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Old 09-17-2016, 07:50 AM   #41
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Originally Posted by twistedtree View Post
Yes, tons of confusion here, partly because of terminology, and partly due to misunderstanding of various pieces of equipment. Here's some of the confusion I'm reading:

- A rate compass and a flux gate compass are magnetic heading devices, though solid state. They report magnetic heading (HDG), and sometimes rate of turn (ROT). Some even report pitch and roll.

- A GPS can not report meaningful COG when there is no movement, or little movement.

- A heading sensor can report HDG regardless of boat movement.

Sorry for being pedantic here. I know lots of people understand all this, but there is also clearly some misunderstanding about what the various devices do and what they enable.
twisted... Well said

- many folks refer to their chart plotter / MFD as the GPS when in fact they are very different. They have the ability to display lots of info from many sources / sensors including depth & engine data but GPS's can't report depth or engine data either.
Unless/until you add sensors a chart plotter w/ built in GPS provides basic GPS info only COG, SOG, CMG, etc

From the Garmin site...
Heading sensor recommendation for use with Garmin chartplotters
Garmin recommends that customers use an Marine Heading Sensor with their Garmin Marine Chartplotter.
This heading sensor can be connected to a chartplotter using either NMEA 0183 or NMEA 2000.
By adding a heading sensor, the chartplotter will have the ability to provide a true heading at all times. Without a heading sensor, the chartplotter will rely on a GPS heading which will only be accurate when the boat is moving. With GPS heading only and while the boat is not moving, the chartplotter will be unable to determine its true heading.
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Old 09-17-2016, 08:07 AM   #42
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You have a compass input and most GPS maps don't. You only get heading from a compass... nothing else. I'd bet that 99.9% of the GPS maps out there do NOT have heading input. It does not come with the unit.

I have a Garmin 4208 and 740s.. neither have heading.
This is a very confusing statement, and I think illustrates why we are talking across each other.

When you refer to a "GPS map", I think you mean a chart plotter. Your Garmin 4208 is such a chart plotter. It accepts a variety of sensor inputs, and uses them to display a chart and your position/progress across that chart.

One of the inputs to the chart plotter is indeed a GPS. This device listens to the sats, computes all the necessary data, and broadcasts it over either NMEA 0183 or NMEA 2000 for anyone who wants to listen. One of the devices listening is you chart plotter. The Garmin 4208 utilizes the Garmin GPS 19x for this purpose. For some plotters, there is a GPS receiver built into the chart plotter rather than being an external device. But even with an internal GPS, all such plotters can also accept data from an external GPS.

You say "You only get heading from a compass... nothing else. I'd bet that 99.9% of the GPS maps out there do NOT have heading input".
Yes, you only get heading (HDG) from a compass. That is correct. But I'll go out on a limb and say that 100% of the chart plotters DO accept a heading input. This comes in over the NMEA 0183 or NMEA 2000 port, and is utilized by the chart plotter in a variety of ways. A heading sensor is not included when you buy a chart plotter, but all chart plotters accept it as a usable and useful input.

Your Garmin 4208 does support heading - you just need to feed it the data.

I think maybe you are using the term "input" differently from the rest of us?
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Old 09-17-2016, 08:17 AM   #43
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Ditto on that great post twisted, thanks. I would slightly disagree, semantically, on your heading sensor comment to the extent that I feel 99% of us do need a compass of some sort to provide heading information, just not necessarily a networked "heading sensor" device.

Getting back to the OP, I forgot to chip in that I think the KVH Azimuth is a really fine piece of kit; a friend had one on his Hatteras 42LRC we cruised the Maritimes on 10 years ago. As long as I had some sort of hand held compass, be it in my binoculars (my favorite) or otherwise, I could see replacing one of the card compasses on board with one.

