Grouping points to make polygon using Python

Question

I have an array of coordinates

Point= [lat,long]

ListPoint= [[38.203655,-76.113281],[38.10,-76.12],..]

I want to make a polygon that contains all these points. There is a project developed by Flicker - The shape of Alpha but the project is not open.

How can I do that?

Answer 1

The following explains (1.) how to construct technically a polygon from points and (2.) different methods to construct a polygon from points.

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1. Tool to create a polygon from point coordinates

I would use the GDAL Python bindings to do that.

To give you a starting point have a look at the following script:

from osgeo import ogr

# you have a list of points
listPoint = [[13.415449261665342, 52.502674590782519],[13.416039347648621, 52.50250152147968],[13.415787220001221, 52.501845158120446],[13.416162729263306, 52.502201097675766],[13.415406346321104, 52.502334982450677],[13.415111303329468,52.50204435400651]]

# Add the points to the ring
ring = ogr.Geometry(ogr.wkbLinearRing)
for point in listPoint:
    lat = point[0]
    lon = point[1]
    print lat, lon
    ring.AddPoint(lat,lon)


# Add first point again to ring to close polygon
ring.AddPoint(listPoint[0][0], listPoint[0][3])

# Add the ring to the polygon
poly = ogr.Geometry(ogr.wkbPolygon)
poly.AddGeometry(ring)
print poly.ExportToJson()

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2. Methods to create a polygon from point coordinates

There are two problems to think about when creating a polygon from points.

• The order of the points
• Points that might be contained by the polygon

Two methods to approach this problem:

1. Convex Hull: It is the smallest convex set containing all points. You can see it as a rubber band wrapped around the “outside” points.

2. Delaunay triangulation: All points are connected to each other.

Answer 2

The closest thing we have in Python to a standard built-in Polygon is defined in "A Python Protocol for Geospatial Data": https://gist.github.com/sgillies/2217756.

The most straightforward way to get a Python polygon is to do this:

# you have a list of points
listPoint = [[13.415449261665342, 52.502674590782519],[13.416039347648621, 52.50250152147968],[13.415787220001221, 52.501845158120446],[13.416162729263306, 52.502201097675766],[13.415406346321104, 52.502334982450677],[13.415111303329468,52.50204435400651]]

polygon = {'type': 'Polygon', 'coordinates': [listPoint]}

If you want to serialize this into GeoJSON for use with web mapping software, you'll need to reverse the order of coordinate pairs (longitude comes first in GeoJSON, KML, etc.):

polygon = {'type': 'Polygon', 'coordinates': [[pt[::-1] for pt in listPoint]]}

import json
print json.dumps(polygon)

@Ustroetz's example does the same thing (ExportToJson() produces the same result as json.dumps(polygon) above, but indirectly through the osgeo.gdal API.