LL to UTM

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 <Roger.Bivand at nhh.no> 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 <e.pebesma at geo.uu.nl> 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.

--
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: Roger.Bivand at nhh.no