[gdal-dev] Spherical Mercator projects and overviews?

[Christopher Schmidt]

On Thu, Apr 09, 2009 at 11:25:04AM -0500, Stephen Woodbridge wrote:
> Frank Warmerdam wrote:
> >Stephen Woodbridge wrote:
> >>Hi all,
> >>
> >>I have a question regarding imagery in spherical mercator projection 
> >>in geotiff images. Do overviews work correctly?
> >>
> >>For example, I use EPSG:4326 and build lots of overviews and I can 
> >>zoom in and out and pan without a problem. Can I do the same thing 
> >>with EPSG:900913 (spherical mercator projection).
> >>
> >>I'm working with LandSat imagery over large area and scale ranges.
> >
> >Steve,
> >
> >I can't see any reason that overviews would not work on spherical
> >mercator when they work on images in other coordinate systems.  The
> >overview handling is independent of the georeferencing.
> >
> 
> Frank,
> 
> Thanks this is what I was hoping the answer would be.
> 
> My fear was that I would run into something like UTM zones and that you 
> can not have one global data set in UTM. So spherical Mercator is 
> projects the world into a single x-y plane similar to the way 
> geographical is treating the global as stretched onto a single x-y 
> plane.  This seems reasonable, I just never thought about it before.

Where 'the whole world' in mercator can't actually be 'the whole world';
you have to pick a reasonable north/south cutoff since the tangent of 90
is infinity/undefined.

-- Chris

> Now I get to reproject everything yet again :)
> 
> Thanks,
>   -Steve
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-- 
Christopher Schmidt
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