[aprssig] Mic-E Position Ambituity in APRS (Robert

Alex Carver kf4lvz at yahoo.com
Tue Jan 8 13:18:19 EST 2008

> Message: 11
> Date: Tue, 8 Jan 2008 12:53:41 -0500
> From: "Robert Bruninga" <bruninga at usna.edu>
> Subject: [aprssig] Mic-E Position Ambituity in APRS
> To: "'TAPR APRS Mailing List'"
> <aprssig at lists.tapr.org>
> Message-ID:
> <043f01c8521f$678c4950$42577a83 at ewlab.usna.edu>
> Content-Type: text/plain;	charset="US-ASCII"
> > I have a D700.  What does *it* do to 
> > the GPS Lat/Lon data when Pos-ambig is
> > turned on? 
> In the normal APRS format.  Unavailable digits of
> precision are
> replaced with a SPACE.
> In the D700 (Mic-E format), unavailable digits of
> precision in
> the latitude are replaced with the letter "Z" (not
> zeros).  The
> Longitude then is assumed to have the same level of
> precision.
> When a person enters his estimated position into a
> D7 or D700
> manually, he only enters those digits he knows. 
> Then he uses
> the position ambiguity menu to INDICATE how many
> extra digits of
> precision he is not using.  The result is exactly
> the same.  
> On decoding of the Mic-E format, the position used
> by the client
> program should not insert "0's" into those unused
> positions, but
> SPACES just like the full APRS format does.  Again,
> this is not
> truncation, but reproducing at the receipent exactly
> what the
> sender intended.

Sorry, Bob, but what you describe is precisely the
definition of truncation.  If I truncate something, I
lop off a piece of it and leave nothing behind.  The
value "34.4567" truncated to two decimal places is
"34.45__".  Rounding is a different story since I may
change one of the digits.  But pure truncation is
exactly what you've done.

The application is going to insert not just zeros but
a whole range of values to plot the result.

If I've got "34.45__" then the full range of values
between 34.4500 to 34.4599 when truncated result in
the value you just transmitted.  So the application
should rightly plot a line from 34.4500 to 34.4599. 
This should happen for latitude and longitude.  Two
perpendicular lines are now generated, sweeping
through a range of values on the surface of a sphere
results in a spherical trapezoid (a degenerate
spherical trapezoid at the poles which is a spherical triangle).

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