[R-sig-Geo] spTransform: datum syntax?

[Agustin Lobo]

Dear list,

I have to transform the datum of an spatial object.
First, I create the object with:
pressp <- SpatialPointsDataFrame(pres[,c(10,6)], data=pres[,c(1,2,11)],
+                proj4string = CRS("+proj=longlat +ellps=WGS84"))

Then I have to transform to UTM but using the International or Hayford 
datum.

The following works:
presspUTM <- spTransform(pressp, CRS("+proj=utm +zone=31))

but then I keep WGS84.

The following also works:
presspUTM <- spTransform(pressp, CRS("+proj=utm +zone=31 +datum=NAD27"))

but this is not the datum I want

Then I've tried:
presspUTM <- spTransform(pressp, CRS("+proj=utm +zone=31 +datum=ED50"))
presspUTM <- spTransform(pressp, CRS("+proj=utm +zone=31 
+datum=International"))
presspUTM <- spTransform(pressp, CRS("+proj=utm +zone=31 +datum=Int"))
presspUTM <- spTransform(pressp, CRS("+proj=utm +zone=31 +datum=Hayford"))

and many other similar names, also tried with ellps=

The doc refers to the proj.4 doc, but cannot find any list of datums 
there, including the pdf docs of proj.4

Does anyone know where I could find a list of supported datums in proj.4
and their correct names for the +datum= argument?

Thanks!

Agus
-- 
Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
LLuis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
email: Agustin.Lobo at ija.csic.es
http://www.ija.csic.es/gt/obster