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

[teyrana]

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:

https://pastebin.com/UxgsV93w

Daniel Williams

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858-220-8134 (Mobile)
github.com/teyrana
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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.

Thoughts? 

Daniel Williams

address@hidden

858-220-8134 (Mobile)
github.com/teyrana
LinkedIn

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.

RGDS
GARY
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