[gdal-dev] Re: help for comparing two WKT

[Even Rouault]

Le dimanche 09 octobre 2011 01:05:21, Etienne Tourigny a écrit :
> Thanks Even
> 
> I see what you mean, several named GEOGCS/datums can match a given
> spheroid.
> 
> I am trying to see if I can assign a named datum given spheroid
> parameters from the CF Convention for the netcdf driver.
> 
> For example, this CF definition corresponds to the WGS84 datum/GEOGCS
>   int crs ;
>     crs:grid_mapping_name = "latitude_longitude";
>     crs:longitude_of_prime_meridian = 0.0 ;
>     crs:semi_major_axis = 6378137.0 ;
>     crs:inverse_flattening = 298.257223563 ;
> 
> -> Are there any numerical differences between GEOGCS/datums that use
> the same spheroid?
> In other words, is it erroneous (numerically) to use another named
> GEOGCS/datum than the one that was intended? (e.g. using WGS_1984
> instead of Hartebeesthoek94).

If using PROJ4 no, they will be equivalent. But if EPSG chose to make them 
different, I guess it probably means that with more precise 
modeling/transformation tools they would be different. It's somehow geodesic 
pedantry.

For example, http://www.fig.net/commission5/reports/wonnacott.pdf explains how 
the Hartebeesthoek94 was built.

> 
> -> Furthermore, is there a precedence than one must or can use in
> order to choose which one that matches?
> I assume that the numerical order of the EPSG codes have nothing to do
> with precedence.

I'm afraid there's no "precedence", apart from an arbitrary choice you could 
made based on what you believe to be the most "usual" / likely one.

EPSG also defines a geographical zone where GEOGCS/PROJCS are valid, so that 
could perhaps be used to guess the most appropriate match when you have other 
elements (such as the bounding box of the raster that uses the SRS), but I'm 
not sure if those values are preserved in the GDAL CSV files.

> 
> regards,
> Etienne