Great circle formulae, True cource and actual heading

As Steve has noted there are two things happening:
(1) The bearing to point B along a great circle/ellipsoid path
from Point A to B varies along the path. (A computer/autopilot
can be set to follow the ever varying bearing.)

Thanks

I was looking for a formula that does the same, so that i can tell my
heading at a certain point on the course.

I will look at the links given.

Again many thanks all for the help.

Sims

In article ,
Sims wrote:

As Steve has noted there are two things happening:
(1) The bearing to point B along a great circle/ellipsoid path
from Point A to B varies along the path. (A computer/autopilot
can be set to follow the ever varying bearing.)

Thanks

I was looking for a formula that does the same, so that i can tell my
heading at a certain point on the course.

I will look at the links given.

Again many thanks all for the help.

Sims

Here is a program that does that:

http://www.freewarepalm.com/astronomy/navigate.shtml

The idea is to compute the great circle heading and distance, divide the
great circle distance by as many legs as desired, project the initial
heading the divided distance, repeat the process from there for one
less leg until no legs remain.

E.g.: from Washington DC to Moskva, RF:

38o 51'N 77o 1' 48"W to 55o 58' 48"N 37o 30'E in 8 legs:

each leg is 607.77 miles:

bearing start
32o 46' 3.86" 38o 51'N 77o 1' 48.00"W
37o 23' 29.85" 46o 3' 58.96"N 70o 11' 54.04"W
44o 5' 15.66" 52o 44' 48.60"N 61o 24' 8.89"W
53o 47' 18.95" 58o 32' 41.37"N 49o 40' 47.65"W
67o 28' 0.55" 62o 53' 15.70"N 34o 1' 55.31"W
84o 58' 38.88" 65o 0' 21.16"N 14o 34' 48.87"W
103o 33' 33.09" 64o 20' 34.61"N 5o 56' 13.18"E
119o 26' 55.57" 61o 5' 21.89"N 23o 46' 41.62"E
55o 58' 48.00"N 37o 30' 0.00"E

Now, try to navigate that accurately. It is hard to do manually.

Randolph J. Herber, , +1 630 840 2966, CD/CDFTF PK-149F,
Mail Stop 318, Fermilab, Kirk & Pine Rds., PO Box 500, Batavia, IL 60510-0500,
USA. (Speaking for myself and not for US, US DOE, FNAL nor URA.) (Product,
trade, or service marks herein belong to their respective owners.)

"Sims" wrote in message ...

As Steve has noted there are two things happening:
(1) The bearing to point B along a great circle/ellipsoid path
from Point A to B varies along the path. (A computer/autopilot
can be set to follow the ever varying bearing.)

Thanks

I was looking for a formula that does the same, so that i can tell my
heading at a certain point on the course.

I will look at the links given.

Again many thanks all for the help.

Sims

As others have pointed out, it's all there on my web-site, but you
have to piece it together.

http://williams.best.vwh.net/avform.htm

To get the magnetic heading at each point:

(1) determine the great circle true course, TC, at each point on the
path. It varies as you progress.
(2) Correcting for magnetic variation (aka declination), you can
determine the magnetic course, MC by MC = TC +- VAR. You can get VAR
from a fit. It varies with time. If you want something quick and
dirty, use the polynomial fit. If you want to use an official model,
such as WMM2000 or IGRF2000, there's source code on my site that will
compute it.
(3) Lastly, to get magnetic *heading* from magnetic course, you need
to solve the standard flight planning wind triangle to find the wind
correction angle, WCA, using the known or forecast wind at that point
on your flight. Coding for this is also on my site. MH = MC +- WCA
(4) Even then you are not strictly finished. You need to correct
the magnetic heading to get a compass heading, CH using the listed
deviation, DEV, on the specific airplane's compass correction card.
CH = MH +- DEV

As others have pointed out, it's all there on my web-site, but you
have to piece it together.

http://williams.best.vwh.net/avform.htm


Many thanks for a great page. Truly a great piece of work.

I have looked at it and found the mag var code.

I am just surprised that they vary so much depending what model is been
used.

What model is the best? And also it seems that by the poles the calculation
are somewhat wrong, is it just my imagination?

Sims

Ed,

I second the compliment on your site. I have found it useful several times.

John Bell
www.cockpitgps.com