How to surround a Polygon object with a corridor of specified width?

Question

I'd like to build a corridor surrounding a polygon, so that the corridor's outer bounds all have the same distance to the inner bounds. Like this:

I've made this lousy graphic in Paint, since I don't even know how this operation is called. I hope you get the idea. To avoid confusion and misplaced suggestions: the outer polygon is NOT a scaled version of the inner. I know I need some transformation for this, I just don't know what kind of.

I'm implementing it in Python, I thought of shapely library, but I'm not sure if that's the one to do it, less so which method might be useful. Thanks in advance

Answer 1

The operation your looking for is called buffering, and is a standard function available in most GIS libraries.

In PostGIS/SpatiaLite it's called ST_Buffer(geometry g1, float radius_of_buffer);

In shapely it's called buffer. An example from the shapely docs:

>>> from shapely.geometry import LinearRing, Polygon
>>> coords = [(0, 0), (1, 1), (1, 0)]
>>> r = LinearRing(coords)
>>> s = Polygon(r)
>>> s.area
0.5
>>> t = Polygon(s.buffer(1.0).exterior, [r])
>>> t.area
6.5507620529190334

OpenGeo has some great examples of using ST_Buffer

Answer 2

For users looking for a non-programmatic answer:

You could use QGIS, buffer your polygon and then use difference.