[3.0, 6.0]
This class allows for the representation of an OGR geometry type in any of several ways:
>>> from django.contrib.gis.gdal import OGRGeomType
>>> gt1 = OGRGeomType(3) # Using an integer for the type
>>> gt2 = OGRGeomType('Polygon') # Using a string
>>> gt3 = OGRGeomType('POLYGON') # It's case-insensitive
>>> print(gt1 == 3, gt1 == 'Polygon') # Equivalence works w/non-OGRGeomType objects
True True
Returns a short-hand string form of the OGR Geometry type:
>>> gt1.name
'Polygon'
Returns the number corresponding to the OGR geometry type:
>>> gt1.num
3
Returns the Django field type (a subclass of GeometryField) to use for storing this OGR type, or None if there is no appropriate Django type:
>>> gt1.django
'PolygonField'
Represents an OGR Envelope structure that contains the minimum and maximum X, Y coordinates for a rectangle bounding box. The naming of the variables is compatible with the OGR Envelope C structure.
The value of the minimum X coordinate.
The value of the maximum X coordinate.
The value of the minimum Y coordinate.
The value of the maximum Y coordinate.
The upper-right coordinate, as a tuple.
The lower-left coordinate, as a tuple.
A tuple representing the envelope.
A string representing this envelope as a polygon in WKT format.
Spatial reference objects are initialized on the given srs_input, which may be one of the following:
Example:
>>> wgs84 = SpatialReference('WGS84') # shorthand string
>>> wgs84 = SpatialReference(4326) # EPSG code
>>> wgs84 = SpatialReference('EPSG:4326') # EPSG string
>>> proj4 = '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs '
>>> wgs84 = SpatialReference(proj4) # PROJ.4 string
>>> wgs84 = SpatialReference("""GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]""") # OGC WKT
Returns the value of the given string attribute node, None if the node doesn’t exist. Can also take a tuple as a parameter, (target, child), where child is the index of the attribute in the WKT. For example:
>>> wkt = 'GEOGCS["WGS 84", DATUM["WGS_1984, ... AUTHORITY["EPSG","4326"]]')
>>> srs = SpatialReference(wkt) # could also use 'WGS84', or 4326
>>> print(srs['GEOGCS'])
WGS 84
>>> print(srs['DATUM'])
WGS_1984
>>> print(srs['AUTHORITY'])
EPSG
>>> print(srs['AUTHORITY', 1]) # The authority value
4326
>>> print(srs['TOWGS84', 4]) # the fourth value in this wkt
0
>>> print(srs['UNIT|AUTHORITY']) # For the units authority, have to use the pipe symbol.
EPSG
>>> print(srs['UNIT|AUTHORITY', 1]) # The authority value for the units
9122
The attribute value for the given target node (e.g. 'PROJCS'). The index keyword specifies an index of the child node to return.
Returns the authority name for the given string target node.
Returns the authority code for the given string target node.
Returns a clone of this spatial reference object.
This method inspects the WKT of this SpatialReference, and will add EPSG authority nodes where an EPSG identifier is applicable.
Morphs this SpatialReference from ESRI’s format to EPSG
Morphs this SpatialReference to ESRI’s format.
Checks to see if the given spatial reference is valid, if not an exception will be raised.
Import spatial reference from EPSG code.
Import spatial reference from PROJ.4 string.
Import spatial reference from WKT.
Import spatial reference from XML.
Returns the name of this Spatial Reference.
Returns the SRID of top-level authority, or None if undefined.
Returns the name of the linear units.
Returns the value of the linear units.
Returns the name of the angular units.”
Returns the value of the angular units.
Returns a 2-tuple of the units value and the units name, and will automatically determines whether to return the linear or angular units.
Returns a tuple of the ellipsoid parameters for this spatial reference: (semimajor axis, semiminor axis, and inverse flattening)
Returns the semi major axis of the ellipsoid for this spatial reference.
Returns the semi minor axis of the ellipsoid for this spatial reference.
Returns the inverse flattening of the ellipsoid for this spatial reference.
Returns True if this spatial reference is geographic (root node is GEOGCS).
Returns True if this spatial reference is local (root node is LOCAL_CS).
Returns True if this spatial reference is a projected coordinate system (root node is PROJCS).
Returns the WKT representation of this spatial reference.
Returns the ‘pretty’ representation of the WKT.
Returns the PROJ.4 representation for this spatial reference.
Alias for SpatialReference.proj.
Returns the XML representation of this spatial reference.
Represents a coordinate system transform. It is initialized with two SpatialReference, representing the source and target coordinate systems, respectively. These objects should be used when performing the same coordinate transformation repeatedly on different geometries:
>>> ct = CoordTransform(SpatialReference('WGS84'), SpatialReference('NAD83'))
>>> for feat in layer:
... geom = feat.geom # getting clone of feature geometry
... geom.transform(ct) # transforming
Dec 23, 2012