[aprssig] APRStt for SAR using UTM grid!

Tom Hayward esarfl at gmail.com
Mon Jun 17 14:23:21 CDT 2013

On Mon, Jun 17, 2013 at 11:24 AM, Robert Bruninga <bruninga at usna.edu> wrote:
> We need to resolve this.  I was told by some SAR folks that for all voice
> reporting during a SAR mission, that the XXX,YYY resolution was for the
> 10km square grid with resolution to 10 meters for the purpose of generally
> knowing where everyone was.  Additional digits were only used if higher
> resolution or a different area was of interest.
> Are you saying that not all groups use this convention?
> Bob, WB4APR

You're just off by one digit.

USNG is the federally mandated coordinate format for SAR in the US.
It's a [relatively] new specification, but it just codifies the
long-time SAR (and military) practice of truncating UTM grids. The
easting 10s-of-km digit is always first, then digits are added to
improve precision. This results in a grid coordinate that repeats
itself every 100km. XXXYYY (100m precision) is most common. Now with
GPS being ubiquitous, XXXXYYYY (10m precison) is becoming more common.

Page 17 of the USNG spec describes how the old practice of truncating
UTM grids has been standardized:
> Truncation of USNG Coordinate Values
> A uniform system of truncation is adopted for the USNG. Truncated coordinates begin
> with the 10,000-meter digit. Truncated coordinate values shall always consist of an even
> number of digits. Table 1 demonstrates how to truncate USNG grid coordinate values and
> compares these with truncated UTM grid coordinates. The portions of the USNG grid
> coordinate that is imbedded in the UTM coordinate value are underlined for illustrative
> purposes.
<snip> Table 1 </snip>
> A similar system of coordinate truncation can be employed for UTM grid coordinates. However, such a
> system of truncation is not part of the definition of the UTM system or the coordinate representation
> standard, ANSI X3.61, and is included here in Table 1 for illustration and uniformity purposes.

If you need to read coordinates off a USGS map (or any other map
marked with UTM) and you don't remember which digit to start with,
it's easy--just start with the digit in the larger font (found near
the tick marks in the margin). These larger font digits are called
principal digits. By themselves they form an XXYY grid (1km


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