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Old 17-06-2015, 07:29   #1
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Garmin Handheld devices - GPS vs. Electronic Compass

So which is better for sailing?

It is my understanding that GPS only works while moving so the electronic compass is more accurate at slower speeds or when standing still (a drifter), but the electronic compass uses more battery power than the GPS function does.

When setting up the Compass function on the unit, what threshold speed do you set for the unit to switch from electronic compass to GPS and after how many minutes of slower than speed?

What is the battery life for GPS vs. Electronic Compass?

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Old 17-06-2015, 07:34   #2
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

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Originally Posted by OldFrog75 View Post
So which is better for sailing?

It is my understanding that GPS only works while moving so the compass is more accurate at slower speeds or when standing still (a drifter), but the compass uses more battery power than the GPS function does.

When setting up the Compass function on the unit, what threshold speed do you set for the unit to switch from Compass to GPS and after how many minutes of slower than speed?

What is the battery life for GPS vs. Electronic Compass?
I'd opt for a binnacle or bulkhead mounted magnetic compass and gps.

Best choice if you ever lose power and have a battery operated GPS and spare batteries.
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Old 17-06-2015, 07:56   #3
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

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I'd opt for a binnacle or bulkhead mounted magnetic compass and gps.

Best choice if you ever lose power and have a battery operated GPS and spare batteries.
Thanks for the suggestion but in this case I'm trying to optimize my handheld GPS unit.
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Old 17-06-2015, 12:09   #4
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

OldFrog,
I'm sure you know this, but just for clarification....a compass tells you what direction something is, or in this case what direction you/your vessel is headed....


If you desire instantaneous electronic heading info, not much beats a new satellite compass....(in the old days gyro compasses ruled)....

If all you need is the normal "fast heading info" (such as when using ARPA or MARPA), then any decent fast heading sensor / fluxgate will work nicely.... (something with a 10hz update rate)

If you primarily desire just COG, then the GPS is fine....


It all depends on what YOU desire...
BUT...



But, to answer your question directly...
Quote:
Originally Posted by OldFrog75 View Post
So which is better for sailing?
Nothing beats a hi-quality, and deviated, steering compass....(I have two Ritche 4.5" Naviagtor's)






Not sure I understand what your exact application is....but the above should help.......


Fair winds...

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Old 17-06-2015, 12:34   #5
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

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OldFrog,

Not sure I understand what your exact application is....but the above should help.......


Fair winds...

John
Honestly, not being very tech savvy it's possible I don't understand myself.

However, it is my understanding that compass headings and bearings on my handheld GPS are more accurate when the readings are taken by the electronic compass than they are when taken with GPS unless you are moving at a pretty good clip because the GPS interpolates your heading based upon movement whereas the electronic compass shows exactly which way you are heading even if standing still.

I can set the Compass function to switch from Electronic to GPS when it rises above a certain threshold speed. Could be anything from 0.1 kts to 99 kts.

Since boats don't go very fast it might not make a difference but I asked the question hoping someone would be able to suggest the 'best' threshold speed for sailing, not only for maintaining accuracy but also within the constraints of battery consumption.

If the difference in accuracy between settings of 3 kts and 10 kts is insignificant but 10 kts burns batteries twice as fast then I set it low and forget about it. But if the difference in accuracy is significant then it might be worth it to burn through batteries faster.
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Old 17-06-2015, 12:45   #6
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

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.whereas the electronic compass shows exactly which way you are heading even if standing still.
No, it doesn't tell you "which way you are heading"...

It will tell you which way N is, all the time, moving or standing still.

That's a definition of a compass.

What GPS do you have?

I must admit, after 35 years in boating, and with the advent of Loran C and now GPS, I have never heard this question. Ever.

Should be interesting.
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Old 17-06-2015, 12:54   #7
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

Heading : where the boat is pointed

Course : where the boat is going

A compass, by pointing where magnetic north is, can be used to determine your heading.

A GPS can never know your heading. It can only calculate your PAST course.

A GPS is therefore a useful tool for determining your course, while a compass is the best tool for determining your heading.

The two sets of data are separate, distinct, and useful in their own rights. At this point it should be clear how meaningless your original question is. Like asking what's most important to a pilot, altitude or speed?
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Old 17-06-2015, 21:05   #8
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

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Heading : where the boat is pointed

Course : where the boat is going

A compass, by pointing where magnetic north is, can be used to determine your heading.

A GPS can never know your heading. It can only calculate your PAST course.

A GPS is therefore a useful tool for determining your course, while a compass is the best tool for determining your heading.

The two sets of data are separate, distinct, and useful in their own rights. At this point it should be clear how meaningless your original question is. Like asking what's most important to a pilot, altitude or speed?
Agreed and well put.

