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A Python Protocol for Geospatial Data

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)
>>> "%s" % a

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

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

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']


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

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


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BTW: with regard to tuples vs lists:

Python has a very flexible type model -- so I'd think the way to go would be to say "sequence". I know I 'like to use Nx2 numpy arrays as a list of coordinates.

Or be a bit more confining and say "anything that you can pass into json.dump and get geoJSON out.

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@sgillies GeoPandas also uses the __geo_interface__ when loading Features into a GeoDataFrame

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Hello @sgillies, It looks like links for [4] and [6] in references are outdated.

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sthagen commented Apr 10, 2023

Hello @sgillies, It looks like links for [4] and [6] in references are outdated.

I would suggest to replace with as helper for random visitors (but Sean will know better replacements):

[4]: Descartes ... but then the source link at PyPI - a fork seems to be benjimin/descartes

[6]: Shapely tutorial per wayback machine snapshot of 2019-Jun-19 ... or try to map to whatever now offers ...

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