Great! Your LL2UTM function was exactly what I wanted. My data sets are each about a city block in size and I mostly work in UTM on this project but I do get a few points in lat long. Its small enough that I have been using that web site I mentioned but it gets tedious after a while and this should be a big help.
I did notice that the R function below is consistent with the web site output only to 5 digits after the decimal place in x and 2 digits after the decimal place in y. I don't know how to determine whether R or the web site is the more accurate but at any rate either seem likely to be well within the accuracy I need since, in truth, I don't need high precision. Thanks. On 3/6/07, Roger Bivand <[EMAIL PROTECTED]> wrote: > On Tue, 6 Mar 2007, Gabor Grothendieck wrote: > > > Thanks. I looked at spTransform prior to posting but don't understand it. > > What I want is to create a function that returns UTM coordinates: > > > > LL2UTM <- function(lat, long, zone = 18) { > > ... > > } > > library(rgdal) > LL2UTM <- function(lat, long, zone=18) { > project(cbind(long, lat), paste("+proj=utm +zone=", zone, sep="")) > } > > gets you there, for example for: > > lat <- seq(30,45,1) > long <- seq(-78,-72,1) > grd <- expand.grid(lat, long) > LL2UTM(grd[,1], grd[,2]) > > The project() interface to PROJ4 does not support datum transfromation, > the spTransform() method does. The cost is having to convert the data to > a Spatial* object: > > SP_grd <- SpatialPoints(cbind(grd[,2], grd[,1]), > proj4string=CRS("+proj=longlat +datum=NAD83")) > SP_grd1 <- spTransform(SP_grd, CRS("+proj=utm +zone=18 +datum=NAD83")) > > returning a SpatialPoints object - use the coordinates() method to > retrieve as a matrix. > > Assuming that your input geographical coordinates are in NAD84/WGS84, and > the output UTM coordinates are in the same datum, project() will be OK. If > you need datum transformation, which may lead to errors of hundreds of > metres if not used, the PROJ4 string needs more detail. If this is at the > block scale, datum transformation will make a difference if the input and > output specifications vary (say placing points on a UTM map not in > WGS84/NAD83). At the continental scale the differences are typically not > great, but then you wouldn't use UTM anyway. > > Hope this helps, > > Roger > > > > > If necessary, zone=18 can be hardcoded in the function and that > > arg removed. > > > > I assume that using spTransform its just a one line body. Can > > you give me the specific line that it should be? > > > > Thanks. > > > > > > On 3/6/07, Edzer J. Pebesma <[EMAIL PROTECTED]> wrote: > > > Gabor, > > > > > > package rgdal provides an interface to the PROJ.4 library for projection > > > of geographical data. Look for the function (or rather method) > > > spTransform. It takes any of the spatial classes provided by package sp, > > > and afaik any of the known projection systems, including UTM and > > > ellipsoids (LL reference "model"). > > > -- > > > Edzer > > > > > > Gabor Grothendieck wrote: > > > > I am currently using this web page to convert LL to UTM (and UTM to LL). > > > > > > > > http://home.hiwaay.net/~taylorc/toolbox/geography/geoutm.html > > > > > > > > All my points are in zone 18. How would I accomplish the same > > > > thing using R? > > > > > > > > Thanks. > > > > > > > > _______________________________________________ > > > > R-sig-Geo mailing list > > > > R-sig-Geo@stat.math.ethz.ch > > > > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > > > > > > > > > > > > > > _______________________________________________ > > R-sig-Geo mailing list > > R-sig-Geo@stat.math.ethz.ch > > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > > > > -- > 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