One instance where I found an old fashion compass handy was when we were at anchor. I had an antique box compass a freind gave me as a gift by my bedside. I had my position info on a Furuno repeater next to the bed, and sometimes a chart plotter on my laptop, but the compass was the quickest cheapest best way to understand the boat had clocked around, and which direction it was currently laying to. A dramatic example of how heading and COG can vary greatly...
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Old 09-17-2016, 08:22 AM   #44
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Magnetic vs Electronic Compass

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Originally Posted by caltexflanc View Post
So your actual heading in that case is coming from the magnetic field driven rate compass, NOT GPS!





Now you are talking about $atellite compa$$/heading $ensor$, not the GPS plotter virtually all cruising boats have. Important to make the distinction to newbies.

That's why I gave the description because so many have bantered about all the different types, and it seems to be confused about which is which and how they use the data.

To the newbie: To gain a clearer picture read your installation manual that came with the GPS and plotter you have. Read up on NEMA output sentences and see which ones you can use in other electronics. Understand the difference between HDG and COG. On some the COG is listed as COB. Heading, course over ground and course over bottom.
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Old 09-17-2016, 08:30 AM   #45
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Originally Posted by caltexflanc View Post
Ditto on that great post twisted, thanks. I would slightly disagree, semantically, on your heading sensor comment to the extent that I feel 99% of us do need a compass of some sort to provide heading information, just not necessarily a networked "heading sensor" device.
I didn't mean to make any judgement about what anyone should pick for their boat. We've seen that it ranges all over the place, and that's fine. My goal was just to clarify how the stuff actually works and what you get/don't get so people can be better informed when choosing.

Personally, I agree with you. I've got a sat compass, a rate compass as a back up, and a traditional magnetic compass, so you know where I land on the spectrum.
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Old 09-17-2016, 08:39 AM   #46
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I just started a poll over here to see how our boats are equipped WRT compasses.

My guess is that 80% or more of our boats have some sort of magnetic compass.
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Old 09-17-2016, 09:14 AM   #47
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Great writeup. Thanks.

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Originally Posted by twistedtree View Post
- A rate compass and a flux gate compass are magnetic heading devices, though solid state. They report magnetic heading (HDG), and sometimes rate of turn (ROT). Some even report pitch and roll.
Silly question: What is a rate compass? Is that a fluxgate with some form of integrated AHRS?
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Old 09-17-2016, 10:12 AM   #48
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Great writeup. Thanks.

Silly question: What is a rate compass? Is that a fluxgate with some form of integrated AHRS?
Not exactly. Here is an excerpt from the Simrad Autopilot Compass Manual.

"The Simrad Rate Compass is in principle a combination of a solid state rate sensor and a fluxgate sensor, where the rate sensor is slaved to the fluxgate sensor.

The rate sensor will momentarily pick up any angular changes in azimuth, but is almost unaffected by roll and pitch.

As the “rate heading” will drift, the fluxgate sensor is needed to maintain the (magnetic) heading. However, as the rate sensor is the primary “heading” source, the fluxgate signal can be more dampened and thus it will not contribute to the instability caused by heavy rolling and pitching.

The Rate Compass gives a more stable and precise autopilot steering on any type of boat. It also eliminates the effect of the distorted horizontal magnetic field at high latitudes and the well known s-ing phenomena when on autopilot steering.

It can also be used as “stand alone” compass to provide a stable heading input to a compass repeater, radar or other equipment."
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Old 09-17-2016, 10:21 AM   #49
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Have two traditional compass units, autopilot with compass, and a chart plotter. Getting any electronics like a cell phone close by a compass causes an issue. Once accidentally left a magnetic screwdriver near the helm which caused a problem with a traditional compass. GPS is reliable and undeterred by magnetic interference - at least anything I can generate.
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Old 09-17-2016, 10:45 AM   #50
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I just started a poll over here to see how our boats are equipped WRT compasses.

My guess is that 80% or more of our boats have some sort of magnetic compass.
Twisted,
Anxious to see your pole, but I'd bet the 100% have a magnetic compass.
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Old 09-17-2016, 10:47 AM   #51
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This is a very confusing statement, and I think illustrates why we are talking across each other.

