Craig Bruce csbruce at cubewerx.com
Mon Apr 3 09:40:10 EDT 2000

"Gerald I. Evenden" <gevenden at capecod.net> wrote:

> Regarding "automatic" UTM by only specifying longitude, there is
> a technical problem: UTM zones allow a half degree overlap (actually
> 3.5 degrees wide) so geographic coordinates laying in this region
> are ambiguous as to which zone to apply.  Secondly, this ambiguity
> always exists for those points on the even 6 degrees.

I'm not sure what other applications might exist for a "automatic" UTM, but
the main one that I can think of is an interactive browser situation where
someone is viewing a rendered image on the screen.  In this situation,
it makes perfect sense for the browser system to automatically select
the UTM zone (and hemisphere) to use based on the center point of the
display window.  Of course, the [a] calculation of the UTM zone of the
center point of the image given its lat/lon is so simple that it doesn't
really need to be part of the Proj library.

zone = floor( (lon0 + 180.0) / 6.0 ) + 1;

> Lastly, even if the longitude could be used as a CM determinator,
> the control stucture for the projection would have to be
> intialized for *each* lat/lon determined.  That is, open and closed
> for each point computed.

In an interactive browser situation, the center point lat/lon would be
determined and this UTM zone (and single control structure) would be used
to transform all of the coordinates.

> Oh yes!  Some zones in the Scandinavian region have funky limits
> and simplistic determination from longitude cannot be used.

In an interactive browsing situation, the user probably wouldn't care
about any special situations.

Dr. Craig Bruce           |  csbruce at cubewerx.com  |             CubeWerx Inc.
Senior Software Developer |           --           |  200 Montcalm, Suite R-13
Ph. (819) 771-7739        |   csbruce at pobox.com    |     Hull, Quebec, J8Y 3B5
Fax (819) 772-4838        | http://www.csbruce.com |  http://www.cubewerx.com/
             "New features, Good quality, On time -- choose two."
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