On Sat, 9 Jul 2011, Dan Bebber wrote:
I have obtained shapefiles for Indian states from here:
http://www.maptell.com/index.php?option=com_remository&Itemid=159&func=fileinfo&filecatid=115&parent=category
Problem: I want to extract centroid coordinates for each State, but there is
some coding problem with the shapefiles that prevents this.
#Code:
#After extracting the shapefiles from the india_state.zip file, then:
You assume that the data source is providing cleaned boundaries, this is
not the case:
library(rgdal)
is0 <- readOGR(".", "india_state")
# reading with readOGR also reads the coordinate reference system
library(maptools)
# use checkPolygonsHoles() to make sure that the holes are correctly
# defined (the input file has a single ring defined as a hole
# which is illogical, holes have to be within something else
slot(is0, "polygons") <- lapply(slot(is0, "polygons"), checkPolygonsHoles)
# next run unionSpatialPolygons() to merge the Polygons objects that
# belong to the same name
is1 <- unionSpatialPolygons(is0, as.character(is0$NAME))
coordinates(is1)
# finally, all Polygons objects store the centroid of the largest
# component, non-hole, Polygon object as a label point, this is
# probably what you want
You don't need to use rgeos directly where wrapper functions are provided
in maptools; using:
gCentroid(is1, byid=TRUE)
returns a SpatialPoints object with centroids accommodating the multiple
Polygon components, if these suit you better.
Please consider R-sig-geo for queries of this kind.
Hope this helps,
Roger
library(maptools)
library(sp)
library(rgeos)
india <- readShapeSpatial("india_state.shp")
#Some states are made up of more than one polygon.
#State names are in the NAME column of the dataframe
india@data
plot(india[india$NAME == "Tamil Nadu",], col = 1:8)
#Extract centroid for a State (this works):
gCentroid(india[india$NAME == "Tamil Nadu",])
#Error arises for this State:
gCentroid(india[india$NAME == "Gujarat",])
#Error says:
#Error in createPolygonsComment(p) :
#rgeos_PolyCreateComment: orphaned hole, cannot find containing polygon for
hole at index 2
#Code ends
I don't know my way around SpatialPolygonsDataFrame objects very well, and have
no idea how to fix the errant hole.
Any help greatly appreciated.
Thanks
Dan Bebber
--
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.biv...@nhh.no
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