When you refer to a "GPS map", I think you mean a chart plotter. Your Garmin 4208 is such a chart plotter. It accepts a variety of sensor inputs, and uses them to display a chart and your position/progress across that chart.

One of the inputs to the chart plotter is indeed a GPS. This device listens to the sats, computes all the necessary data, and broadcasts it over either NMEA 0183 or NMEA 2000 for anyone who wants to listen. One of the devices listening is you chart plotter. The Garmin 4208 utilizes the Garmin GPS 19x for this purpose. For some plotters, there is a GPS receiver built into the chart plotter rather than being an external device. But even with an internal GPS, all such plotters can also accept data from an external GPS.

You say "You only get heading from a compass... nothing else. I'd bet that 99.9% of the GPS maps out there do NOT have heading input".
Yes, you only get heading (HDG) from a compass. That is correct. But I'll go out on a limb and say that 100% of the chart plotters DO accept a heading input. This comes in over the NMEA 0183 or NMEA 2000 port, and is utilized by the chart plotter in a variety of ways. A heading sensor is not included when you buy a chart plotter, but all chart plotters accept it as a usable and useful input.

Your Garmin 4208 does support heading - you just need to feed it the data.

I think maybe you are using the term "input" differently from the rest of us?
Twistedtree,

Good point and I apologize for not being clearer. In the marine world I should be saying "chart plotter", which, when said, most folks believe it's a form of a gps mapping display for their boat, and most have options of additional inputs for more information. I'd bet the bulk of us use them for charting and depth.

I do have my 4208 connected to a NMEA backbone. Yes, mine will accept heading, but today I have no use for it. Perhaps if it makes sense for an additional piece of equipment, like MARPA on a radar, I'll get one.

And just for correction.... magnetic north is not the only place we can get heading from. The Satellite compass triangulated from 2 or more GPS receivers and compares the time delay to give us true heading input. That has it's advantages for operation at high latitudes or world travel, but most of us don't need or use true heading.

Now on a Trawler forum, I'll stand corrected as perhaps there's a lot more folks that have more sophisticated electronics than the average recreational boater, excepting the die hard fishermen.
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Old 09-17-2016, 10:55 AM   #52
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So your actual heading in that case is coming from the magnetic field driven rate compass, NOT GPS!

Now you are talking about $atellite compa$$/heading $ensor$, not the GPS plotter virtually all cruising boats have. Important to make the distinction to newbies.
Absolutely right. In my case, the 'Heading' information is coming from the Rate Compass and the 'GPS Heading' is what is being calculated by the GPS.

If you will refer back to the picture in post #27, the 'GPS Heading' is 048M while the 'Heading' is 054M. This shot was taken during a right turn where the calculated 'GPS Heading' was lagging behind the actual 'Heading'.

Because the Rate Compass is energized whenever the autopilot is powered on, in standby, it serves as an excellent steering compass, even when the GPS is turned off. The autopilot displays this 'Heading' (and rudder) information on the screen in a very readable format.

Finally, as mentioned elsewhere, this information is also shared with, and can usually be displayed on any other devices connected to the network.

BTW, I do have a standard magnetic compass, but I find this easier to read, especially when it gets lumpy and things start bouncing around.

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Old 09-17-2016, 11:17 AM   #53
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I don’t know guys, but I suppose the rest of you have much more reliable electronics than I have had, or perhaps just better luck with Mr Murphy:

• Seas were about 2 stories when the oil pressure gauge developed a small leak;
• Engine oil then traveling down the copper tubing and dripped directly onto the autopilot relay on my Wood Freeman, rendering it inoperable;
• Of course that didn’t matter because shortly thereafter the seas grew to 3 stories and there’s no way the AP could have steered in those;
• A year later the old flasher stopped working;
• 6 months later the speed log stopped working. Wasn’t algae or seaweed in the spinning turbine wheel. Never figured it out;
• Learned my lesson about using “old fashioned” technology, so on another boat I upgraded to the latest and greatest Lowrence electronics;
• My two year old sounder/plotter/GPS stopped communicating with external circuits. The DSC radio no longer received position input. Neither Lowrence nor Finland were able to fix. Warranty was expired therefore the solution = $$$;
• Learned my lesson about using integrated electronics!
• Learned my lesson about relying on electronics 100%. The whiskey compass will always be my primary, plus I find it hard to steer by digital numbers anyway.

