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

[Rick Reeves]

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

-- 
Rick Reeves	
Scientific Programmer / Analyst	
National Center for Ecological Analysis and Synthesis
UC Santa Barbara
reeves at nceas.ucsb.edu
www.nceas.ucsb.edu
805 892 2533

-------------- next part --------------
A non-text attachment was scrubbed...
Name: reeves.vcf
Type: text/x-vcard
Size: 339 bytes
Desc: not available
URL: <https://stat.ethz.ch/pipermail/r-sig-geo/attachments/20070828/a78ca738/attachment.vcf>