[R-sig-Geo] Handling invalid geometries from the ‘mapdata’ package

[Roger Bivand]

On Mon, 28 Mar 2011, Roger Bivand wrote:

> On Mon, 28 Mar 2011, Karl Ove Hufthammer wrote:
>
>> Roger Bivand wrote:
>> 
>>>> pls1 <- lapply(pls, checkPolygonsHoles)
>>> Error in RGEOSBinPredFunc(spgeom1, spgeom2, byid, "rgeos_equals") :
>>> TopologyException: side location conflict at 78.9364 33.4081
>>> 
>>> then I can reproduce it, and will examine whether it can be handled - the
>>> checkPolygonsHoles() function also uses GEOS, but probably should warn
>>> after failing try(gEquals(...)).
>>> 
>>> Was this your error?
>> 
>> Yes.
>> 
>> I also eventually managed to get a segfault when trying various stuff like 
>> this, but it may have been unrelated. (At least, I can’t reproduce it.)
>
> Thanks. Updating to maptools 0_8-5 and rgeos 0_1-3 (sources on CRAN now) 
> should provide better protection against non-conformant data causing a 
> seg-fault in GEOS - GEOS really likes its polygons *very* tidy.

The attached plot shows this issue, which is in the area between northern 
India, Kashmir, and China, so perhaps reality is more complicated than 
GEOS prefers? I've only plotted the border given in the data set for 
China, which includes the odd reversed straight line. If I include other 
borders, I see a line starting at right angles to the northern end of this 
line and heading WSW, which may be another demarcation line. Apart from 
this case, the rest of worldHires got through OK - the maptools code on 
SVN now reports:

> pls1 <- lapply(pls, checkPolygonsHoles)
Warning message:
In checkPolygonsGEOS(x, properly = properly) :
   Polygons object China, Polygon 1: Error in RGEOSBinPredFunc(spgeom1, 
spgeom2, byid, "rgeos_equals") :
   TopologyException: side location conflict at 78.9364 33.4081

instead of failing.

Roger

>
> Roger
>
>> 
>> -- 
>> Karl Ove Hufthammer
>> 
>> _______________________________________________
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>
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no
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