Great circle formulae, True cource and actual heading

[Sims]

Hi,

I am trying to write a formula to compute flight headings.
If i use 2 lat/lon i can get a "true course" but due to magnetic variations
the true course is not really all that useful, (knowing that my true course
is 118deg will not tell me that i should fly 93 deg).

What i mean is if i go from A to B i can calculate the True course, ('tc')
but my actual heading varies a lot depending on my position along the
course.

Is there a reliable formula to calculate my actual heading at a certain
point along the course?

Many thanks in advance.

Sims

[Dave Patton]

"Sims" > wrote in
:

> Hi,
>
> I am trying to write a formula to compute flight headings.
> If i use 2 lat/lon i can get a "true course" but due to magnetic
> variations the true course is not really all that useful, (knowing
> that my true course is 118deg will not tell me that i should fly 93
> deg).
>
> What i mean is if i go from A to B i can calculate the True course,
> ('tc') but my actual heading varies a lot depending on my position
> along the course.
>
> Is there a reliable formula to calculate my actual heading at a
> certain point along the course?
>
> Many thanks in advance.
>
> Sims

At least in sci.geo.satellite-nav this sort of question about
formulae has been asked numerous times, including quite recent
threads, so a search via Google Groups would likely have already
found you your answer.

Take a look at Ed Williams page about Aviation Formulary:
http://williams.best.vwh.net/avform.htm

--
Dave Patton
Canadian Coordinator, the Degree Confluence Project
http://www.confluence.org dpatton at confluence dot org
My website: http://members.shaw.ca/davepatton/
Vancouver/Whistler - host of the 2010 Winter Olympics

[Dale DePriest]

Sims wrote:

> Hi,
>
> I am trying to write a formula to compute flight headings.
> If i use 2 lat/lon i can get a "true course" but due to magnetic variations
> the true course is not really all that useful, (knowing that my true course
> is 118deg will not tell me that i should fly 93 deg).
>
> What i mean is if i go from A to B i can calculate the True course, ('tc')
> but my actual heading varies a lot depending on my position along the
> course.
>
> Is there a reliable formula to calculate my actual heading at a certain
> point along the course?
>
> Many thanks in advance.
>
> Sims
>
>

You mean your magnetic heading. You have your actual heading. You need
a table of magnetic variations and interprolate from the table based on
your location as there is no mathmatical model that will provide this
information reliably.

Dale
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs

[red rover]

"Dale DePriest" > wrote in message
> >
>
> You mean your magnetic heading. You have your actual heading. You need
> a table of magnetic variations and interprolate from the table based on
> your location as there is no mathmatical model that will provide this
> information reliably.
>

I found the post a little confusing because it seemed to be mixing
the problem of the course heading changing due to the great circle
route (mentioned in the subject) with heading changes due to changing
declination mentioned in the body. Two very different issues.

Steve

[Craig Prouse]

Dale DePriest wrote:

> You need
> a table of magnetic variations and interprolate from the table based on
> your location as there is no mathmatical model that will provide this
> information reliably.

Given a table of magnetic variations at known locations, there are
statistical methods to derive a useful mathematical model. The web site
cited in a previous article provides such a model as a polynomial.

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

[Dale DePriest]

red rover wrote:

> "Dale DePriest" > wrote in message
>
>>You mean your magnetic heading. You have your actual heading. You need
>>a table of magnetic variations and interprolate from the table based on
>>your location as there is no mathmatical model that will provide this
>>information reliably.
>>
>
>
> I found the post a little confusing because it seemed to be mixing
> the problem of the course heading changing due to the great circle
> route (mentioned in the subject) with heading changes due to changing
> declination mentioned in the body. Two very different issues.
>
> Steve
>
Yes, it was my mistake, I misread the original post by not comparing the
text to the subject line. I tried to cancel but too late, Sorry

Dale


--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs

[red rover]

"Dale DePriest" > wrote in message
...
> >
> Yes, it was my mistake, I misread the original post by not comparing the
> text to the subject line. I tried to cancel but too late, Sorry
>
> Dale
>
Well I actually meant the original post was confusing, not your
reply.

[Ron McConnell]

Sims wrote:

> flight headings.
> If i use 2 lat/lon i can get a "true course"
> but due to magnetic variations ...
> actual heading varies a lot depending on my position along the
> course. ...formula to calculate my actual heading at a certain
> point along the course?

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.)

