rgeos::gArea() to all polygons within sp polygons DF
When working with polygons with holes sp objects do not necessarily contain information on which holes belong to which polygons which is something that rgeos needs to know to properly calculate properties like area. You can use the createSPComment function to use rgeos to assign holes to polygons which will create the comment attribute mentioned in the warning. I believe the area slot of the Polygons object also ignores any holes and so it should match the result from gArea when ignoring orphaned holes as well, but this is the area of the containing polygon not the area of the containing polygon - area of any holes. -Colin
On Apr 9, 2012, at 11:12 AM, 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 Agus El d?a 8 de abril de 2012 17:26, Edzer Pebesma <edzer.pebesma at uni-muenster.de> escribi?:
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
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-- Edzer Pebesma Institute for Geoinformatics (ifgi), University of M?nster Weseler Stra?e 253, 48151 M?nster, 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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