[R-sig-Geo] a projection question

[Roger Bivand]

On Sun, 22 Dec 2013, Barry Rowlingson wrote:

> First thought is to try and project a sample point and catch any error
> messages, but you can't be sure that your sample point is valid in the
> given coordinate system.
>
> Second thought was to look at the proj4 and rgdal packages to see if
> they implemented checking of PROJ.4 strings. Couldn't see it.

?checkCRSArgs

checkCRSArgs("+init=epsg:4326")
checkCRSArgs("+init=apsg:4326")
checkCRSArgs("+init=epsg:4326999")
checkCRSArgs("+init=epsg:-1")

Been there a long time.

Roger

>
> So third thought was to look at the PROJ.4 API and see what its got:
>
> http://trac.osgeo.org/proj/wiki/pj_init_plus
>
> which is a C function to initialise a projection. So I wrote a little wrapper:
>
> #include <proj_api.h>
>
> void ptest(char *proj[], int *status){
>  projPJ out;
>  if(!(out = pj_init_plus(proj[0]))){
>    *status = 1;
>  }else{
>    *status = 0;
>   /* need to pj_free the out object here... I think... */
>  }
> }
>
> with an R wrapper: isValidProjection <-
> function(s){.C("ptest",s,status=as.integer(-1))$status==0}
>
> compiled it, linked with libproj.so and tested it:
>
>
>> isValidProjection("+init=epsg:4326")
> [1] TRUE
>
> - user typo:
>
>> isValidProjection("+init=apsg:4326")
> [1] FALSE
>
> - try more than +init strings:
>
>> isValidProjection("+proj=tmerc +lon_0 +datum=WGS84")
> [1] TRUE
>
> - some bogus EPSG codes:
>
>> isValidProjection("+init=epsg:4326999")
> [1] FALSE
>> isValidProjection("+init=epsg:-1")
> [1] FALSE
>
> Looks like it works. The tricky part of all this is linking the C code
> with libproj.so which I did by doing:
>
> R CMD SHLIB projtest.c /usr/lib/libproj.so
>
> on the command line. Might work for you, but it might be worth asking
> Roger, as maintainer or rgdal, or Smon Urbanek as maintainer of
> project, to add it as a top-level R function.
>
> Maybe I'll check out the rgdal source and push a change for Roger...
>
> Barry
>
>
>
>
> On Sat, Dec 21, 2013 at 10:19 PM, Hodgess, Erin <HodgessE at uhd.edu> wrote:
>> Hello everyone:
>>
>> I'm setting up a function in which the user enters the projection (as part of other stuff).
>>
>> What is the best way to determine if the entered projection is correct, please? (so I can send back an error message right away).
>>
>> Thanks,
>> Erin
>>
>>
>>         [[alternative HTML version deleted]]
>>
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-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
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