[gdal-dev] how to pick the correct projection

[Even Rouault]

Le mercredi 09 décembre 2015 19:23:16, Djordje Spasic a écrit :
> Hi,
> 
> I am downloading an open source .asc elevation files from
> opentopography.org.​ I attached one of them in this email. Once downloaded
> .asc elevation file need to be projected from WGS84 datum (EPSG 4326) to
> Universal Transverse Mercator. Here is an example of how this looks like
> in python GDAL for Barcelona Spain downloaded elevation model: gdalwarp
> -s_srs EPSG:4326 -t_srs EPSG:32631 -r cubic BarcelonaWGS84.asc
> projectedToUTM.tif ​So in this case the EPSG:32631 has been used as a
> projection (it hasn't been picked by me), it was googled.
> 
> But how do I know which projection to pick, depending on the location for
> which the elevation data is downloaded? I googled a bit, and there are few
> articles online, which suggest picking the right EPSG projection based on
> UTM zone to which the downloaded elevation model belong
> to:https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_
> system#/media/File:Utm-zones.jpg
> 
> But I am not sure I understand how to choose the EPSG projection based on
> UTM zone.
> 
> How can this be done?

Formula given in 
http://stackoverflow.com/questions/9186496/determining-utm-zone-to-convert-from-longitude-latitude

long2UTM <- function(long) {
    (floor((long + 180)/6) %% 60) + 1
}

Take the longitude of the central point of the source raster as input

Just thinking it could be interesting to have gdalwarp accept -t_srs AUTO:42001 to do that automatically:
http://cite.opengeospatial.org/OGCTestData/wms/1.1.1/spec/wms1.1.1.html#auto_projections.42001

> 
> Thank you for the reply.
> 
> Kind regards,
> Djordje Spasic

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