The "irksome" part comes in when the GPS manual says "Heading" in one of the menu boxes, which is NOT necessarily what the COMPASS on the boat is saying, which is the true boat heading - the compass.

GPS heading is not always boat heading. Same term/word used, but understand the differences.
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Old 17-06-2015, 21:31   #9
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

As others have put in several ways, your handheld Garmin unit will provide you with two quite different sets of data.

1. a compass.
2. past/current course

Having it switch automatically between these some predetermine speed would annoy me tremendously so I would probably manually switch it if/when I so desired.

FWIW, here is another explanation of the difference between the two :

Compass heading is the angle between where the bow of you boat is facing and where North is. It has no relation to the where you have been or where you are going, It only changes whenever you bow points towards a new direction. Of course, knowing your compass heading can be very helpful in determining where you are going but it doesn't give you that information directly.

Course is the direction of the progress of the boat referenced to North. If the boat isn't moving, you don't have any course. Course may or may not be the same as the compass heading.
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Old 18-06-2015, 00:45   #10
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

I appreciate the comments but the question must have some relevance since the unit has the ability to be changed from Electronic Compass to GPS at a user entered speed.

The original default speed is 10 kts or 10 mph. Above that speed the electronic compass switches off and the GPS takes over computing heading, bearing, course, degrees to a waypoint, etc.

I currently have it set at 3 kts. Up to 3 kts the compass, bearing, heading etc are determined by the electronic compass. Above 3 kts the electronic compass turns off and the GPS takes over.

Since boats frequently don't sail above 10 kts the default setting would more or less mean the unit is always using the electronic compass rather than GPS for determining all relevant directions.

I was trying to find out if there is an optimum setting for sailing since the electronic compass uses more battery power than the GPS.

Again, this is my understanding of how it works but I'm happy to be educated. Like I said, it must be relevant otherwise the unit would not have the ability for the user to change the threshold speed that turns the electronic compass off.
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Old 18-06-2015, 01:40   #11
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

Highlighting the Compass tab in the Setup menu there is a field labeled, "Heading Data Source. Use GPS if speed is above (blank)."

The next field is labeled, "Use Compass if below (blank) speed for at least (blank) seconds."

At the bottom of the page it says, "Set the speed above which to use GPS for heading. The compass will use no battery power while GPS is being used for heading."

"Set the time to wait before using the compass for heading if the compass is turned on..."

FWIW, I do know the definitions of course, heading, bearing, track, etc.

I think I'll just leave the compass "on" and set the threshold speed at 10 kts. That way it will pretty much always be engaged. I'll just have to buy batteries more often.

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Old 18-06-2015, 03:21   #12
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

In a handheld device, the compass tells you the heading of the device (not the boat), while the GPS tells you the direction the device is moving. So unless the device is aligned exactly with the direction of motion, the two are not the same. On a boat, unless you have the handheld mounted in some sort of fixture, it's unlikely to be exactly aligned while you're holding it in your hand.

Generally, a solid state compass uses very little power compared to a GPS receiver, but for boat navigation I would use the GPS course and a hard mounted compass as others have recommended. I think the handheld compass would be useful to take bearings on fixed features to get a position fix the old fashioned way. But that seems redundant since you can get your position fix from the GPS.
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Old 18-06-2015, 09:27   #13
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

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Originally Posted by ka4wja View Post


If you desire instantaneous electronic heading info, not much beats a new satellite compass....(in the old days gyro compasses ruled)....

If you primarily desire just COG, then the GPS is fine....


It all depends on what YOU desire...

But, to answer your question directly... Nothing beats a hi-quality, and deviated, steering compass....

Not sure I understand what your exact application is....but the above should help.......


Fair winds...

John
Thanks, John

This thread didn't go exactly the way I had intended but maybe it's because of the way I worded the question or maybe it's because other handheld users a) have a device that doesn't have these functions or b) have the functions but never used them or were generally unaware they existed.

Regarding needs/desires maybe I should have been more specific but I didn't think the question needed the detail - perhaps I was wrong:

I am on a racing boat. There is no binnacle. There is no deviated steering compass - high quality or otherwise. All information is retrieved electronically in some fashion or by eyes on the water.

What I want/need - Heading, Bearing to the next Waypoint, Degrees to next Waypoint, SOG, Distance to next Waypoint, VMG, etc.

I was trying to find out the optimum way to set my unit to get the Heading, which I assume in turn gives me Bearing and Degrees to the next Waypoint.

When I asked fellow crew members how they had their units set up no one knew what I was talking about. Seems most people have always just turned the unit on and trusted the information given without reading the fine print in their owner's manuals. I confess, I did the same thing but I recently had to update my software version which required a full system reboot and installation and it was during that process I discovered this "feature" and had no clue what to set it on. I called the Garmin Help Line and they explained how the feature works but the representative I talked to had no idea what the optimum setting for sailboats should be, hence I came to CF to ask the experts.

