[Proj] MGRS notation in extended UTM zones?

[Charles Karney]

Mikael,

Take a look at

http://geographiclib.sourceforge.net/html/classGeographicLib_1_1MGRS.html

to see the rules that GeographicLib uses for its MGRS class.  You can 
perform the conversions with the web interface

http://geographiclib.sourceforge.net/cgi-bin/GeoConvert

When I started writing this class (Aug 2008) I tried using GeoTrans as
the authority; see

http://earth-info.nga.mil/GandG/geotrans/index.html

However it quickly became clear the GeoTrans used inconsistent rules.  A
big problem was that MGRS coordinates were obtained by rounding eastings
and northings instead of by truncation.  As a result a conversion from
UTM to MGRS might result in an MGRS coordinate which could not be
converted back to UTM.  Partly as a result of my prodding, GeoTrans 3.0
and later fix this particular problem (but others remain).

So instead I devised my own set of rules.  In response to Mikael's
query, my zone overlap rules are as generous as possible consistent with
staying within the MGRS lettering scheme.  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 can expand on my reasons for these choices if anyone's interested.
Also I can unearth my most recent E-mails with the GeoTrans developers
to document what I perceive as the shortcomings of the current version
of GeoTrans.


On 06/21/11 05:29, Mikael Rittri wrote:
> I have a question on MGRS notation and overlapping zones.
>
> On maps with the UTM/UPS grids, the displayed grid
> for one zone may extend a bit into an adjacent zone:
>
>      "Overlap: On large-scale maps and trig lists,
>       the data for each zone, datum, or ellipsoid
>       overlaps the adjacent zone, datum, or ellipsoid
>       a minimum of 40 kilometers. The UTM grid extends
>       to 80°30'S and 84°30'N, providing a 30-minute
>       overlap with the UPS grid."
>           -- DMA TECHNICAL MANUAL 8358.1
> 		Datums, Ellipsoids, Grids, and Grid Reference Systems.
> 		Section 2-6.3.1: Universal Transverse Mercator (UTM) Grid.
>
>      "Overlap: The UPS grid extends to 83°30'N and 79°30'S,
>       providing a 30-minute overlap with the UTM grid."
> 		same reference,
> 		Section 2-6.3.2: Universal Polar Stereographic (UPS) Grid.
> 		http://earth-info.nga.mil/GandG/publications/tm8358.1/tr83581b.html#ZZ26
>
> I am not sure how this should be applied to MGRS notation.
> Perhaps not at all?
>
> For example, grid zone 30X (covering 6°W to 0°E,
> and 72°N to 84°N) contains a 100-km square named WS,
> but it only about 44.3 km wide, because it is clipped
> by the east zone boundary. So, should the MGRS reference
>
>    30XWS5000050000
>
> be considered legal? Is would represent a position
> about 5.7 km into UTM zone 31, but maybe that's okay
> due to the 40 km overlap of UTM zones? Or is the overlap
> intended only to make measurements easier on a printed
> map, and not intended for MGRS notation?
>
> I guess some common sense judgement is required here.
> A software system could reject all MGRS references that
> would be the slightest bit outside their proper UTM zone,
> and the system would then catch more mistakes, but it might
> also reject some slight, innocent trespassing across the
> UTM zone boundaries.
>
> I just wonder if there is some standard, or recommendation,
> about how this error-checking should be done.

-- 
Charles Karney <charles.karney at sri.com>
SRI International, Princeton, NJ 08543-5300
Tel: +1 609 734 2312