[gdal-dev] Re: gdal to define projection, EPSG:3412

[John Callahan]

Thank you Frank.  I hadn't thought about file formats supporting 
projections or not.  Typically, you think about the software or 
algorithm not supporting a projection or coordinate system, not the 
format itself. 

- John



Frank Warmerdam wrote:
> John Callahan wrote:
>> Thanks Frank/Hermann for your responses.  However, something odd 
>> seems to be going on here. I feel like I'm missing something on how 
>> GDAL handles coordinate system information.
> >
>> Using the same input data set, and converting to different formats, 
>> the description of the output coordinate systems are different. Is 
>> this expected?   Here are the gdal_translate commands and their 
>> respective output files gdalinfo descriptions below.
>
> John,
>
> I think the key thing is that coordinate system encoding mechanisms
> tend to vary from file format to file format.  A few points of note:
>
>  o GeoTIFF has two general ways of encoding - one is to just embed
>    the EPSG PCS code for a whole coordinate system.  The other is
>    to encode the projection method along with the parameters.  You
>    will tend to get the first when specifying the coordinate system
>    with EPSG:n and the second when using PROJ.4 strings.
>
>  o Many geospatial formats have a particular way of representing
>    projections, and may not support all projection methods, preserving
>    datums, preserving names, etc.
>
>> [1] gdal_translate -a_srs "+proj=stere +lat_0=-90 +lat_ts=-70 
>> +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m 
>> +no_defs" -of GTiff nt_200712_f13_v01_s.bsq nsidc_test.tif
>>
>> [2] gdal_translate -a_srs "+proj=stere +lat_0=-90 +lat_ts=-70 
>> +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m 
>> +no_defs" -of HFA nt_200712_f13_v01_s.bsq nsidc_test.img
> ...
>> Fortunately, I can display these images fine.  They overlay nicely in 
>> QGIS with some vector data I have.   However, the problem occurs when 
>> projecting to EPSG:4326 (or similar) coordinate system; the output 
>> files are incorrect/missing data.  (It displays data from roughly lat 
>> -39.25 only down to roughly -54.75 when data exists all the way down 
>> to -90.)
>>
>> One possible cause could be that gdalinfo reports (for all of these 
>> datasets) latitude_of_origin as -70.  From 
>> http://nsidc.org/data/atlas/epsg_3412.html, the latitude of origin 
>> should be -90 and the latitude of standard parallel should be -70.  
>> Or the problem could be somewhere in gdalwarp, or specifically with 
>> the Hughes 1980 ellipsoid.  I have no idea.  Thanks.
>
> It sounds like there may be another issues with regard to the latitude
> of origin in addition to the issue with GeoTIFF not working properly with
> 3412.  If you can isolate and file a ticket that might be helpful.
>
> I'm sorry I haven't been following some of the recent srs threads very
> closely.
>
> Best regards,