On Thu, 11 Jun 2009, Demmler J. wrote:
Dear R fellow-users,
I'm new to using the "maptools" and "rgdal" libraries in R, so I'm very
sorry if this is something really obvious. I'm trying to get an ESRI
shape file (Wales lower super output areas) into the right format. I
think a "proper" shapefile should just have two columns with the
coordinates, so this one is a bit odd. I'm not quite sure what the
problem is, if it is the dimensions or the class of the dataset (or
both).
Your shapefile contains 1935 observations, and the geometric form
(support) of the observations is as polygons. As read, they are in the
OSGB National Grid coordinate reference system. It seems that you believe
that the data is in geographical coordinates, but it is not. If the two
attributes GEOEAST and GEONORTH are geographical coordinates, you need to
extract them, cbind() them to make a two column matrix, then make a
SpatialPoints object - watch the datum of the CRS(), it will bite - and
spTransform() that.
But it isn't obvious that you want to do that, or that GEOEAST, GEONORTH
are in geographical coordinates (or POPEAST, POPNORTH for that matter) -
do summary() of the object you have read and look at those variables. Find
out from the data provider what they are, I suspect that the GEOEAST,
GEONORTH pair is the centroid of the polygon, and that POPEAST, POPNORTH
is the population-weighted centroid of the polygon for the population
distribution at a known date.
Hope this helps,
Roger
library(maptools)
library(rgdal)
ogrInfo(".","Wales_low_soa_2001")
Driver: ESRI Shapefile number of rows 1935
Feature type: wkbPolygon with 2 dimensions
Number of fields: 7
name type length typeName
1 GEONORTH 2 31 Real
2 POPNORTH 2 31 Real
3 GEOEAST 2 31 Real
4 NAME 4 254 String
5 LABEL 4 254 String
6 POPEAST 2 31 Real
7 ZONECODE 4 254 String
wales_ll <- readOGR(".","Wales_low_soa_2001")
OGR data source with driver: ESRI Shapefile
Source: ".", layer: "Wales_low_soa_2001"
with 1935 rows and 7 columns
Feature type: wkbPolygon with 2 dimensions
I also tried ReadShapePoly, but I can't see any noticable difference.
class(wales_ll)
[1] "SpatialPolygonsDataFrame"
attr(,"package")
[1] "sp"
I would like to do something like this (I also tried to just use the GEONORTH
and GEOEAST columns, but wasn't very successful either):
proj4string(wales_ll) <- CRS("+proj=longlat ellps=WGS84")
Error in `proj4string<-`(`*tmp*`, value = <S4 object of class "CRS">) :
Geographical CRS given to non-conformant data
summary(wales_ll)
Object of class SpatialPolygonsDataFrame
Coordinates:
min max
r1 146597.1 355312.8
r2 164538.4 395994.1
Is projected: TRUE
proj4string :
[+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000
+y_0=-100000 +ellps=airy +units=m +no_defs]
etc.
and then later I would like to do:
EPSG <- make_EPSG()
EPSG[grep("British National Grid", EPSG$note), 1:2]
code note
2791 27700 # OSGB 1936 / British National Grid
wales.gb <- spTransform(wales_ll, CRS("+init=epsg:27700"))
and possibly at some point writeOGR to kml...
Any help with this is highly appreciated!
Thanks Joanne
-------------------------------------------------------------------
Linux (Ubuntu 9.04), installed from pre-compiled binary
sessionInfo()
R version 2.8.1 (2008-12-22)
i486-pc-linux-gnu
locale:
LC_CTYPE=en_GB.UTF-8;LC_NUMERIC=C;LC_TIME=en_GB.UTF-8;LC_COLLATE=en_GB.UTF-8;LC_MONETARY=C;LC_MESSAGES=en_GB.UTF-8;LC_PAPER=en_GB.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_GB.UTF-8;LC_IDENTIFICATION=C
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] rgdal_0.6-8 maptools_0.7-23 sp_0.9-37 foreign_0.8-30
loaded via a namespace (and not attached):
[1] grid_2.8.1 lattice_0.17-25
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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.biv...@nhh.no
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