[gdal-dev] Re: UTM zones exceptions

[Frank Warmerdam]

Gé Gé wrote:
> Hi all and Franck,
> 
> I use OGR and GDAL in a software that has to manipulate and convert UTM 
> and WGS84 coordinates and maps.
> OGR/GDAL is very powerful and I'm very happy with it.
> 
> Nevertheless I have a (maybe stupid) question concerning UTM zones 
> exceptions, for instance for Norway where the 32V is a bit wider as
> it should be:
> http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system#Exceptions
> http://en.wikipedia.org/wiki/File:LA2-Europe-UTM-zones.png
> 
> Do I have to take this exceptions into account when I indicate an UTM 
> zone to OGR&GDAL (for instance with OSRSetUTM() or SetUTM() functions)
>  or do I have to work with OGR & GDAL as if there was no exception ?

Gérard,

GDAL/OGR/PROJ.4 does not currently have a function which can be used to
suggest the most appropriate UTM zone for a particular lat/long location,
so the exceptions are not relevant.

 >
 > (more pragmatically, if I'm on Norway west coast, do I indicate 31V or
 > 32V zone to OGR/GDAL?)

If you need to match a local map, then read the map to determine what it
uses.  If you just want to pick the most appropriate zone to minimize
distortion then using the normal six degree wide zone boundaries should
be fine.

I presume the exceptions exist to avoid having small slices of a country
mapped in a different zone than the rest and so they are effectively just
conventions about what zone to use for local mapping projects.

Best regards,
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