# [R-sig-Geo] spTransform problem

Roger Bivand Roger.Bivand at nhh.no
Thu Nov 16 09:26:00 CET 2006

```On Wed, 15 Nov 2006, Duncan Golicher wrote:

> 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.

Because spTransform uses PROJ.4, this is a PROJ.4 problem. I wonder
whether PROJ.4 provides NAD grids for Southern Mexico, or whether you will
rather need to find the +towgs= arguments directly? If PROJ.4 doesn't
knowing if it helps, this looks relevant:

http://article.gmane.org/gmane.comp.gis.proj-4.devel/1582/match=mexico

with +towgs84=-12,130,190. I don't think just putting +datum=NAD27 will
work, as you've found. Is there a Grids&Datums for Mexico (can't see one
on: http://www.asprs.org/resources/grids/)?

Roger

>
> Three small functions.
>
> 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"))}
>
> coordinates(points)<-~x+y
> points at proj4string<-CRS("+proj=utm +zone=15 +datum=NAD27 +units=m")
>
> 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)
>
>
> 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
>
>
> SpatialPoints:
>              x        y
> [1,] -92.60617 16.70272
> [2,] -92.60633 16.70454
> Coordinate Reference System (CRS) arguments:  +proj=longlat +datum=NAD27
>
> 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
>
>
>           x         y
> 1 0.5857494 -112.2658
> 2 0.5855395 -112.2768
>
>
>
> Duncan Golicher
>

--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of