[OpenLayers-Users] Find UTM Zone from x,y

[Barend K ö bben]

Hi Linda,

I think your question starts with a basic mis-assumption:

> Basically if I pass in the x,y coordinates in WGS_84 or any other meter
> projection

WGS84 is a officially a Datum, not a projection. A Datum is a global or
ellipsoidal object closely matching the Earth's shape. A projection is
literaly what it says: a projection of that global object on a flat surface.

(for a good overview, I suggest http://kartoweb.itc.nl/geometrics/)

What most people mean if they say "WGS84 projection" is "Geographic
coordinates (in latitude, longitude) on the WGS_84 Datum". Therefore it's
coordinates are NOT x,y but lat,lon  and it's NOT a meter projection but a
(decimal) degrees description of a vector to a point on the surface of a
global object (actually the lat and lon are angles with respect to the
globe's origin...).
In more simpel terms: lat,lon does not have a cartesian grid with an origin
and equal values along two axes as its mathematical basis, as the UTM and
other (meter) projections have.

> I would like it to return the zone that those coordinates are in.
You can easily do that if indeed you have lat,lon coordinates as described
above. As the zones of UTM are just 60 zones divided over the 360 degrees of
longitude, each zone is 360/60=6 degrees of longitude. Zone 1 is from -180
to 174, zone 31 is from 0 (Greenwich) to 6, etcetera. The zones each have a
N-S divide over the equator, so if the latitude is positive, the zone is
zone xN, if negative it's zone xS.
So my city Enschede is at lat,lon= 52.2166667 , 6.9 and therefore in zone
31N.

For "any other meter projection" it's not so easy, because the projections
all use different origins of their grid. The best is to calculate the
projected x,y back to lat,lon on the WGS84 (in PROJ4-based systems use the
toWGS84 part of the projection description) and then to the UTM zone...

Yours,
__ 
Barend Köbben
International Institute for Geo-information
Sciences and Earth Observation (ITC)
PO Box 6, 7500AA Enschede (The Netherlands)
ph: +31 (0)53 4874253; fax: +31 (0)53 4874335