[gdal-dev] Testing if two spatial references are the same

[Simon Perkins]

Hmm, looking at the projection string in more detail and comparing with 
the OGR source code, I see that it doesn't include a "PROJECTION" node, 
which OGR.IsSame() checks for.

PROJCS["unnamed",
   GEOGCS["NAD83",
       DATUM["North_American_Datum_1983",
           SPHEROID["GRS 1980",6378137,298.2572221010002,
               AUTHORITY["EPSG","7019"]],
           AUTHORITY["EPSG","6269"]],
       PRIMEM["Greenwich",0],
       UNIT["degree",0.0174532925199433],
       AUTHORITY["EPSG","4269"]],
   UNIT["US survey foot",0.3048006096012192,
       AUTHORITY["EPSG","9003"]],
   AUTHORITY["EPSG","26942"]]

So, is the TIFF file this came from missing some information? It was 
just a file I found on the web... It does have an EPSG code, so should 
IsSame() be checking for that?

Cheers,

Sy



Simon Perkins wrote:
> I have a file, which is giving me some strange results with the 
> OGRSpatialReference.IsSame() method. Here's a short test program:
>
> #include "gdal_priv.h"
> #include "ogr_spatialref.h"
> #include "ogr_core.h"
> #include <iostream>
>
> using namespace std;
>
> int main(int argc, char** argv)
> {
>    GDALAllRegister();
>    const char* filename = argv[1];
>    GDALDataset* dataset = (GDALDataset*) GDALOpen(filename, GA_ReadOnly);
>    cout << "Opened file: " << filename << "\n";
>    const char* projWkt = dataset->GetProjectionRef();
>    cout << "Raw Projection WKT: " << projWkt << "\n";
>    OGRSpatialReference* spatialRef = new OGRSpatialReference(projWkt);
>    char* prettyWkt = new char[4096];
>    spatialRef->exportToPrettyWkt(&prettyWkt, 0);
>    cout << "Pretty Projection WKT:\n" << prettyWkt << "\n";
>    cout << "Result from s.IsSame(s): " << 
> spatialRef->IsSame(spatialRef) << "\n";
>    return 0;
> }
>
>
> Basically, the code opens a raster file, grabs the projection Well 
> Known Text string, constructs an OGRSpatialReference using that 
> string, and then compares that spatial reference to itself using the 
> IsSame() method. One would expect that a spatial reference is the same 
> as itself, so this method should return 1.
>
> Here's the output from a particular file:
>
> Opened file: C:/Images/Demo/K10.tif
> Raw Projection WKT: 
> PROJCS["unnamed",GEOGCS["NAD83",DATUM["North_American_Datum_
> 1983",SPHEROID["GRS 
> 1980",6378137,298.2572221010002,AUTHORITY["EPSG","7019"]],AU
> THORITY["EPSG","6269"]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433], 
>
> AUTHORITY["EPSG","4269"]],UNIT["US survey 
> foot",0.3048006096012192,AUTHORITY["EP
> SG","9003"]],AUTHORITY["EPSG","26942"]]
> Pretty Projection WKT:
> PROJCS["unnamed",
>    GEOGCS["NAD83",
>        DATUM["North_American_Datum_1983",
>            SPHEROID["GRS 1980",6378137,298.2572221010002,
>                AUTHORITY["EPSG","7019"]],
>            AUTHORITY["EPSG","6269"]],
>        PRIMEM["Greenwich",0],
>        UNIT["degree",0.0174532925199433],
>        AUTHORITY["EPSG","4269"]],
>    UNIT["US survey foot",0.3048006096012192,
>        AUTHORITY["EPSG","9003"]],
>    AUTHORITY["EPSG","26942"]]
> Result from s.IsSame(s): 0
>
> Any ideas what's going on here? Is it something specific to this 
> particular projection string? Is there a better way of checking 
> equivalence of projected coordinate systems, e.g. using the EPSG code? 
> What would I have to check to ensure equivalency?
>
> Regards,
>
> Simon
>
>
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