README.md

geojson-rewind

A Python library for enforcing polygon ring winding order in GeoJSON

The GeoJSON spec mandates the right hand rule:

A linear ring MUST follow the right-hand rule with respect to the area it bounds, i.e., exterior rings are counterclockwise, and holes are clockwise.

This helps you generate compliant Polygon and MultiPolygon geometries.

Note: Co-ordinates in the input data are assumed to be WGS84 with (lon, lat) ordering, as per RFC 7946. Input with co-ordinates using any other CRS may lead to unexpected results.

Installation

pip install geojson-rewind

Usage

As a Library

Enforce RFC 7946 ring winding order (input/output is a GeoJSON string):

>>> from geojson_rewind import rewind

>>> input = """{
...      "geometry": {   "coordinates": [   [   [100, 0],
...                                             [100, 1],
...                                             [101, 1],
...                                             [101, 0],
...                                             [100, 0]]],
...                      "type": "Polygon"},
...      "properties": {"foo": "bar"},
...      "type": "Feature"}"""

>>> output = rewind(input)

>>> output
'{"geometry": {"coordinates": [[[100, 0], [101, 0], [101, 1], [100, 1], [100, 0]]], "type": "Polygon"}, "properties": {"foo": "bar"}, "type": "Feature"}'

>>> type(output)
<class 'str'>

Enforce RFC 7946 ring winding order (input/output is a python dict):

>>> from geojson_rewind import rewind

>>> input = {
...     'geometry': {   'coordinates': [   [   [100, 0],
...                                            [100, 1],
...                                            [101, 1],
...                                            [101, 0],
...                                            [100, 0]]],
...                     'type': 'Polygon'},
...     'properties': {'foo': 'bar'},
...     'type': 'Feature'}

>>> output = rewind(input)

>>> output
{'geometry': {'coordinates': [[[100, 0], [101, 0], [101, 1], [100, 1], [100, 0]]], 'type': 'Polygon'}, 'properties': {'foo': 'bar'}, 'type': 'Feature'}

>>> type(output)
<class 'dict'>

On the Console

# Enforce ring winding order on a GeoJSON file
$ rewind in.geojson > out.geojson

# fetch GeoJSON from the web and enforce ring winding order
$ curl "https://myserver.com/in.geojson" | rewind

Versioning

geojson-rewind follows semantic versioning. For this project, the "API" also includes:

• CLI flags and options
• CLI exit codes

In line with common practice in the python community, geojson-rewind will drop compatibility with unsupported python versions without incrementing the major version.

Acknowledgements

geojson-rewind is a python port of Mapbox's javascript geojson-rewind package. Credit to Tom MacWright and contributors.