# [Proj] MGRS/USNG support?

Gerald I. Evenden gerald.evenden at verizon.net
Fri May 5 19:59:20 EDT 2006

```On Friday 05 May 2006 2:25 pm, Oscar van Vlijmen wrote:
> >>> Let the projection code calculate what the nearest central meridian is,
> >>> merely based on the input of the longitude of each point.
> >
> > From: Gerald I. Evenden
> > UTM allows for a 1/2 degree overlap or extension beyond its normal
> > longitude range.  Given that factor, how do you know which CM to use?
>
> That's a shortcoming, but to calculate a 'regular' CM would be the best
> thing to do.
>
> > Similarly, if the point lies exactly on the division between zones, which
> > CM do you use?
>
> This is a matter of choosing one and documenting what the code does.
> I believe the usual choice is zone 1 for lon=-180, zone 2 for lon=-172, ...
> zone 31 for lon=0, zone 32 for lon=6, etcetera.
>
> > Also, some zones in the Scandinavian region extend beyond the normal
> > +/-3 degree limit, so again, explicit specification of the zone is
> > required.
>
> This is NOT required! It can be done based on latitude and longitude alone.
> Others have done it, I have done it, so it can be done again.

Using the argument that "others have done" holds no weight.

> Admittedly, usage of tmerc/utm in this fashion would be rare, but it
> certainly is requested every now and then.
> Some people do have lat/lon data without zone designations. If the UTM or
> GK setup of a mutizonal region is highly regular -- without overlaps,
> ambiguities, singularities etcetera -- an algorithmically derived CM can be
> used without any problem.

I am afraid that I am adamantly against doing any such "conversion" and using
the name UTM in the same breath.  One of the nice things about UTM is
that it is a standard, a good standard, and does not need any tweeking.
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--
Jerry and the low-riders: Daisy Mae and Joshua
"Cogito cogito ergo cogito sum"
Ambrose Bierce, The Devil's Dictionary
```