(2) The Earth's magnetic variation (a.k.a. declination) that affects
a magnetic compass heading varies with latitude and longitude
and also with time. This magnetic field offsets the
true great circle bearing at a given point along the path.

If one follows a rhumb line course instead of a great circle path,
the rhumb line bearing is constant. This is easier for a human
to follow at the expense of having a longer path.
One can find this on a Mercator projection map.
Again the magnetic variation offsets the rhumb line bearing
at a given point.

As Dave notes, Ed Williams' site discusses these issues.

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


The World Magnetic Model (WMM) maintained by the U.S. DoD
and others

http://www.ngdc.noaa.gov/seg/WMM/DoDWMM.shtml

predicts the components of the Earth's steady state magnetic field
for a given latitude and longitude for a given date.
The horizontal component direction corresponds to the magnetic
variation. The target accuracy is one degree over a 5-year period.

My freeware DOS command line program GCGC* calculates the great circle
bearings between two points (WGS-84 default) and then uses
the WMM to also calculate the magnetic bearings at the end points.
If one needs the true and magnetic bearings along the flight path
(certainly a reasonable thing to need in flying), one would recalculate
the great circle path to point B from the position at the moment.
One can use the included Direct function (Lat1/Long1, Bearing 1-2,
and distance 1-2 to get Lat2/Long2) to calculate positions
along the great circle path by splitting it into pieces.

I also have a program (gcb12) that does rhumb lines at my web site,
but I haven't gotten around to adding the magnetic bearings.

Cheers, 73,

Ron McConnell
w2iol

N 40º 46' 57.9" W 74º 41' 21.9"
Magnetic Variation = 13.0º W in October 2003
FN20ps77GU46 [FN20ps77GV75]

* GCGC executable and source at

http://home.earthlink.net/~rcmcc

[Sims]

>
> 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

[Sims]

"red rover" > wrote in message
.. .
>
> "Dale DePriest" > wrote in message
> > >
> >
> > You mean your magnetic heading. You have your actual heading. You need
> > a table of magnetic variations and interprolate from the table based on
> > your location as there is no mathmatical model that will provide this
> > information reliably.
> >
>
(Sorry to reply here your post was cancelled).

Yes i do mean the magnetic heading.
Sorry I was not clear.

Where can i get a table to compute my magnetic heading?

Thanks

Sims.

[Dave Martindale]

"Sims" > writes:

>I guess i was looking for a formula to calculate my True Course +Magnetic
>variation at any given point on the globe.

Formulas can tell you the true course needed at all points along the
great circle route, because it's just geometry. But the local magnetic
variation is not predictable by a formula. Just look at a map of lines
of constant variation - they're too irregular (and they move with time).

The best you can do is a table plus interpolation, or a set of functions
empirically fitted to measured data. Either way, it won't be just a
simple formula.

On the other hand, even if you knew the magnetic course you need to
follow exactly, that still wouldn't tell you the heading to fly or
sail, since the heading also needs to provide wind correction.

If you have a GPS along on your trip, it can show you whether you are to
the left or right of the ideal great-circle path between two waypoints.
The correct heading is whatever heading keeps you along the centerline
of that path. There's no better definition of correct heading.

Dave

[John Bell]

>
> On the other hand, even if you knew the magnetic course you need to
> follow exactly, that still wouldn't tell you the heading to fly or
> sail, since the heading also needs to provide wind correction.
>
> If you have a GPS along on your trip, it can show you whether you are to
> the left or right of the ideal great-circle path between two waypoints.
> The correct heading is whatever heading keeps you along the centerline
> of that path. There's no better definition of correct heading.
>

The GPS itself will indicate the amount to turn in order to reach the
correct heading. COURSE or DTK (synonyms) will change along the route as a
reflection of great circle effects. Likewise, the BEARING will also change
for great circle. While what you said is correct, a good way of staying on
course is to compare TRACK to BEARING or keep TURN to zero. This makes it
easier to find the heading that keeps the vehicle along the centerline of
the path.

[Dale DePriest]

Sims wrote:

> "red rover" > wrote in message
> .. .
>
>>"Dale DePriest" > wrote in message
>>
>>>You mean your magnetic heading. You have your actual heading. You need
>>>a table of magnetic variations and interprolate from the table based on
>>>your location as there is no mathmatical model that will provide this
>>>information reliably.
>>>
>>
> (Sorry to reply here your post was cancelled).
>
> Yes i do mean the magnetic heading.
> Sorry I was not clear.
>
> Where can i get a table to compute my magnetic heading?

