Calculating area of polygons

Dear list,

I would like to calculate the area of a polygon. I have found areapl() in splancs but am having trouble using it.

x<-1
poly <- readShapePoly("a1_l4_aerial_ws.shp", IDvar="cat", proj4string=CRS("+proj=tmerc +south +ellips=WGS84 +datum=WGS84 +lon_o=31 +k_0=1 +units=m +no_defs"))
poly_add <- poly[poly$cat[x], ]
areapl(poly_add)
Error in `[.data.frame`(x at data, i, j, ..., drop = FALSE) : 
  undefined columns selected

I have tried various combinations of commands with all returning either an error or 0. I know the area of the polygon is 4.760465 square meters (area slot) but cant seem to replicate this using areapl(). Is it possible to access the area slot directly instead of computing the area with areapl()? 

Many thanks,
Wesley 


Wesley Roberts MSc.
Researcher: Earth Observation (Ecosystems)
Natural Resources and the Environment
CSIR
Tel: +27 (21) 888-2490
Fax: +27 (21) 888-2693

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