[R] conversion of LL coordenates to UTM problems (ED50-WGS84 format)

2006-10-18 Thread Berta 
Hi R-Users, 
I have plotted a region whose polygon coordinates are given in shp format  ED50 
UTM (zone=30) ) using  readShapePoly in library(maptools).

Now I need to plot a set of points in that region (my.dataframe, with X and Y 
geographic coordinates), which have been read using GPS in Longitud-Latitud 
form (using WGS84 system), so I first need to convert these Longitud-Latitud 
data into UTM data. To do so, I use package PBSmapping:

attr(my.dataframe,  projection) -LL
attr(my.dataframe, zone) - 30
my.dataframe-as.PolySet(my.dataframe)
my.dataframeUL-convUL(my.dataframe)

obtaining the UTM coordinates. But when I plot them in the polygon region, 
points seem to be badly located. 
I suspect it is due to the ED50 UTM of the polygon versus WGS84 coordenates of 
the points. If this is the problem, is there a way to convert them into the 
same format? It it is not the problem, what am I doing wrong?

Thanks a lot in advance. 
Berta. 

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Re: [R] conversion of LL coordenates to UTM problems (ED50-WGS84 format)

2006-10-18 Thread Roger Bivand 
On Wed, 18 Oct 2006, Berta wrote:

 Hi R-Users, I have plotted a region whose polygon coordinates are given
 in shp format ED50 UTM (zone=30) ) using readShapePoly in
 library(maptools).
 
 Now I need to plot a set of points in that region (my.dataframe, with X
 and Y geographic coordinates), which have been read using GPS in
 Longitud-Latitud form (using WGS84 system), so I first need to convert
 these Longitud-Latitud data into UTM data. To do so, I use package
 PBSmapping:
 
 attr(my.dataframe,  projection) -LL
 attr(my.dataframe, zone) - 30
 my.dataframe-as.PolySet(my.dataframe)
 my.dataframeUL-convUL(my.dataframe)
 
 obtaining the UTM coordinates. But when I plot them in the polygon
 region, points seem to be badly located.  I suspect it is due to the
 ED50 UTM of the polygon versus WGS84 coordenates of the points. If this
 is the problem, is there a way to convert them into the same format? It
 it is not the problem, what am I doing wrong?

If the differences are in a few hundreds of metres, not transforming 
between ellipsoid definitions is the most likely cause.

If you are willing to use the rgdal package, you will find the ED50 UTM 
zone 30 string is:

+proj=utm +zone=30 +ellps=intl +units=m towgs84=-87,-98,-121,0,0,0,0

and the WGS84/LL string is 

+proj=longlat +datum=WGS84

Then for your GPS data as a SpatialPointsDataFrame with its coordinate 
reference system set, spTransform() should do what you want. See also the 
note by Edzer Pebesma and myself in R News in November 2005 on sp classes.

Please consider following this up on the R-sig-geo list.

 
 Thanks a lot in advance. 
 Berta. 
 
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-- 
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]

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