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Old 18-06-2015, 10:42   #14
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

Sailboats are always moving...so use a gps.

You are required (in most countries) to have a magnetic compass as part of your safety gear. Most production boats have a compass built in.

A garmin etrex will run 24 hours on a single pair of AA batteries. But you don't have to run the gps all the time. I crossed the atlantic with a garmin etrex, and only changed the batteries once.

I recommend a good garmin handheld gps on a ram mount with 12v power cable.
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Old 18-06-2015, 11:39   #15
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Re: Garmin Handheld devices - GPS vs. Electronic Compass

Oldfrog,
Thanks for the additional information....it explains a lot...AND it also allows us to actually answer your questions!!

The short answer:
Use the compass!!



The long answer:

Please forgive any bluntness on my part, I mean no offense...
BUT...

But, it seems that you are not as familiar with navigation, GPS or otherwise, as you mentioned earlier (where you wrote: "FWIW, I do know the definitions of course, heading, bearing, track, etc."), as you are assuming that your heading has anything to do with a bearing....it does NOT....

And, I think this misconception is the primary cause of the confusion here!


As everyone has already written, your "heading" is merely where the boat is pointed....and has NO relationship at all to bearing, etc...

Bearing (whether to a "waypoint" or to anything) is based on WHERE you are and WHERE the other thing is....and this has NO relation to your "heading" at all!


Not having a compass on-board makes steering a boat on a course pretty impossible, unless you can "see the mark" and drive the boat to this visual mark....but actually following a course to a "mark", "waypoint", etc., usually requires a compass...
So, I understand your desire to have a compass!!!

But, understand that with a handheld GPS, I believe the "compass" is its internal fluxgate compass (good), which might not be too accurate due to the deviations caused by its surroundings....
And, while we that use fixed fluxgates / heading sensors, etc. for our autopilots / radars, etc. usually only need to "swing" our compasses once, you may end up needing to do this everytime you go out....(and I'm not sure that you want to spend the time to do so).....you may NOT need to do this but just once....but I'm not on your boat, nor have I used a handheld GPS' compass....so, I'm just making sure you understand what might be needed...

Assuming your GPS's compass is accurate enough without swinging thru / auto-deviating (or that you do a swing / auto-deviate, each time), then it should be good for you to use in order to know where you boat is pointing....




Btw, my all-time favorite feature of modern navigation (GPS) is the VMG calculations/display!!!
(in the old days, we had to do trig problems, etc. to calculate our VMG's!!!)
And, for racing...well you should be watching VMG all the time (upwind, at least!)....


--- So, your GPS will give you a "bearing" (usually abbreviated as BTW) and "distance" (DTW), to your waypoint....which it constantly calculates, and constantly updates, using the waypoint's location versus your boats location)

--- It will also give you your VMG (Velocity Made Good), which is simply how fast you are actually going TOWARD your waypoint....and is also constantly updated....

--- And, of course the GPS also calculates Speed Over Ground (SOG), but for racing (or even for any upwind sailing), this is a fairly meaningless number....it is VMG that matters!!!




--- Your "heading" is the direction your boat is pointed....and is usually NOT "calculated" but simply "measured", using a compass....

--- If you can maintain sufficient speed, AND maintain a fairly constant heading (good helmsman!), then a GPS can "calculate" your heading (using GPS signals), based on where you HAVE been....



As I answered above in the "short answer", you will use the "compass", not the "GPS" for heading!!


Quote:
Originally Posted by OldFrog75 View Post
I am on a racing boat. There is no binnacle. There is no deviated steering compass - high quality or otherwise. All information is retrieved electronically in some fashion or by eyes on the water.

What I want/need - Heading, Bearing to the next Waypoint, Degrees to next Waypoint, SOG, Distance to next Waypoint, VMG, etc.

I was trying to find out the optimum way to set my unit to get the Heading, which I assume in turn gives me Bearing and Degrees to the next Waypoint.
NO....
This is NOT the way it (your GPS) works...

When I asked fellow crew members how they had their units set up no one knew what I was talking about. Seems most people have always just turned the unit on and trusted the information given without reading the fine print in their owner's manuals. I confess, I did the same thing but I recently had to update my software version which required a full system reboot and installation and it was during that process I discovered this "feature" and had no clue what to set it on. I called the Garmin Help Line and they explained how the feature works but the representative I talked to had no idea what the optimum setting for sailboats should be, hence I came to CF to ask the experts.



I do hope this helps....(and I do also hope you see why it is always good to provide as much info as you can....especially when posting something on-line....remember we're not teenagers "texting" each other 100's of times each day....and I'm not a mind reader!)

Fair winds..

John
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