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Driving Doglegs
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 |
Driving Doglegs
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. |
Driving Doglegs
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 |
Driving Doglegs
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 |
Driving Doglegs
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Driving Doglegs
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! |
Driving Doglegs
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Driving Doglegs
@ 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 |
Driving Doglegs
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 |
Driving Doglegs
Karen, thanks for the info ...
btw. you still owe me some ETec info ;) I havent ruled out that motor yet, no maintenance, light, little fuel use, quiet and lotsa power is just soooo tempting :) ... Would you please email me (real addy posted)? ( I think your posted email here is eithr fake or not checked)... Matt |
Driving Doglegs
Shortwave Sportfishing wrote: On 25 Dec 2005 12:06:41 -0800, wrote: 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 ... If I had to guess, just a guess, I don't think it will deal with set and drift in the sense that it will correct a course for you. What it will do is show you where you are drifting off course, but I don't think it will automatically create a corresponding plot change or set up a corresponding course correction (think of it as aiming a rifle and correcting for a cross wind - inches to the right or left of the target, etc.) from the git go by entering set and drift. I will test this in summer and report back ... My Raymarine RC400 Chartplotter will do it just fine, but then with Navionics Gold chart, it's over $700. The garmin unit with map and memory is even more .. :( Matt |
Driving Doglegs
I'm not a Garmin fan - I've had three or four really unfortunate customer service problems with them with one that was really bad and a rather spectacular example of how not to deal with a customer. After that last one, I have never purchased another Garmin unit again and I never will. I am sorry to hear that :( I have just the opposite experience ... Have my 3rd GPS and also a Garmin Fishfinder .. Their electronics, displays and user friendly menues are the best out there (IMO)... Customer service has been good too ... Just their manuals stink to the point where they are useless .... They have some really tech support guys but with Garmins sales going up over the last years they are now a little overrun with the phone droid tech support .. Like: Me: Hello, .. I have such and such question about this and that function of a 276C ... Phone Driod: *** Deer-Caught-In-The-Headlight-Silence **** ... Have turned the unit on ... ? Me: ... well thanks for your help anyways ... *click* Matt |
Driving Doglegs
IMO, for the gps to compute a compass course to compensate for wind and
current it would have to have some means to acquire the inputs. In theory, it could compute the cross track error and recommend a course to compensate. Perhaps long stretches between waypoints in deep open water would be a situation in which such calculations would be possible. The type of boating in which most of us seem to participate with relatively inexpensive gps it may not be quite so practical. Coastal cruising and river and lake boating in my area is full of changing conditions due to the shallow bottom conditions causing frequent changes in the speed and direction of currents and the surrounding coastal structure which has a similar effect on wind direction. I'm pretty happy with the cross track error output on my gps. After a few corrections on a long course I find it pretty easy to find a steady heading that requires little additional human input. Butch "Gene Kearns" wrote in message ... On 23 Dec 2005 22:07:18 -0800, wrote: 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? Yes, Matt, that is the solution... or part of it, anyway. First note the compass direction that is required to arrive at the destination. Next, proceed on course and watch for the amount of error that accrues. Alter your heading a few degrees (as appropriate) and hold that heading. You may have to make a few more adjustments along the way. Net result is that if your destination was at a GPS course of 000 degrees and you had a wind from 090... then without correction you would end up tracking a path of (something like) 352 degrees... and missing your destination, or having to sail "Direct TO" from the point you figured out you were lost. If you pilot a course of 008, then you should come out at your destination. Bear in mind that your boat is "facing" a heading of 008, but is actually traveling a straight line to 000. -- _ ___c \ _| \_ __\_| oooo \_____ ~~~~|______________/ ~~~~~ ~~~ ~~~~~~ ~~~ ~~~~~ ~~~~~~~~~ ~~~~ }((((o ~~~~~~ }{{{{o ~~~~~~~ Grady-White Gulfstream, out of Southport, NC. http://myworkshop.idleplay.net/ Homepage* http://www.thebayguide.com/rec.boats Rec.boats at Lee Yeaton's Bayguide |
Driving Doglegs
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GPS Issues: was Driving Doglegs
Tom,