Try here for a start.

http://www.ngdc.noaa.gov/seg/WMM/DoDWMM.shtml

Dale

>
> Thanks
>
> Sims.
>
>

--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs

[Dave Patton]

"Sims" > wrote in
:

>
> "red rover" > wrote in message
> .. .
>>
>> "Dale DePriest" > wrote in message
>> > >
>> >
>> > You mean your magnetic heading. You have your actual heading. You
>> > need a table of magnetic variations and interprolate from the table
>> > based on your location as there is no mathmatical model that will
>> > provide this information reliably.
>> >
>>
> (Sorry to reply here your post was cancelled).
>
> Yes i do mean the magnetic heading.
> Sorry I was not clear.
>
> Where can i get a table to compute my magnetic heading?

You might take a look at the Canadian Geological Survey
Geomagnetism website:
http://www.geolab.nrcan.gc.ca/geomag/home_e.shtml
After all, the North magnetic pole is in Canada ;-)

--
Dave Patton
Canadian Coordinator, the Degree Confluence Project
http://www.confluence.org dpatton at confluence dot org
My website: http://members.shaw.ca/davepatton/
Vancouver/Whistler - host of the 2010 Winter Olympics

[Dave Martindale]

"John Bell" > writes:

>The GPS itself will indicate the amount to turn in order to reach the
>correct heading. COURSE or DTK (synonyms) will change along the route as a
>reflection of great circle effects. Likewise, the BEARING will also change
>for great circle. While what you said is correct, a good way of staying on
>course is to compare TRACK to BEARING or keep TURN to zero. This makes it
>easier to find the heading that keeps the vehicle along the centerline of
>the path.

I haven't heard of a GPS receiver that will tell you what heading to fly
or sail to compensate for wind or current. A GPS-only unit has no way of
knowing what your current heading is in the first place. It can tell
you how much to turn to align your heading with the desired course, but
that assumes no wind or current. In practice, you'll have to make
adjustments to that heading, while watching the cross-track error.

Dave

[Dale DePriest]

Dave Martindale wrote:

> "John Bell" > writes:
>
>
>>The GPS itself will indicate the amount to turn in order to reach the
>>correct heading. COURSE or DTK (synonyms) will change along the route as a
>>reflection of great circle effects. Likewise, the BEARING will also change
>>for great circle. While what you said is correct, a good way of staying on
>>course is to compare TRACK to BEARING or keep TURN to zero. This makes it
>>easier to find the heading that keeps the vehicle along the centerline of
>>the path.
>
>
> I haven't heard of a GPS receiver that will tell you what heading to fly
> or sail to compensate for wind or current. A GPS-only unit has no way of
> knowing what your current heading is in the first place. It can tell
> you how much to turn to align your heading with the desired course, but
> that assumes no wind or current. In practice, you'll have to make
> adjustments to that heading, while watching the cross-track error.
>
> Dave

The III pilot does permit you to make the adjustment manually so that
you can use the GPS for guidance but it does not and cannot make this
adjustment automatically since it has no sensors to detect this.

Dale

--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs

[Randolph J. Herber]

In article >,
Craig Prouse > wrote:
>Dale DePriest wrote:

>> You need
>> a table of magnetic variations and interprolate from the table based on
>> your location as there is no mathmatical model that will provide this
>> information reliably.

>Given a table of magnetic variations at known locations, there are
>statistical methods to derive a useful mathematical model. The web site
>cited in a previous article provides such a model as a polynomial.

>http://williams.best.vwh.net/avform.htm#Var

http://geomag.usgs.gov/chartsdo.html

http://geomag.usgs.gov/models.html

http://www.ngdc.noaa.gov/IAGA/wg8/igrf.html

http://www.ngdc.noaa.gov/IAGA/wg8/table1.txt

http://www.freewarepalm.com/calculator/magcalc.shtml

http://www.garmin.com/manuals/GPSIIIPlus_OwnersManual.pdf page 78 (in document)
(page 88 in pdf file) gives 4 heading adjustment choices: auto magnetic
(using a magnetic field model driven by the current location), true, grid
(driven the UTM locatin) or user magnetic (the user states the desired
adjustment). I suppose that you could use this to include a wind adjustment?!

Other than that; get your aviation weather report and your E6B.

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.)

