Guys,

I had the discussion how one could avoid driving doglegs with a GPS.

A dogleg is the course that occurs when you "aim" for your target
waypoint while wind or current pushes you from the side ... If you
follow bearing to target you would arch around the target and finally
meet it but it would NOT be a straght line..

Question is:

What does one have to do to get to the target on a staright line? Lets
assume we dont make lots of intermediate waypoints.

My 276C has CDI (cross track thing or whatever) havent been able to
test it but wonder if thats the solution?

Matt

xpost

wrote in message
oups.com...
Guys,

I had the discussion how one could avoid driving doglegs with a GPS.

A dogleg is the course that occurs when you "aim" for your target
waypoint while wind or current pushes you from the side ... If you
follow bearing to target you would arch around the target and finally
meet it but it would NOT be a straght line..

Question is:

What does one have to do to get to the target on a staright line? Lets
assume we dont make lots of intermediate waypoints.

My 276C has CDI (cross track thing or whatever) havent been able to
test it but wonder if thats the solution?

Matt

xpost


Well, couldn't you make some assumptions for set and drift of current and
expected time to travel, then plug in a waypoint that matches the new
corrected magnetic course? No, I guess not, since that fancy gps thing will
continue to correct you back to an arc and your false target instead of
allowing a straight line course that brings you to your real target.

I've only used paper, pencil, erasers, plotters and rulers to sail anywhere
(still haven't even used an autopilot) and never thought about the dilemma
created by gps in the presence of set and drift. I guess a gps controlled
autopilot would make so many corrections that the doglegs would be
insignificant. Cant wait to hear the answer.

my suggstion was to point to the target and leave the wheel straight
and go.

current will turn you to one side and the angle will be displayed on
GPS as off course you then have to steer the same angle to the other
side of your direct line course.

This should get you on a straight line to the target but is a little
cumbersome ...

Matt

wrote in message
oups.com...
Guys,

I had the discussion how one could avoid driving doglegs with a GPS.

A dogleg is the course that occurs when you "aim" for your target
waypoint while wind or current pushes you from the side ... If you
follow bearing to target you would arch around the target and finally
meet it but it would NOT be a straght line..

Question is:

What does one have to do to get to the target on a staright line? Lets
assume we dont make lots of intermediate waypoints.

My 276C has CDI (cross track thing or whatever) havent been able to
test it but wonder if thats the solution?

Matt

xpost


Not sure what kind of GPS you are using, but I assume it is a chart plotter.
Most I am familiar with will draw a line (track) from your position to your
destination or waypoint. If you are drifting due to current or wind, your
position cursor should show that you are moving off the line, even though
your bow is still heading toward the waypoint. At least that's how the
Raymarine and Furuno systems I have used work.

Eisboch

wrote:
Guys,

I had the discussion how one could avoid driving doglegs with a GPS.

A dogleg is the course that occurs when you "aim" for your target
waypoint while wind or current pushes you from the side ... If you
follow bearing to target you would arch around the target and finally
meet it but it would NOT be a straght line..

Question is:

What does one have to do to get to the target on a staright line? Lets
assume we dont make lots of intermediate waypoints.

My 276C has CDI (cross track thing or whatever) havent been able to
test it but wonder if thats the solution?

Matt

xpost



About the only way is to use lots & lots of waypoints. So the GPS will
go to each in turn & ensure you're path over the bottom is as intended.

Numerous "commercial" boats have come to grief for relying too much on
the GPS taking the boat to a far off waypoint, at times depending on all
sorts of things the boat can get well off track & even end up
approaching the waypoint from a totally different bearing than was
originally intended, hopefully a reef etc doesn't get in the way.

The expensive systems will maintain track but they do it by calcing
lots of close waypoints & seeing that most systems come with lots & lots
of waypoint capacity you might as well use it, even for a seemingly safe
long open water leg.

Set & drift etc are of course contributors to being off course:-) but
the GPS knows none of this all it knows is where the boat is "over the
bottom" at any given instant & the outputs to give the autopilot to take
you straight to the next waypoint from "that" point. Again as above the
result over time is you can end up a long way from where you thought
you'd be traveling even though you will get to the right place in the
end, save you don't run aground on the way.

