help calculating "true bearing to destination" value in $GPRMB string

Hi everyone,

In the $GPRMB data string, the 11th byte gives the "true bearing to
destination". I need to verify this parameter and was wondering how to
calculate it based on the LAT and LONG data. I think the following
formula is probably the most accurate and efficient way of doing this,
right?

TB2 = acos([sin(LAT_START)*sin(LAT_DEST) +
cos(D/60)]/[sin(D/60)*cos(LAT_START)]

where D = 60*acos[sin(LAT_START)*sin(LAT_DEST) +
cos(LAT_START)*cos(LAT_DEST)*cos(LONG_START-LONG_DEST)]

and LAT_DEST and LONG_DEST are the lattitude and longtitude coordinates
to the destination.


Thanks in advance,
weg

Hi weg,

For a good discussion and the formulae you want, see
http://www.angelfire.com/nt/navtrig/C1.html .

weg
As you have probably already discovered you have an error in the
formula for TB2. It should be
TB2 =ArcCos [(Sin(Lat_Dest) - Sin(Lat_Start)*Cos(D/60))/(Sin(D/60) *
Cos(Lat_Dest))]
This gives the correct azimuth (or bearing) if it is to the northeast
or southeast. If the azimuth is to the southwest or northwest the TB2
must be adjusted according to
If Long_Dest is east of Long_Start then TB2= TB2
If Long_Dest is west of Long_Start then TB2= 360 - TB2

The formula for D which is the arc distance in minutes of angle from
start to dest is correct.

TB2 calculated this way is also called the intended track or desired
track. I am not a navigator but it seems to me that "true bearing to
destination" would be calculated from your present position not your
starting position. Hopefully someone with a navigation background could
clear this up.

Regards
Jerry