
Dear Miltinho,

You might want to take a look at this website:

http://spatialreference.org/ref/epsg/?search=SAD69

It allows you to export an existing coordinate system into a proj4 string, e.g.:

Proj4js.defs["EPSG:29170"] = "+proj=utm +zone=20 +ellps=aust_SA +units=m +no_defs ";

This you can then edit this by hand, e.g. to modify the datum:

"+proj=utm +zone=20 +ellps=aust_SA +towgs84=-57,1,-41 +units=m +no_defs " 


cheers,

Tom Hengl
http://spatial-analyst.net

PS: please be careful when using the spatialreference.org website - I discovered that the proj4 string often does not completely correspond to the CRS in use.



-----Original Message-----
From: r-sig-geo-bounces@stat.math.ethz.ch on behalf of milton ruser
Sent: Tue 9/2/2008 8:04 PM
To: r-sig-geo@stat.math.ethz.ch
Subject: [R-sig-Geo] Utm to latlong conversion (Datum problem)
 
Dear all,

I know that it is a knew issue, but I have some troubles
on the conversion of UTM to Latlong of my shape file.
In fact, it apears to work but my input file is on South America Datum
(SAD69),
but I can´t find the how to indicate the datum. I tryed to include
"+datum=SAD69"
on the proj4string, but I received the following error: "unknown elliptical
parameter name"
Any help are welcome.

Best wishes,

miltinho astronaura
brazil
-------------

require(sp)
require(maptools)
require(rgdal)

setwd("G:\\HDExt\\R_scripts_genericos\\converte_utm_latlong")

SP<-readShapePoly("area_infl_ind.shp", IDvar=NULL,
proj4string=CRS("+proj=utm +zone=22 +units=m +south"))

SPll<-spTransform(SP, CRS("+proj=longlat +datum=WGS84"))

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