Last thing for people with ingredient X:-) that can save time & fuel,
depending on the trip passage leg etc, & of course safety (i.e. what
reefs or other there are to hit if you're deliberately off track)
Consider disregarding tidal flow; if your trip is of reasonable length
often what the tidal flow steals from you on say the flood you'll gain
back for free on the ebb. No point fighting the tide all day just to
cover an arbitrary line across the seabed. Lots of people think the
shortest distance is the chosen track but it's really the shortest
distance "through the water", so even though your course over the ground
seems longer (as given by say GPS) your actual distance through the
water can actually be substantially less (as given by say an accurate
sumlog)

Always a bit bemused when people bemoan the inaccuracy of their silly
old low tech sumlogs because they rarely agree with the GPS, when in
fact often both are correct, the GPS records miles over the seabed, the
sumlog miles through the water.


K

wrote in message
Guys,

I had the discussion how one could avoid driving doglegs with a GPS.

Matt....

What you're talking about is the difference between "homing" and "tracking".
The GPS unit ought to be able to handle this auto-magically for you, if
you're coupled to an autopilot. The concept is that you input a correction
right from the start, so you're constantly compensating for the forces
driving you off course. You don't need a bazillion waypoints, or anything
like that. The correction factor is adjusted (as required) to maintain your
track to target. It's actually quite easy to do manually, and most GPS
units will at the very least give you a screen that displays track, so you
can steer a correction yourself. Check a navigation text, (or take a
course - CGAux, for example) to be sure you understand the concept and
mechanics of track and correction, then check your GPS manual to see how to
set it up.

Merry Christmas!

On 23 Dec 2005 22:07:18 -0800, wrote:

A dogleg is the course that occurs when you "aim" for your target
waypoint while wind or current pushes you from the side ... If you
follow bearing to target you would arch around the target and finally
meet it but it would NOT be a straght line..

====================================

With a GPS this is fairly easy and there are at least two ways to do
it.

Method 1: Set a waypoint for your destination; observe your bearing
to the waypoint and your Course Over Ground (COG); adjust your steered
course so that they are equal to each other; readjust as necessary.

Method 2: Set a From/To route using your present position as "From"
and your destination as "To"; Observe your Cross Track Error (XTE),
and adjust course to keep XTE near zero.

Both methods will result in a straight line course over ground
depending on your steering accuracy.

Many GPS have a "highway mode" Which is a pretty good graphic of the direct
course to destination with your position relative to same. Pretty easy to
adjust your steering to maintain a steady course along the highway.

Butch

PS: Only for us non USPS amateurs, of course.

B
"Wayne.B" wrote in message
...
On 23 Dec 2005 22:07:18 -0800, wrote:

A dogleg is the course that occurs when you "aim" for your target
waypoint while wind or current pushes you from the side ... If you
follow bearing to target you would arch around the target and finally
meet it but it would NOT be a straght line..

====================================

With a GPS this is fairly easy and there are at least two ways to do
it.

Method 1: Set a waypoint for your destination; observe your bearing
to the waypoint and your Course Over Ground (COG); adjust your steered
course so that they are equal to each other; readjust as necessary.

Method 2: Set a From/To route using your present position as "From"
and your destination as "To"; Observe your Cross Track Error (XTE),
and adjust course to keep XTE near zero.

Both methods will result in a straight line course over ground
depending on your steering accuracy.

@ All

thanks for the replies

To clarify: I know how to navigate without GPS and with cross current
by useing vectors...

My question is purely:

can a GPS (alone, no autpilot) show you the course or where you have
to point your bow (bearing) in order to drive a straight line to your
target. WITHOUT creating waypoints or "manual" input like adjusting for
cross track errors ...

Since the GPS is a more powerful computer as the one that got the
Apollo spaceships to the moon and back, that should be possible

My garmin GPS 276C has a "CDI" which i beleive does just that but the
description in the manual is useless esp. in regards how it works and
what not ...

So I was wondering if the GPS can do more as what we can do on a map
....

Mat

So I was wondering if the GPS can do more as what we can do on a map

Ah yes, the wonderful Garmin manuals - one of their more successful
customer interfaces that they don't even bother to improve. Well done
Garmin!! Another success in dealing with the average joe.

No kidding ... its one of the "useful" kind which describes functions
by how to activate them in the menu (duh) but does not waste time
explaining what exactly it does and how it works .... who would want to
knwo that ?




Well, according to the manual, which I just downloaded, it either does
it through setting Waypoints and creating a Route, or it doesn't do it
that way at all, but by using the "highway" setting.

I can't make heads or tales out of the manual without having a 276c in
front of me to experiment with.

Well ... its too cold to use the boat for a real life test ... I tested
this CDI in simulator mode .... but that only works so well and doesnt
simulate a cross current ...

What it does with NO cross current is, it steers you back to the direct
line from start to target IF you follow the little CDI icon .... hard
to say what it will do with cross current ...

Matt