sgillies/geo_interface.rst

## geo_interface.rst

      
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              geo_interface.rst
            
          
    Author: Sean Gillies
Version: 1.0

Abstract

This document describes a GeoJSON-like protocol for geo-spatial (GIS) vector data.

Introduction

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.

__geo_interface__

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.


Examples

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'


Implementations

Python programs and packages that you have heard of – and made be a frequent
user of – already implement this protocol:

ArcPy [3]
descartes [4]
geojson [5]
PySAL [6]


Shapely

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


References


[1] http://docs.scipy.org/doc/numpy/reference/arrays.interface.html


[2] https://tools.ietf.org/html/rfc7946


[3] https://desktop.arcgis.com/en/arcmap/latest/analyze/arcpy-functions/asshape.htm


[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