Hello,
In short: they are probably in UTM (metres) for whatever UTM zone your
counties are closest to.
Shape files sometimes have this information in a .prj file.
If you use rgdal,
library(sp)
library(rgdal)
d <- readOGR("FiveNWStateCounties.shp", "FiveNWStateCounties")
proj4string(d)
If proj4string is not NA, then you can use spTransform:
d.ll <- spTransform(d, CRS("+proj=longlat"))
Otherwise, you can project that matrix of coordinates with (if "m" is a
matrix of your polygon centroids)
x.ll <- project(m, "insert non-NA p4string here", inv = T)
Do you have a .prj file, or other information that refers to UTM, or
State Plane,with parameters?
HTH, Mike.
Rick Reeves wrote:
> Hello List:
>
> I'm getting to know the polygon centroid - generating functions
> get.Pcent() (maptools) and calcCentroid() (PBSmapping),
> and would like to know the units of the centroid point coordinates.
>
> Working with maptools version:
>
> library(maptools)
> MapPolysMapClass = read.shape("FiveNWStateCounties")
> MapPolyCents = get.Pcent(MapPolysMapClass)
>
> Here is a portion of my input polygon file (CNTRDLONG/LAT computed
> elsewhere),
>
> Browse[1]> MapPolysMapClass$att.data
> STATEFIPS COUNTYFIPS COUNTYNAME LSAD LSAD_TRANS CNTRDLONG
> CNTRDLAT F_AREA
> 1 53 073 Whatcom 06 County
> -121.7137 48.82591 5584761843
> 2 53 073 Whatcom 06 County
> -123.0569 48.98870 12378754
> 3 30 029 Flathead 06 County
> -114.0498 48.29519 13613065142
> 4 30 053 Lincoln 06 County
> -115.4052 48.54250 9518449902
> 5 16 021 Boundary 06 County
> -116.4629 48.76703 3309965666
>
> .and here are the corresponding polygon centroids
>
> [1,] -1762352.7 1282662.2
> [2,] -1846616.8 1326565.8
> [3,] -1256556.1 1099405.3
> [4,] -1343898.6 1146606.3
> [5,] -1410325.9 1187932.0
>
> The PBSmapping calcCentroid function (using PolySet inputs) generates
> the same numbers.
>
> So, what units are these coordinates? I havent found the answer in the
> maptools or PBSmapping package manuals.
> Side question: (How) can I translate them to Lat/Long coordinates?
>
>
> Thanks,
> Rick Reeves
>
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>
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