[R-sig-Geo] rgeos::gArea() to all polygons within sp polygons DF

[Roger Bivand]

On Mon, 9 Apr 2012, Agustin Lobo wrote:

> Thanks Edzer,
>
> b2 <- gArea(BorneoLCg, byid=T)
>
> I get many warnings such as:
> 40: In RGEOSMiscFunc(spgeom, byid, "rgeos_area") :
>  Polygons object missing comment attribute ignoring hole(s). See
> function createSPComment.
>
> is this normal?
>
> The result is identical to
> b <- sapply(BorneoLCg at polygons, function(x) x at area)
>
> while it is different than the one provided by QGIS (known to be wrong for very
> large polygons only because of a format problem) and postgis (assumed
> to be correct). Please give a look to:
> http://hub.qgis.org/issues/5332

Please always provide sessionInfo() output, and a copy of the rgeos 
startup messages (showing which version of GEOS is used). With rgeos 0.2-5 
and GEOS runtime version: 3.3.2-CAPI-1.7.2, I have:

bn <- readOGR(".", "BorneoLCv2google")
b2 <- gArea(bn, byid=TRUE)
> print(b2[25967], digits=16)
            25966
8550196769.93012
> print(b2[26570], digits=16)
             26569
173647473841.3217

so the hole warnings were the reason for the difference. SVN revision 327 
of 16 March enforced the use of createSPComment() in gArea() - the user 
was expected to denote holes properly themselves before this change. 
Update your rgeos and try again.

Please update the QGIS #5332 to state that from rgeos 0.2-5, the values 
are correct. The area slot in the Polygons object is a gross area used to 
order plotting (because until recently R polygons simply overplotted one 
another), so it is not guaranteed to be a planar area measure for 
polygons with holes.

Roger

>
> Agus
>
>
> El día 8 de abril de 2012 17:26, Edzer Pebesma
> <edzer.pebesma at uni-muenster.de> escribió:
>> Agus, have you tried
>>
>> gArea(BorneoLCg, byid = TRUE)
>>
>> ?
>>
>> On 04/08/2012 04:52 PM, Agustin Lobo wrote:
>>> Hi!
>>>
>>> How do I  apply gArea()) to all polygons within an spatial pol. DF?
>>> I've tried
>>> areas <- apply(BorneoLCg,gArea)
>>> and
>>> areas <- sapply(BorneoLCg,gArea)
>>>
>>> but this is not correct.
>>>
>>> I want to compare to
>>> areas <- sapply(BorneoLCg at polygons, function(x) x at area)
>>>
>>> Thanks
>>>
>>> Agus
>>>
>>> _______________________________________________
>>> R-sig-Geo mailing list
>>> R-sig-Geo at r-project.org
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>
>> --
>> Edzer Pebesma
>> Institute for Geoinformatics (ifgi), University of Münster
>> Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
>> 8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
>> http://www.52north.org/geostatistics      e.pebesma at wwu.de
>>
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>
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-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
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