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Re: [gpsd-dev] [RFC] $GPVTG & magnetic course

From: teyrana
Subject: Re: [gpsd-dev] [RFC] $GPVTG & magnetic course
Date: Wed, 30 May 2018 18:39:09 -0400

Another idea for extract the $GPVTG message data was to stored the compass variation in existing variables, like so:

> session->gpsdata.navdata.compass_variation = true_track - mag_track;

Yay or nay?

p.s. More context:

Daniel Williams
858-220-8134 (Mobile)

On Mon, May 28, 2018 at 10:37 PM, teyrana <address@hidden> wrote:
Ah, right.   I forget VTG was track, not heading.

Option B is to calculate the magnetic declination by subtracting Magnetic Track from True Track.  And we could store *that* quantity in the navdata_t -> compass_declination.   That would allow us to extract the information from this message, without adding new fields.


Daniel Williams
858-220-8134 (Mobile)

On Mon, May 28, 2018 at 7:24 PM, Gary E. Miller <address@hidden> wrote:
Yo teyrana!

On Sat, 26 May 2018 18:43:26 -0400
teyrana <address@hidden> wrote:

> I was looking through gps.h, and I notice that " struct navdata_t {"
> @ line 1849 has a 'compass_heading' field. Are you okay with storing
> the GPVTG data there?

'heading' and 'track' are not the same.

'heading' is where you are pointed.  'track' is where you actually went.

Imagine you are in a sailboat pointed north, with a hurricane force
wind from the east.  You heading is north, but your track will be to
the west.

> p.s. FYI: compass 'dip', (aka compass inclination) is in the 'struct
> attitude_t', while the other compass deviation & variation are in the
> different struct: 'navdata'.

Yeah, a mess.

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

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