[aprssig] UTM grid (was: APRStt for SAR using...)

[Tom Russo]

On Thu, May 16, 2013 at 03:03:10PM +0100, we recorded a bogon-computron collision of the <steve at daniels270.eclipse.co.uk> flavor, containing:
> I collected a lot of information a year or so ago, and if I recall you need
> the reference point and ellipsoid scale for each area.

Given a geodetic datum (and WGS84 is sorta the standard for APRS already),
the ellipsiod is defined for all UTM zones in the same way.

The globe is divided into 60 six-degree zones (numbered 1-60), starting from
180W-174W as zone 1, increasing to the east.  The central meridian of each
zone is the reference point for each zone, and is assigned a "false easting"
of 500000m.  Once you've determined which 6-degree zone contains your longitude,
conversion from lat/lon to UTM in that zone is straightforward based
on the difference between that longitude and the zone's central meridian, 
though the formulas are pretty hairy.

UTM coordinates within a zone are always positive, and decrease going west 
from 500000m at the central meridan, and increase to the east.  Since the
zones are so large, it would be unnecessary for zone information to be
included in the strictly local APRStt input --- for any given pair of 
northing and easting in the vicinity of the search, there can be only one zone 
to which they would refer (even if the search straddles a zone boundary, there 
would be no ambiguity --- low valued eastings would be in the higher numbered 
zone, high valued eastings in the lower).  One would simply need to code the
local APRStt system (which would presumably be temporary infrastructure anyway)
to do the math.

The problem with this scheme of Bob's is that for it to work, field resources
still need a GPS to get either their UTM coordinates or their Lat/Lon
(depending on how one decides to set up APRStt).  Then they need to 
type in those coordinates into a DTMF pad, just so they can be on APRS, 
presumably so that those responsible for having full incident situational 
awareness get the information.  And if they're not using a GPS, but rather
using crude map-based methods to pick off coordinates, there is not much value
to having them type in high-precision UTM coordinates either.

As one of the New Mexico State Police incident commanders for search
and rescue, I would see almost no benefit to asking teams to follow this 
procedure over simply radioing their positions by voice on request, and having 
our comms specialists at base enter the data into APRS in the comfort of the 
incident communications center so it can be shared around the incident 
management team easily.  We *already* have them call in that information,
and where we use APRS we do so to *simplify* information transfer and 
decrease radio chatter, not add complexity to team assignments.

APRS in an automated portable tracker has a benefit both to incident management
(who get to see the data without pulling it constantly) and to the deployed 
resource (who get to have that data pushed without having their work 
assignments constantly interrupted to pull out a GPS and read it off over the 
air).  APRStt sounds like the worst of both worlds --- we still have to task 
field resources to do manual data entry, still need to handle correcting 
broken manual data (usually by calling back and getting clarification) AND it 
is using a technique that is more time consuming and at least as error prone 
as relaying coordinates by voice to a human sitting in front of a better data 
entry system.

This sounds like a solution desperately in search of a problem.  I would rather
see time and energy spent working to get better, lighter, and more reliable
two-way, automatic APRS devices into searcher's hands.  Two-way devices
tied to mapping GPS units allow not only searchers to report their current
locations to the IMT, but for positional data to be transmitted back to them
from the IMT (via objects) to aid them in the completion of their assignments
without adding extra procedures for them to follow.

> -----Original Message-----
> From: aprssig-bounces at tapr.org [mailto:aprssig-bounces at tapr.org] On Behalf
> Of Lynn W. Deffenbaugh (Mr)
> Sent: 16 May 2013 13:39
> To: TAPR APRS Mailing List
> Subject: [aprssig] UTM grid (was: APRStt for SAR using...)
> 
> On 5/16/2013 7:45 AM, Robert Bruninga wrote:
> > Now all I gotta do is refresh myself on the UTM grid to LATLONG
> conversion.
> 
> Copy me on what you learn because the last time I looked there isn't 
> jsut one UTM grid, but several localized versions that use different 
> reference points and conversion equations.

-- 
Tom Russo    KM5VY   SAR502   DM64ux          http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236        http://kevan.org/brain.cgi?DDTNM
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