[R-sig-Geo] Creating random (but believable) geographies

Barry Rowlingson b.rowlingson at lancaster.ac.uk
Mon Dec 28 11:33:55 CET 2009

I've recently been working on a web site for tropical disease mapping
in Africa. As a demo, I'm not using real data, but I have been using
real countries. I don't really want to use real countries just in case
there's any misunderstanding that this is simulated case numbers.

 So I've thought about constructing random polygons to represent a
fake country and its administrative subdivisions. Any ideas how to do
this? Here's some thoughts so far:

 * Generate some random points, create voronoi polygons, then wiggle
the edges to make them look more natural. Problem may be stopping the
wiggling from causing edges to overlap, but maybe that just needs a
bunch of point-in-poly tests... Hmmm...

 * Overlay two existing unassociated polygon geometries, such as UK
counties and Ethiopian districts, compute intersections and rebuild
the polygon topology. Then you should end up with polygonal regions
with outlines composed of both sets of polygons. You might end up with
lots of tiny polygons where two lines closely overlap along their
length, but maybe a post-processing step can fix this.

 * Raster-based methods: similar to the voronoi method, start with a
number of seed points on a grid and grow by some random method - then
do raster-vector conversion.

 Obviously there's lots of possible options such as creating islands
and holes, but I thought I'd start with the basics.

 If anyone has any existing fake geographies in digital form then it
might be an idea to create a repository of them...


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