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 provide the NAD transformation for your location, I'd search. Without 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. > > NAD27_WGS84ll<-function(points){ > coordinates(points)<-~x+y > [EMAIL PROTECTED]<-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 > [EMAIL PROTECTED]<-CRS("+proj=utm +zone=15 +datum=NAD27 +units=m") > spTransform(points, CRS("+proj=longlat +datum=NAD27"))} > > NAD27_WGS84<-function(points){ > coordinates(points)<-~x+y > [EMAIL PROTECTED]<-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 [EMAIL PROTECTED],@alaska,@ntv2_0.gsb,@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 > -- 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: [EMAIL PROTECTED] _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
