Author: Sean Gillies Version: 1.0
This document describes a GeoJSON-like protocol for geo-spatial (GIS) vector data.
Python has a number of built-in protocols (descriptors, iterators, etc). A very
simple and familiar one involves string representations of objects. The
built-in str() function calls the __str__() method of its single
argument. By implementing __str__(), instances of any class can be printed
by any other Python program.
>>> class A(object): ... def __str__(self): ... return "Eh!" ... >>> a = A() >>> str(a) 'Eh!' >>> "%s" % a 'Eh!'
What if we could do something like this for geo-spatial objects? It might, for example, let any object be analyzed using any other hypothetical software package like this:
>>> from some_analytic_module import as_geometry >>> as_geometry(obj).buffer(1.0).area # obj is a "point" of some kind 3.1365484905459389
The hypothetical as_geometry() function of the hypothetical
some_analytic_module module would access relevant data of its single argument
using an agreed upon method or attribute.
Following the lead of numpy's Array Interface [1], let's agree on
a __geo_interface__ property. To avoid creating even more protocols, let's
make the value of this attribute a Python mapping. To further minimize
invention, let's borrow from the GeoJSON format [2] for the structure of this
mapping.
The keys are:
- type (required)
- A string indicating the geospatial type. Possible values are "Feature" or a geometry type: "Point", "LineString", "Polygon", etc.
- bbox (optional)
- A tuple of floats that describes the geo-spatial bounds of the object: (left, bottom, right, top) or (west, south, east, north).
- properties (optional)
- A mapping of feature properties (labels, populations ... you name it. Dependent on the data). Valid for "Feature" types only.
- geometry (optional)
- The geometric object of a "Feature" type, also as a mapping.
- coordinates (required)
- Valid only for geometry types. This is an
(x, y)or(longitude, latitude)tuple in the case of a "Point", a list of such tuples in the "LineString" case, or a list of lists in the "Polygon" case. See the GeoJSON spec for details.
First, a toy class with a point representation:
>>> class Pointy(object):
... __geo_interface__ = {'type': 'Point', 'coordinates': (0.0, 0.0)}
...
>>> as_geometry(Pointy()).buffer(1.0).area
3.1365484905459389
Next, a toy class with a feature representation:
>>> class Placemark(object):
... __geo_interface__ = {
... 'type': 'Feature',
... 'properties': {'name': 'Phoo'},
... 'geometry': Pointy.__geo_interface__ }
>>> from my_analytic_module import as_feature
>>> as_feature(Placemark())['properties']['name']
'Phoo'
Python programs and packages that you have heard of – and made be a frequent user of – already implement this protocol:
Shapely [7] provides a shape() function that makes Shapely geometries from
objects that provide __geo_interface__ and a mapping() function that
writes geometries out as dictionaries:
>>> from shapely.geometry import Point
>>> from shapely.geometry import mapping, shape
>>> Point(0.0, 0.0).__geo_interface__
{'type': 'Point', 'coordinates': (0.0, 0.0)}
>>> shape(Point(0.0, 0.0))
<shapely.geometry.point.Point object at 0x...>
>>> mapping(Point(0.0, 0.0))
{'type': 'Point', 'coordinates': (0.0, 0.0)}
The Shapely version of the example in the introduction is:
>>> from shapely.geometry import shape >>> shape(obj).buffer(1.0).area 3.1365484905459389
where obj could be a geometry object from ArcPy or PySAL, or even a mapping
directly:
>>> shape({'type': 'Point', 'coordinates': (0.0, 0.0)}).buffer(1.0).area
3.1365484905459389
| [1] | http://docs.scipy.org/doc/numpy/reference/arrays.interface.html |
| [2] | http://geojson.org |
| [3] | http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//000v00000153000000 |
| [4] | https://bitbucket.org/sgillies/descartes/src/f97e54f3b8d4/descartes/patch.py#cl-14 |
| [5] | http://pypi.python.org/pypi/geojson/ |
| [6] | https://pysal.readthedocs.io/en/latest/users/tutorials/shapely.html |
| [7] | https://github.com/Toblerity/Shapely |