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Re: [gpsd-users] Position accuracy of AIS data in gpsd json?

From: Gary E. Miller
Subject: Re: [gpsd-users] Position accuracy of AIS data in gpsd json?
Date: Thu, 9 Aug 2018 15:37:16 -0700

Yo Sven!

On Thu, 9 Aug 2018 20:03:59 +0200
Sven Geggus <address@hidden> wrote:

> Gary E. Miller schrieb am Donnerstag, den 09. August um 19:41 Uhr:
> > Where exactly are you looking?  In the JSON, or in one of the
> > clients?  
> gpsd_json.c see below.

So it is where gpsd is preparing the JSON.  Which fields are you
concerned with?

> > > Looking at the spec, the accuracy of the messages should be
> > > 0.0001’ which would be 0.0000017°  
> > 
> > Citation?  Which spec?  I'm guessing that would be 0.617 feet at the
> > equator.  Only survey grade GPS and RTK GPS are that accurate.  
> Well, when using scaled=false one will get an Integer which can then
> be converted to degrees by means of deviding its by 60000.0 which
> equals a resolution of 0.000016666667 degrees. (see AIS_LATLON3_DIV
> in gps.h). I have been using scaled=true which gave me discretised
> positions.

Lost me.  By citation I mean an external specification we can comply
to.  And I have no idea which feilds you are concerned about.

Examples please.

No practical user level GPS is going to be accurate to 0.617 feet today.
So the extra digits are ust meaningless random numbers.

> So the _resolution_ provided by the NMEA packages is this exact no
> matter how exact the gpv receiver of the vessel is in reality.

No point shipping meaningly random digits to the user.

> > That would exceed the accuracy of any AIS I ever heard of.  Maybe 
> > %.5f would be best.  
> I somewhat feel that the resolution of NMEA should also be reflected
> in the corresponding json output.

The NMEA spec (GPGLL) is for 0.01 degrees.  WAY less than you ask for.
Less than what gpsd does.

But the spec is pretty meaningless, every GPS implements it differently.
What does YOUR NMEA look like?

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin

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