So was it just my bad luck, or is it more likely that sh*t happens and usually in the worst sea conditions?????
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Old 09-17-2016, 11:29 AM   #54
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Just for clarification....

Heading: (where we’re pointed)
This is where my nose points - and seeing as my nose is attached to my head, this is where my head (and thus my machine) is pointing relative to North. It is usually expressed as magnetic heading, but could be true heading. We get heading from a compass, not from GPS.
Course: (future)
This is my INTENDED path of travel that I have calculated taking into consideration winds, variation, current and such. We can obviously plot this on the chart plotter in advance to get us where we are going, avoiding obstacles and shallow water. This is the path we want to get to a destention.
Track: (history)
This is my ACTUAL path traveled over ground - just like a set of tracks I would leave behind in the snow or sand, relative to magnetic North. This is what the GPS on the chart plotter tells us we have done… it’s history. However, is nothing changed it will project the track ahead of us giving us the tract that will be made good. We get track from a GPS, not a compass or map.

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Old 09-17-2016, 11:31 AM   #55
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My guess is that 80% or more of our boats have some sort of magnetic compass.
At present, mine is magnetic but I'm scrapping that for the KVH 1000. (as previously mentioned)
The Fluxgate is in the body of the compass. I can't begin to put in words just what a great product this is! It's been mentioned by several people that they never use their "whiskey compass" anymore. With the KVH, you'll be using it all the time....even with your chat plotter!


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Old 09-17-2016, 11:44 AM   #56
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At present, mine is magnetic but I'm scrapping that for the KVH 1000. (as previously mentioned)
The Fluxgate is in the body of the compass. I can't begin to put in words just what a great product this is! It's been mentioned by several people that they never use their "whiskey compass" anymore. With the KVH, you'll be using it all the time....even with your chat plotter!


http://www.jamestowndistributors.com...540042%20F.pdf
Codger,

You keep making posts like this I'm liable to get one.....

I do worry sometimes when I get out of the bunk in the morning and find the sun has come up on the wrong side.... oh, the boat moved... that would solve that.
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Old 09-17-2016, 11:50 AM   #57
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Twistedtree,

Good point and I apologize for not being clearer. In the marine world I should be saying "chart plotter", which, when said, most folks believe it's a form of a gps mapping display for their boat, and most have options of additional inputs for more information. I'd bet the bulk of us use them for charting and depth.

I do have my 4208 connected to a NMEA backbone. Yes, mine will accept heading, but today I have no use for it. Perhaps if it makes sense for an additional piece of equipment, like MARPA on a radar, I'll get one.

And just for correction.... magnetic north is not the only place we can get heading from. The Satellite compass triangulated from 2 or more GPS receivers and compares the time delay to give us true heading input. That has it's advantages for operation at high latitudes or world travel, but most of us don't need or use true heading.

Now on a Trawler forum, I'll stand corrected as perhaps there's a lot more folks that have more sophisticated electronics than the average recreational boater, excepting the die hard fishermen.
Any decent navigator, human or electronic can quickly change true to magnetic and visa versa....

No big deal....TVMDC
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Old 09-17-2016, 12:08 PM   #58
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That is rich, coming from someone who thinks most people have no clue. When you are in a hole, stop digging. Satellite compasses do, in fact, provide heading information (very accurately), even when stationary. And all the major chart plotter brands accept heading information -- it comes in as NMEA data.

-- Not to be confrontational
Most people don't, just read the posts....
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Old 09-17-2016, 12:14 PM   #59
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So far..just parrying jabs....no blows landed....
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Old 09-17-2016, 12:21 PM   #60
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Twisted,

Anxious to see your pole, but I'd bet the 100% have a magnetic compass.

You're right, I miss-spoke. I meant some sort of electronic compass.
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