plot projected xyz

Hello!

I'm having trouble making a map of terrain elevations.
I have a collection of (lon,lat, z) points. These points have regular
spacing (20km) when projected, but not in geographical coordinates.

So I project the data and make a SpatialGRidDataFrame object with sp and
rgdal packages.
How can I plot it with lon,lat axes? By costumizing the axes?
Is there a way to define the projection I am working on and do all the
work with lon,lat coordinates?

See ?gridlines and ?gridat in the sp package to construct a grid and label 
it. If you construct the grid in geographical coordinates and project it 
using spTransform() methods in rgdal to the projection of the data, you 
can overplot your projected data with a longlat grid:

library(rgdal)
scot_BNG <- readOGR(system.file("vectors", package = "rgdal")[1],
   "scot_BNG")
scot_LL <- spTransform(scot_BNG, CRS("+proj=longlat +datum=WGS84"))
grd_LL <- gridlines(scot_LL, ndiscr=100)
summary(grd_LL)
grd_BNG <- spTransform(grd_LL, CRS(proj4string(scot_BNG)))
grdtxt_LL <- gridat(scot_LL)
grdtxt_BNG <- spTransform(grdtxt_LL, CRS(proj4string(scot_BNG)))
plot(scot_BNG)
plot(grd_BNG, add=TRUE, lty=2)
text(coordinates(grdtxt_BNG),
   labels=parse(text=as.character(grdtxt_BNG$labels)))

is an example.

Roger

Thanks in advance,
Ana Rosa

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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.Bivand at nhh.no