Have been comparing Lowrance LMS-337CDF vs Garmin 198C. Results have been interesting, Many swear by Garmin, any Garmin, while a small minority seem to prefer Lowrance. Probably depends on where you started given the reliability of both brands. I currently have a Lowrance LMS-160 purchased in 1999 with which I've been pretty happy overall. I've never had any issues until last Spring when I started getting vertical lines on the display. The unit was remains useful but I needed an excuse to upgrade to a WAAS unit and now I have one. I down loaded the virtual 337 and owner's manual and have been playing with it for about a week. I also compared both units side by side at my local Boater's World store. The display on the Lowrance is superior with a 480X480 display but a map chip must be purchased at around $100 for a Navionics Gold. The Garmin has a complete coastal Blue Map installed but who needs all the extra mapping? Lawrance also has a chip for about $80 (???) which has good coastal coverage for a wide area and even covers our Mobile Bay Delta very well. The Garmin is more expensive but by the time a couple of chips for the Lowrance have been purchased the prices are similar. The Lowrance includes a dual freq transducer with temp and water speed included. The Garmin xducer must be added and if dual freq, temp and speed are included it gets pretty expensive. On balance the Lowrance is a little less expensive and has a better display, IMO. The downloadable virtual 337 is a great teaching device. It also has a couple of Navionics maps included to give the user an idea of the detail possible on the unit. I'm not in love with either owner's manual but the Lowrance manual seems slightly easier to understand. Both companies could take lessons from the military on how to write manuals. I believe I'll be buying the 337 from Lowrance. The display quality and the fine detail of the Navionics Gold maps are the major discriminators but the lower price is also attractive. Butch "K. Smith" wrote in message ... wrote: Snipped! |
Driving Doglegs
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Driving Doglegs
On Mon, 26 Dec 2005 00:33:08 GMT, "Butch Davis"
wrote: IMO, for the gps to compute a compass course to compensate for wind and current it would have to have some means to acquire the inputs. ======================== Exactly right. |
Driving Doglegs
wrote in message 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 ? I've had a Garmin 48 handheld for several years, and that unit will display a "highway" page and tell me the crosstrack error. Its then a simple matter to manually compensate your heading to offset the drift and/or recover the original track, and the display continues to show your position relative to the originally calculated track line. When dealing with these manuals, it is well to remember that their aim is to instruct on the operation of the unit, not to teach navigation. |
Driving Doglegs
Gene Kearns wrote:
On Mon, 26 Dec 2005 12:33:13 -0500, Wayne.B wrote: On Mon, 26 Dec 2005 00:33:08 GMT, "Butch Davis" wrote: IMO, for the gps to compute a compass course to compensate for wind and current it would have to have some means to acquire the inputs. ======================== Exactly right. Anybody know of a GPS with this capability? I'm not sure what you are trying to accomplish, but inputting wind speed and current wouldn't help since both affect different boats in different ways. A smaller, shorter boat, for example, is less affected by wind than a taller, longer boat. The purpose of a GPS is to do more than a compass. It will help you correct for wind and current if you simply use it to get to your waypoint. Dan |
Driving Doglegs
"Dan Krueger" wrote in message ink.net... Gene Kearns wrote: On Mon, 26 Dec 2005 12:33:13 -0500, Wayne.B wrote: On Mon, 26 Dec 2005 00:33:08 GMT, "Butch Davis" wrote: IMO, for the gps to compute a compass course to compensate for wind and current it would have to have some means to acquire the inputs. ======================== Exactly right. Anybody know of a GPS with this capability? I'm not sure what you are trying to accomplish, but inputting wind speed and current wouldn't help since both affect different boats in different ways. A smaller, shorter boat, for example, is less affected by wind than a taller, longer boat. The purpose of a GPS is to do more than a compass. It will help you correct for wind and current if you simply use it to get to your waypoint. Dan I guess I am missing something here. Assuming the discussion is about a decent chartplotter type GPS, why all the questions about wind and drift? As I recall using the Raymarine system on my Navigator, I would transfer my planned track or route from the paper charts to the chartplotter in the morning before getting underway. The route would consist of several waypoints, usually placed in the area of a navigation buoy on the paper charts. I'd then just follow the tracks from waypoint to waypoint, either by manually steering the boat or by autopilot. If the boat marker started drifting away from the track due to wind or current, I'd see it and correct for it. The autopilot can be used in two modes. One simply steers to a course heading with no correction for wind or drift. The "Navigate" mode keeps the boat on the intended waypoint track, automatically correcting for wind or drift and automatically steering the boat to a new course as required by the waypoint settings. Worked fine for 1500 miles to Florida. So what am I missing? Eisboch |
Driving Doglegs
On Mon, 26 Dec 2005 14:00:59 +1100, "K. Smith" wrote:
there's a further assumption that the quantum AND direction of off track forces will remain constant throughout the leg; both rarely happen. ================================= Not true. If you monitor COG vs bearing to mark and keep them equal, the "off track forces" can vary all they want, same thing if you are monitoring (and minimizing) XTE. The old fashioned way was to estimate current set over the entire leg, plot a graphical solution, and steer a constant offset. At the end of the leg you got to find out the accuracy of your estimate and calculations. With GPS you get constant feedback. |
Driving Doglegs
On Tue, 27 Dec 2005 00:11:51 GMT, Gene Kearns
