You were absolutely right. If I use south everything is fine. That once again
proves the danger of intuition. I simply assumed since almost all of the map is
on the northern half that the projection should use north as well. But that is
apparently wrong. I couldn't find any description of these definition string
parameters which could have been informative. Since I am not a geographer
either, I probably used the wrong terms for searching.
Anyway, now it works.
Thanks so much for the help, Jim!
Best regards,
Werner
----- Ursprüngliche Mail ----
Von: Jim Regetz <[EMAIL PROTECTED]>
An: Werner Wernersen <[EMAIL PROTECTED]>
CC: [EMAIL PROTECTED]
Gesendet: Mittwoch, den 20. August 2008, 03:08:48 Uhr
Betreff: Re: converting coordinates from utm to longitude / latitude
Werner Wernersen wrote:
> It would be nicer to convert directly the entire shapefile object to long/lat
> coordinates but if that is not possible, I will convert the other points to
> UTM.
> Hence, I am playing around with rgdal.
>
> library(rgdal)
> SP <- SpatialPoints(cbind(32.29252, -0.3228500),
> proj4string=CRS("+proj=longlat"))
> spTransform(SP, CRS("+proj=utm +zone=36"))
>
>
>> spTransform(SP, CRS("+proj=utm +zone=36"))
> SpatialPoints:
> coords.x1 coords.x2
> [1,] 421274.4 -35687.37
> Coordinate Reference System (CRS) arguments: +proj=utm +zone=36 +ellps=WGS84
>
> This result corresponds with what I get when using convUL() but my map of
> that area in UTM coordinates does not extend to the negative.
> An external program converts the point to x=420994 y=9964407 which also
> seems correct with respect to the map. Fore sure, I am using the function
> wrongly somehow. Can anyone give me a hint?
It looks like you are specifying 36S in your external program, and
(implicitly) 36N in R. Using your SP from above, note the following:
> spTransform(SP, CRS("+proj=utm +zone=36 +north"))
SpatialPoints:
coords.x1 coords.x2
[1,] 421274.4 -35687.37
Coordinate Reference System (CRS) arguments: +proj=utm +zone=36
+ellps=WGS84
> spTransform(SP, CRS("+proj=utm +zone=36 +south"))
SpatialPoints:
coords.x1 coords.x2
[1,] 421274.4 9964313
Coordinate Reference System (CRS) arguments: +proj=utm +zone=36 +south
+ellps=WGS84
The latter gets in the ballpark of output from your external program.
I'd speculate that the remaining differences owe to different
assumptions about e.g. the datum (WGS84 for the R statements given
above), but I must admit I'm not a geographer.
Hope that helps,
Jim
> That's very much appreciated!
>
> Thanks,
> Werner
>
>
>
> ----- Ursprüngliche Mail ----
> Von: Werner Wernersen <[EMAIL PROTECTED]>
> An: [EMAIL PROTECTED]
> Gesendet: Dienstag, den 19. August 2008, 20:28:29 Uhr
> Betreff: converting coordinates from utm to longitude / latitude
>
> Hi,
>
> is there a function in R to convert data read with read.shape and which is
> originally in UTM coordinates into longitude / latitude coordinates?
> I found the convUL() function from the PBSmapping package but I have no idea
> how I could apply that to the read.shape object.
>
> Many thanks,
> Werner
>
>
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