Re: Coordinates of AIS targets

[Gary E. Miller]

Yo Владимир!

On Mon, 8 May 2023 20:09:56 +0300
Владимир Калачихин <v.kalachikhin@gmail.com> wrote:

> 07.05.2023 22:29, Gary E. Miller пишет:
> >> the OpenCPN and SignalK understands my AIS sentences and display
> >> targets in correct coordinates, but gpsd not.  
> > Odd, I'd need to see your raw AIS to make a gues.
> >   
> 
> I found that coordinates in 27'th sentence rounded to one decimal
> place. But for 18'th sentence the accuracy is maintained.

I looked at the standard:

https://www.itu.int/rec/R-REC-M.1371/en

The transmitted message 18 precision is 1/600,000th of a degree
(0.00000166...)  Message 18 output format for lat/lon was %.6f.  I
bumped that up to %0.7f to maintain precision.

> The coordinates are stored in 27'th sentence with precision of 1/600
> of degrees, so gpsd must return it as least with two decimal places
> precision.

Per M.1371:

1/10 minute, which is 1/000 degrees.  So I made that %.4f.

The transmitted message 27 precision is 1/600th of a degree (0.00166..).
It was %.1f in gpsd, I changed that to %.4f.

I also adjusted some of the other lat/lon message's precisions.

> In attachment the AIS flow with 27 and 24 sentences only.

18 and 27 only.

Is that different than you last one?  If so, how?  Which is best to
use?

I always prefer untouched captures.  For best test coverage.

> The 
> coordinates in 27 sentence has full precision.
> I try:
> $ gpsfake digitrafficAIS27Y.log
> and in other terminal:
> $ gpspipe -w --scaled

Easier: gpsdecode < digitrafficAIS27Y.log

Changes pushed.  Please test.  I also added your digitrafficAIS.log as
a regression test, but would prefer a full, unedited, log.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

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