[gdal-dev] Fwd: OSRExportToProj4 losing datum

[Roger Bivand]

On Sat, 3 Nov 2012, Even Rouault wrote:

> Selon Roger Bivand <Roger.Bivand at nhh.no>:
>
>> Even,
>>
>> I've opened #4880 on this - it is problematic in the R-spatial setting
>> because the CRS (coordinate reference system) object recorded in each
>> Spatial object uses the PROJ.4 string to represent spatial reference. A user
>> can create a CRS, write out a raster (which expands the description to
>> include , read it back, and the CRS are not the same. The same does not
>> appear to happen with OGR write/read (ESRI Shapefile driver). Is the
>> conclusion that calling programs must first find "NAD83" in the raster input
>> WKT, if it is there set the environment variable if the default GDAL
>> behaviour is not desired, export to Proj4, then remove the environment
>> variable if set? Does this only affect NAD83 - it does seem to be limited to
>> this datum, as WGS84 does not show the same behaviour.
>
> Roger,
>
> Hum, I somehow regret of having tried to fix the issue that was originally
> raised in trac.osgeo.org/gdal/ticket/3450 since it seems that all GRASS and now
> RGdal developers hate me now ;-) But hopefully they will realize that they
> should consider using more reliable ways of representing SRS.
>
> In the case of NAD83, +datum=nad83 is expanded as
> GEOGCS[\"NAD83\",DATUM[\"North_American_Datum_1983\",SPHEROID[\"GRS
> 1980\",6378137,298.257222101,AUTHORITY[\"EPSG\",\"7019\"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY[\"EPSG\",\"6269\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.0174532925199433,AUTHORITY[\"EPSG\",\"9108\"]],AUTHORITY[\"EPSG\",\"4269\"]]
> . This expansion is done at import time (importFromProj4).
>
> This explains why at export time the +datum isn't exported since there's a
> TOWGS84 node.
>
> You would also have a similar behaviour with +datum=WGS72 since +datum=WGS72
> expansion implicitely adds a TOWGS84 node.
>
> In the case of NAD83, I'm not sure why SetWellKnownGeogCS("NAD83") adds the
> TOWGS84[0,0,0,0,0,0,0]. If this was removed, then I believe that exportToProj4()
> would again generate +datum=nad83. But I'm now more than hesitant in touching
> anything in that area, since this could potentially break something else.
>
> Sorry to repeat myself, but you should NOT use proj4 strings as a way of
> representing a SRS. It is just not reliable. It is not, and has never been. You
> must realize that proj4 only knows 10 datum names (see pj_datums.c). All other
> datums must be translated into +ellps= +towgs84= anyway. You shouldn't assume
> that round-tripping importFromProj4() / exportToProj4() will be perfect. proj4
> strings are only usefull for proj4, to do SRS transformations. They are not
> meant at being a comprehensive way of representing SRS. I'm not sure how hard it
> is for RGdal, but you should consider migrating to using WKT strings / model.
> WKT has been designed for that.

Thanks, Even.

It isn't easy to make design decisions, and in the case of sp, the R 
spatial class definition package, our concern was that spatial data 
analysts were most often unaware of spatial reference issues. PROJ.4 
strings are a low-threshold text string entry point for users who have no 
grasp of other representations, and when we began (2003), were among the 
only games in town. Inside R, the objects cannot reliably be pointers to 
C/C++ objects cross-platform; indeed, it is only a few month since we 
achieved PROJ.4/GDAL CRAN (archive network) support for OSX on two intel 
architectures. I'll try out adding a user-settable argument in calling 
functions to set and unset the environment variable, but this isn't very 
satisfactory.

I agree that in a better world, in sp 2.0, we should replace the 
representation in the CRS class with (maybe) WKT. But I don't think that 
this will happen any time soon, and in any case, we would still need to be 
able to let users to enter the simpler PROJ.4 representations, as writing 
WKT is much more error-prone.

Best wishes,

Roger

>
> Best regards,
>
> Even
>

-- 
Roger Bivand
Department of Economics, NHH 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