[Mogens Beltoft]

Dave Martindale wrote:
> "John Bell" > writes:
>
>> The GPS itself will indicate the amount to turn in order to reach the
>> correct heading. COURSE or DTK (synonyms) will change along the
>> route as a reflection of great circle effects. Likewise, the
>> BEARING will also change for great circle. While what you said is
>> correct, a good way of staying on course is to compare TRACK to
>> BEARING or keep TURN to zero. This makes it easier to find the
>> heading that keeps the vehicle along the centerline of the path.
>
> I haven't heard of a GPS receiver that will tell you what heading to
> fly
> or sail to compensate for wind or current. A GPS-only unit has no way
> of knowing what your current heading is in the first place. It can
> tell
> you how much to turn to align your heading with the desired course,
> but that assumes no wind or current. In practice, you'll have to make
> adjustments to that heading, while watching the cross-track error.

We use GPS receivers in our gliders, and most of them can find the wind
direction and speed automatically by detecting that you have made a full 360
degree turn (happens often for gliders when searching for thermals), and
analysing the 360 degree turn you have made for drift direction and speed.

And I'm certain that the information is used to display a wind-corrected
heading.

We use Pocket-Nav http://www.cambridge-aero.com/pocketNAV.htm and others.

/Mogens

[Randolph J. Herber]

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.)

[Ed Williams]

"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

[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
>

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

[Dave Martindale]

"Mogens Beltoft" > writes:

>We use GPS receivers in our gliders, and most of them can find the wind
>direction and speed automatically by detecting that you have made a full 360
>degree turn (happens often for gliders when searching for thermals), and
>analysing the 360 degree turn you have made for drift direction and speed.

>And I'm certain that the information is used to display a wind-corrected
>heading.

So which GPS receivers do this for you?

>We use Pocket-Nav http://www.cambridge-aero.com/pocketNAV.htm and others.

That seems to be an application that runs on an external computing device,
not something that a GPS receiver itself provides. Interesting, but
different.

Dave

[John Bell]

Ed,

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

John Bell
www.cockpitgps.com

[John Bell]

Dave,

>A GPS-only unit has no way of knowing what your current heading is in the
first place.

I agree with this statement.

>I haven't heard of a GPS receiver that will tell you what heading to fly or
sail to compensate for wind or current.

Neither have I.

>It can tell you how much to turn to align your heading with the desired
course, but that assumes no wind or current. In practice, you'll have to
make
>adjustments to that heading, while watching the cross-track error.

I agree that a GPS will not tell you an absolute heading to fly, but it will
tell you a relative heading. The GPS will give you a value of TRACK and it
will give you a value for BEARING. The idea is to adjust your heading so
that TRACK matches BEARING. Many GPS receivers have a data field called
TURN --this is just the difference between TRACK and BEARING. If the GPS
indicates a TURN of 5 degrees left, adjust your heading 5 degrees left. If
you follow TURN or match TRACK and BEARING, you should track directly to the
active waypoint.

I agree that the GPS could care less what your heading is. Example: If you
have a TRACK of 85 and a BEARING of 80, the TURN would be L5. Turn 5 left
so that TRACK matches BEARING. Let's say that you have done that and the
TRACK and BEARING are now both 80. The GPS has no idea if you are heading
065 for a left crosswind, 95 for a right crosswind, or 080 for no wind.

If you were to follow a heading to match the GPS value of BEARING, a curved
path would result from a crosswind or cross current. This is often refereed
to as homing and is considered bad technique.

For large turns the change in heading does not always match the change in
TRACK. For example, if you were flying north into a northerly wind and
turned to 090, the change in track would be more than the change in heading.
The headwind would change to a crosswind. However for small adjustments,
the change in heading is proportional to the change in track.

There is a slight variation in this technique for panel mounted aviation
receivers. The difference is that they usually do not offer TURN, but do
offer a field called Track Angle Error, TKE. TKE is the difference between
TRACK and DTK or COURSE (synonyms). TKE gets you parallel to the course as
opposed to directly to the active waypoint. In this case, cross-track error
is necessary. However by turning so that TKE is zero will result in a
constant cross-track error. It may then be necesary to adjust the heading a
little more to reduce the cross-track error to zero.

The method that you describe of adjusting your heading to manage cross track
is not incorrect. However, I thing that the ability to get an exact heading
that compensates for wind or currents by comparing the TRACK to BEARING is
one of the most powerful features that GPS has to offer aviators and
boaters.

I have more details in the navigation section of either of my online books
at www.cockpitgps.com.