wrote: Anybody know of a GPS with this capability? ===================================== Some of the better sailboat systems like Ockam or B&G offer direct digital readout of current direction and speed. They are calculated numbers based on observing SOG vs speed through the water, and COG vs magnetic compass heading. The instrument system resolves all the vectors and spits out the numbers. It is fairly slick but requires constant calibration and cleaning of the speed sensors which are mechanical flow transducers in through hull fittings. My Furuno Navnet, integrated chart plotter/GPS/RADAR, displays a heading vector and a separate COG vector on the RADAR screen. It has an electronic compass sensor to provide heading information. Since it knows your heading, it can also overlay chart data with the RADAR data which can sometimes be useful. It needs heading information to rotate the chart into the proper orientation. I find this function particularly useful for quickly determining whether a RADAR target is a NAVAID such as a buoy, or another boat. |
Driving Doglegs
On Mon, 26 Dec 2005 19:55:05 -0500, "Eisboch" wrote:
Worked fine for 1500 miles to Florida. So what am I missing? Nothing, that's the way most folks do it. The GPS displays COG (Course Over Ground) which is the combined effect of Boat Speed through the water, Heading, Current Speed and Current Direction, plus any set due to a crosswind. It is impossible to separate the various components without having a very sophisticated instrumentation system such as the ones that many racing sailboats carry. If you set both a "From" waypoint (sometimes done automatically), and a "To" waypoint (destination), then the GPS can also calculate XTE (CrossTrack Error). Most GPS units today can direct the autopilot to minimize XTE and thus maintain a straight course regardless of current or wind. |
Driving Doglegs
"Wayne.B" wrote in message ... On Tue, 27 Dec 2005 00:11:51 GMT, Gene Kearns wrote: Anybody know of a GPS with this capability? ===================================== Some of the better sailboat systems like Ockam or B&G offer direct digital readout of current direction and speed. They are calculated numbers based on observing SOG vs speed through the water, and COG vs magnetic compass heading. The instrument system resolves all the vectors and spits out the numbers. It is fairly slick but requires constant calibration and cleaning of the speed sensors which are mechanical flow transducers in through hull fittings. My Furuno Navnet, integrated chart plotter/GPS/RADAR, displays a heading vector and a separate COG vector on the RADAR screen. It has an electronic compass sensor to provide heading information. Since it knows your heading, it can also overlay chart data with the RADAR data which can sometimes be useful. It needs heading information to rotate the chart into the proper orientation. I find this function particularly useful for quickly determining whether a RADAR target is a NAVAID such as a buoy, or another boat. I had very mysterious readings for a while until it was discovered that the electronic compass (I think it's called a flux gate compass) had been mounted in the engine room about 6 inches away from the electric motor for a washing machine. If I deviated more than 10 degrees off course, the boat marker would swing around so it appeared we were doing 19 knots in reverse. Another neat feature of the overlays (haven't tried it yet though) is that you can set up the Raymarine system to automatically command the autopilot to change course if the radar shows something big and solid (like another boat) on a collision course and within a programmed range. I am sure the Furuno system has something similar. I briefly had an Egg Harbor with the Furuno setup and never really learned the system. I find the Raymarine to be much more intuitive. Eisboch |
Driving Doglegs
the original question was that if a computer (GPS) could work its magic
and do a no brains naviagtion by telling you where to steer to go from point A to B WITH cross wind/current. WITH NO MANUAL INPUT OR READING THIS PARAMETER OR THAT ERROR Simply (and only) by giving you degree where you need to point your bow. Matt |
Driving Doglegs
wrote in message oups.com... the original question was that if a computer (GPS) could work its magic and do a no brains naviagtion by telling you where to steer to go from point A to B WITH cross wind/current. WITH NO MANUAL INPUT OR READING THIS PARAMETER OR THAT ERROR Simply (and only) by giving you degree where you need to point your bow. Matt My answer: Yes, *BUT* it may take you a hell of a long time to get there. Eisboch |
Driving Doglegs
On Mon, 26 Dec 2005 22:07:16 -0500, "Eisboch" wrote:
the original question was that if a computer (GPS) could work its magic and do a no brains naviagtion by telling you where to steer to go from point A to B WITH cross wind/current. WITH NO MANUAL INPUT OR READING THIS PARAMETER OR THAT ERROR Simply (and only) by giving you degree where you need to point your bow. =========================== That's not a GPS, it's called a navigation computer or calculator. It will still require manual inputs, and they will be estimates at best. The question as stated is of theoretical interest only since the constraints imposed are quite artificial in the real world. In the real world your GPS will in fact tell you where to point your bow but it is a feedback mechanism, not a one shot calculation. Even NASA makes mid course corrections based on observed vs calculated. |
Driving Doglegs
On Mon, 26 Dec 2005 20:57:02 -0500, "Eisboch" wrote:
I had very mysterious readings for a while until it was discovered that the electronic compass (I think it's called a flux gate compass) had been mounted in the engine room about 6 inches away from the electric motor for a washing machine. ========================== The flux gate sensor should be mounted as far away from iron, steel or electrical wiring as possible. I ended up putting mine in a locker where I store only life jackets. I think the Furuno user interface may have improved in recent years. I had very little difficulty teaching Mrs B how to use the target tracking functions and she is still struggling with computers and the internet. Target tracking with the ARPA functions is way cool, everybody loves it. |
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