[R-sig-Geo] Units of centroid point coordinates generated byget.Pcent and calcCentroid?

[Michael Sumner]

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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