[Proj] MGRS notation in extended UTM zones?

[Mikael Rittri]

Well argued. I hadn't thought of your motivation (2), 
but it makes sense. I think I am convinced.  

Thanks again,

Mikael 
 
-----Original Message-----
From: Charles Karney [mailto:charles.karney at sri.com] 
Sent: den 21 juni 2011 16:18
To: Mikael Rittri
Cc: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] MGRS notation in extended UTM zones?

On 06/21/11 07:35, Mikael Rittri wrote:
> Thanks, Charles.
>
> I suppose this means that there are no official guidelines, unless one 
> would accept GeoTrans as an authority. But I don't have much 
> confidence in GeoTrans (although I haven't investigated it in detail, 
> as you have).
>
>> In the east-west direction this means that UTM eastings in 
>> [100km,900km) are legal.
>
>> The rule for the latitude band letter is that if a latitude band 
>> boundary intersects a 100 km MGRS block then either letter is valid 
>> for any coordinate in that block.
>
> I agree that the second rule is necessary (more or less) and harmless.
> But I am less sure about the first rule.
>
>> I can expand on my reasons for these choices if anyone's interested.
>
> Yes, I would be interested in more details.

Motivations for the rules for allowed MGRS conversions:

(1) Repeated conversions should be stable.  Given that UTM/UPS -> MGRS conversion is by truncation, then MGRS -> UTM/UPS should return the center of the square (at the appropriate precision).

(2) Truncating a legal MGRS coordinate should result in a legal MGRS coordinate; similarly adding precision to an legal MGRS coordinate should result in a legal MGRS coordinate.

(3) MGRS lettering boundaries should be adhered to since these supply redundancy checks.  Thus zone 02 can only use column letters J-R.  It shouldn't be extended further east by using letter S, because zone 03 uses column letters S-Z and doing so would lose a check.  Similarly the use of latitude band letters should be as strict as possible consistent with (2).  (These rule allow various bits of the MGRS code, e.g., the UTM zone number, to be dropped with the nearest point of possible confusion being reasonably far away.)

A consequence of rule (2) is that the extent of a numeric zone should follow the boundaries of the 100km MGRS blocks.

The maximum extent of a UTM zone [100km,900km] in the E-W direction (due to rule 3), and GeographicLib imposes a limit in the N-S direction in order to guarantee a 0.5 deg overlap with UPS.  This defines a rectangular regions in projected space for each UTM zone (and similarly for the S and N UPS zones).

The only remaining question is to make the legal regions smaller than these in order to make the overlap region no more than about 0.5 deg.
The boundary would then zig-zag to follow the 100km block boundaries.  I elected not to do this in GeographicLib, because

(a) this would be tricky to get right (especially with the exceptions for Norway and Svalbard)

(b) there's nowhere I can since in the US DoD documentation that says that the overlap should be restricted to 0.5 deg.  GeoTrans once (and may still) enforced this overlap restriction but that's for a bad
reason: that the GeoTrans UTM algorithm is inaccurate much beyond 3 deg from the central meridian

(c) the sort of coordinate transformations I need to do, e.g., integrate Lidar collects in different UTM zones, a stricter overlap region would inevitably "get in my way".  Being generous allows, for example, a user to use "doubled" UTM zones in Greenland and the north of Canada similar to the double-wide Svalbard zones.

(d) if restrictions are needed then they can be enforced by a separate layer outside GeographicLib.