[R-sig-Geo] spTransform problem

[Duncan Golicher]

Can anyone tell me what I am doing wrong when using spTransform?
I need to transform points in Southern Mexico from UTM NAD27 to Long Lat 
WGS84.

Three small functions.

NAD27_WGS84ll<-function(points){
coordinates(points)<-~x+y
points at proj4string<-CRS("+proj=utm +zone=15 +ellps=clrk66 +datum=NAD27 
+units=m")
spTransform(points, CRS("+proj=longlat +datum=WGS84"))}

NAD27_NAD27ll<-function(points){
coordinates(points)<-~x+y
points at proj4string<-CRS("+proj=utm +zone=15 +datum=NAD27 +units=m")
spTransform(points, CRS("+proj=longlat +datum=NAD27"))}

NAD27_WGS84<-function(points){
coordinates(points)<-~x+y
points at proj4string<-CRS("+proj=utm +zone=15 +datum=NAD27 +units=m")
spTransform(points, CRS("+proj=utm +zone=15 +datum=WGS84 +units=m"))}

When I test them I get strange results.

x<-c(541987,541970)
y<-c(1846598,1846799)

points<-data.frame(x,y)

NAD27_WGS84ll(points)

SpatialPoints:
             x        y
[1,] -92.60617 16.70272
[2,] -92.60633 16.70454
Coordinate Reference System (CRS) arguments:  +proj=longlat +datum=WGS84 
+ellps=WGS84 +towgs84=0,0,0

NAD27_NAD27ll(points)

SpatialPoints:
             x        y
[1,] -92.60617 16.70272
[2,] -92.60633 16.70454
Coordinate Reference System (CRS) arguments:  +proj=longlat +datum=NAD27 
+ellps=clrk66 +nadgrids=@conus, at alaska, at ntv2_0.gsb, at ntv1_can.dat

The results are the same, even though one function involves a datum 
shift and the other doesn't.

The difference between NAD27 and WGS84 in Southern Mexico should be 
around 200m N and 20m E. But I get

points-as.data.frame(NAD27_WGS84(points))

          x         y
1 0.5857494 -112.2658
2 0.5855395 -112.2768


Thanks for any advice.

Duncan Golicher
-- 

Dr Duncan Golicher
Ecologia y Sistematica Terrestre
Conservación de la Biodiversidad
El Colegio de la Frontera Sur
San Cristobal de Las Casas, 
Chiapas, Mexico

Email: dgoliche at sclc.ecosur.mx 

Tel: 967 674 9000 ext 1310
Fax: 967 678 2322

United Kingdom Skypein; 020 7870 6251
Skype name: duncangolicher