--John Bell

[Dave Martindale]

"John Bell" > writes:

>I agree that a GPS will not tell you an absolute heading to fly, but it will
>tell you a relative heading. The GPS will give you a value of TRACK and it
>will give you a value for BEARING. The idea is to adjust your heading so
>that TRACK matches BEARING. Many GPS receivers have a data field called
>TURN --this is just the difference between TRACK and BEARING. If the GPS
>indicates a TURN of 5 degrees left, adjust your heading 5 degrees left. If
>you follow TURN or match TRACK and BEARING, you should track directly to the
>active waypoint.

That's correct, with a couple of caveats.

The first is that in an aircraft at least, you normally fly a heading
based on the directional gyro, since it gives you a stable reading. The
GPS TRACK doesn't reflect heading changes instantly, while the DG does.
So you maintain a heading, then see what happens to the track, then
possibly adjust the heading.

The second is that this method takes you directly to the destination,
no matter where you are now, no matter how much you've drifted off your
original course. Sometimes that's fine, but sometimes you want to
regain your originally-planned ground track (e.g. it takes you between
mountains, or avoids submerged rocks, or avoids a restricted area).
For these cases, what you really want is to look at cross-track error
(the deviation from your planned route) and get back on the planned
track.

>The method that you describe of adjusting your heading to manage cross track
>is not incorrect. However, I thing that the ability to get an exact heading
>that compensates for wind or currents by comparing the TRACK to BEARING is
>one of the most powerful features that GPS has to offer aviators and
>boaters.

Why is it not correct? Keeping the cross-track error zero takes you
directly to your next waypoint, factoring in any wind or current
correction that is necessary, and it lets you follow your originally-
planned ground track. It's the best you can do. It's like following a
VOR radial, except that the virtual "VOR" can be placed anywhere (it's
just a waypoint), and the "needle deflection" vs. track error has
constant gain all along the route.

The method of matching track to bearing will also take you directly to
the destination if you never get off course. But once you do get off
course, this method takes you along a new "direct" path to the next
waypoint. This is *faster*, but it isn't always safe. Navigating to
minimize cross-track error is safer, but potentially slower. Both
result in exactly the same ground track if you never get off the planned
route. You can say that either is better than the other, depending on
circumstances. I don't see how you can say one method is "not
correct".

Dave

[John Bell]

Dave,

I actually agree with your points. In fact, many of the points that you
make are ones that I make in my chapters on navigation in both of my texts
at www.cockpitgps.com.

The only thing that I substantially disagree with you is in that I disagreed
with you in the first place. In answer to the comment: "I don't see how
you can say one method is 'not correct'." I had actually written that your
original post was "not incorrect." I apologize for the double negative, but
my intent was to emphasize that I was not contradicting your original post
so the double negative seemed appropriate in context. I feel confident in
my ability to navigate with a GPS. However, I have never bragged about my
writing abilities.

My point is that although old methods of navigation based on cross track
error are still correct, the GPS gives a unique capability to more
accurately find a very precise heading to compensate for winds and cross
currents because of its ability to sense an actual track.

I think that cross track error is a very useful and at times essential piece
of data. I see the method that you describe and the method that I describe
as being complimentary, not contradictory.

--John Bell, www.cockpitgps.com

[Dave Martindale]

"John Bell" > writes:

>The only thing that I substantially disagree with you is in that I disagreed
>with you in the first place. In answer to the comment: "I don't see how
>you can say one method is 'not correct'." I had actually written that your
>original post was "not incorrect."

Oops. I was in a hurry, and misread that as "not correct". My fault.

Dave

[Mogens Beltoft]

Dave Martindale wrote:
> "Mogens Beltoft" > writes:
>
>> We use GPS receivers in our gliders, and most of them can find the
>> wind direction and speed automatically by detecting that you have
>> made a full 360 degree turn (happens often for gliders when
>> searching for thermals), and analysing the 360 degree turn you have
>> made for drift direction and speed.
>
>> And I'm certain that the information is used to display a
>> wind-corrected heading.
>
> So which GPS receivers do this for you?
>
>> We use Pocket-Nav http://www.cambridge-aero.com/pocketNAV.htm and
>> others.
>
> That seems to be an application that runs on an external computing
> device, not something that a GPS receiver itself provides.
> Interesting, but different.

The GPS receiver is just an standard "mouse", so NMEA data is analysed by
PocketNav to provide the information.

/Mogens