[R-sig-Geo] Cleaning up self-intersections

[Lyndon Estes]

Hi Barry,

Thanks very much for the tip.  I will try and see if I can implement
that using system calls from my R script, and will report back.

Thanks, Lyndon

On Fri, Dec 14, 2012 at 7:04 PM, Barry Rowlingson
<b.rowlingson at lancaster.ac.uk> wrote:
> On Fri, Dec 14, 2012 at 8:16 PM, Lyndon Estes <lestes at princeton.edu> wrote:
>
>> Given the type of work we are asking of people, we are getting many
>> "unclean" polygons (self-intersects, overlaps, non-noding
>> intersections, etc).  I have been writing in various patches to deal
>> with these as I have been going, but I turn now for some advice on
>> cleaning up operations. An example taken from the workflow illustrates
>> a typical problem I encounter that I am looking for an efficient way
>> to handle. This example is followed by my two questions.
>
>> So, here''s my first question.  Is there some other of cleaning
>> self-intersections up that preserves the area better than the examples
>> using gBuffer and gSimplify, and which might be more efficient and
>> provide a results that more closely approximates the original map than
>> rasterizing and polygonizing? e.g. Is it possible to explode polygons
>> at the point(s) of self-intersection, in order to create multiple
>> valid polygons?
>>
>> My second question: am I barking up the wrong tree here?  Should I
>> preferentially use something like GRASS's v.clean before I even read
>> into R?  I haven't until now because 1) I don't know GRASS very well,
>> 2) I want to minimize the number of different routines/software in the
>> workflow (which currently involves python, postgis/postgres, R,
>> openlayers).
>
>  The most promising thing I've seen for cleaning up polygon topology
> is pprepair:
>
> https://github.com/tudelft-gist/pprepair
>
>
> which won a best paper prize at OSGIS-UK this year. Its not simple to
> run properly, which is why I say most promising, and not best.
>
> If run on your extreme self-intersection 'bow tie' polygon, it
> produces a shapefile with a feature for each part of the bow tie. I
> can't currently find a way of tieing those two features back to the
> source bow-tie feature since the attributes aren't propogating
> properly. Also, it seems to merge two of the large features in your
> examples. Its a standalone C++ program that needs the CGAL library to
> work, and there's no R interface. It operates on Shapefiles (or
> possibly any OGR source?).
>
> Overall, cleaning up messy digitizing is a hard problem. Obviously
> overlaps between polygons are wrong, and pprepair does a great job of
> assigning them to one or other of the polygons, but gaps are trickier
> to assign - they might be real gaps (like a river between regions) or
> they could be digiitzing errors. Its possible that some buffering
> could help here, before doing pprepair.
>
> Barry



-- 
Lyndon Estes
Associate Research Scholar
Princeton University
+1-609-258-8308 (o)
+1-609-258-2799 (f)
+1-202-431-0496 (m)
lestes at princeton.edu
www.princeton.edu/~lestes