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Old 15-03-2011, 09:48   #46
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Re: Deviation Table from GPS

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Originally Posted by goboatingnow View Post
lack of accuracy doesnt mean the display is jumping around, ( thats more correctly called jitter). Its just that the steady CoG figure you are seeing ( steady as its heavily damped), is not the correct CoG, ie the boat isnt going where the Cog says it is.
Dave
Dave, I think you edited this post a little from the first version that was in the alert email - "ie the boat isnt pointing where the Cog says it is."
- - I think the use of "pointing" is more appropriate as that is what the magnetic compass is actually showing.
- - However, where the boat is "pointing" and where the GPS COG is saying you are going in relation to the mud/bottom underneath your boat will not agree - unless - there is no wind/leeway, no current/set & drift, and no propeller "walk" if you are motoring.
- - Getting all these factors "zeroed out" is not easy so even disregarding GPS system errors, what you see on the magnetic compass and where the boat is actually going in relation to the physical Earth "COG" are rarely the same. That is why I advocate "locking" the boat in a static location/heading with anchors, dock-lines, or whatever has the best chance of getting a reasonably accurate deviation measurement.
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Old 15-03-2011, 10:42   #47
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Re: Deviation Table from GPS

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Originally Posted by David M View Post
Just do it the old fashioned way. It's easy and one more navigation merit badge that you can add to your sash that the purely GPS cub scouts will never have the chance to earn.
Right on,David M!
All these posts should inform the OP that using the gps is a lot more difficult than just swinging the boat per the simple directions included with a cheap compass.Use a handbearing compass held up high away from any ferrous or electrics. etc. By the way,watch out for untwisted dc wiring near the compass.
"Steering" is altogether different than GPS.
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Old 15-03-2011, 10:57   #48
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Re: Deviation Table from GPS

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Originally Posted by goboatingnow View Post
Furuno quote CoG accuracy on their GPS navigators as +-3% under 17 knots and +- 1% for over 17knots, thats upto 6 degree error on a sailboat!!

Dave
Or what the average helsman steers inthe average chop...yet every time I put a buoy in as a waypoint and run on autopilot...if I don't manually steer around..I'd hit many of buoys...

So all the complaints of using GPS to correct a compass seem pretty rediculous to me...I'd like to see anyone hand steer a compass course and hit a buoy from 1000 miles away...any yes I know how the GPS does it...so unfair??? I think not...I think too many are trying to cure a headache with brain surgery.
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Old 15-03-2011, 11:06   #49
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Re: Deviation Table from GPS

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Originally Posted by HappySeagull View Post
Right on,David M!
All these posts should inform the OP that using the gps is a lot more difficult than just swinging the boat per the simple directions included with a cheap compass.Use a handbearing compass held up high away from any ferrous or electrics. etc. By the way,watch out for untwisted dc wiring near the compass.
"Steering" is altogether different than GPS.
Actually it's not....given a calm, non-tidal current area...swinging the compass and building a deviation card is pretty simple...I did it for several boats in our company and everything seems to work just fine...except for like I said before about most small boats...every time they get underway...there's enough metal changing position to throw the deviation table out the window...

So I do it the easy yet equally effective way because I get paid by the hour...my boss likes efficiency that still meets the requirements.
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Old 15-03-2011, 11:13   #50
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Re: Deviation Table from GPS

"GPS COG is typically accurate to 3,"
I think that relies on assumptions, multiple assumptions, and would appreciate any information to the contrary.
GPS "results" are going to be affected by many things. The one we can control is the length of the baseline, i.e. using a waypoint that is 1 or 10 or 100 or 1000 miles away. One mile may not be enough and a thousand may start rhumb line issues. And different magnetic variations. Somewhere in between, perhaps 10 perhaps 50, should be enough to generate more reasonable baselines.
But that brings to the forefront something GPS makers (Garmin springs to mind!) are a bit paranoid about, they don't want to reveal trade secrets and usually they will refuse to discuss which algorithms they are using, what timing they have chosen, and any specifics of how their calculations are made.
After some of these issues were raised in another venue, I tried to call Garmin to ask very simply, which way did they calculate something? Which process were they using, beause there were several that would produce similar but different results? And hell yes, you could hear the tech at the other end get all icy and cold because he was SURE i must be a competitor trying to steal their secrets, they couldn't even comprehend that some users just might want to know which results, and which errors, were going to be displayed. Or assumed.

So OK, maybe steering by GPS compass is a very bad idea. Compared to...what, using your deviation card in both Northern and Southern hemispehres, without stopping at reswing the magnetic compass? Or, do you just switch deviation cards when you switch from the compass you used in the Northern hemisphere, to the correctly compensated one for the Southern hemisphere? <G>


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Old 15-03-2011, 11:38   #51
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Re: Deviation Table from GPS

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Maybe be off topic but why do you think you can't make your own lifejackets?
Bottom end jackets aren't exactly rocket science; making one's own table is even easier.
As I have said in my post - I do not think that in a situation when something is required by the law the item can be self-made.

As has been pointed out by one poster above, this is not the case in his country.

b.
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Old 15-03-2011, 11:53   #52
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Re: Deviation Table from GPS

I would spend a day or two swinging the compass to known ranges either natural or man made. Thus taking the time to reduce the deviation as low as I can get it with compesating magnetics. Then every time you run a range you can check the Deviation table, to ensure that there is no induced magnetism onto your boat.
And a compass check against the GPS, Adding/subtracting in the leeway your are making from the GPS heading to again checking your deviation card. Because the GPS will show the actually course you are making, even if your bow is pointed elsewhere.
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Old 15-03-2011, 11:56   #53
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Re: Deviation Table from GPS

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As I have said in my post - I do not think that in a situation when something is required by the law the item can be self-made.

As has been pointed out by one poster above, this is not the case in his country.

b.
Not positive...but I think you can make your own nav lights and shapes as well as sound producing equipment...as long as it meets the nav rules (yes there are already approved ones for sale) I think you are OK...I'm sure there are more items in the commercial side like bilge alarms ect that we used to make up and as long as they met the inspectors approval...good enough.
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Old 15-03-2011, 11:59   #54
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Re: Deviation Table from GPS

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Originally Posted by psneeld View Post
Not positive...but I think you can make your own nav lights and shapes as well as sound producing equipment...as long as it meets the nav rules (yes there are already approved ones for sale) I think you are OK...I'm sure there are more items in the commercial side like bilge alarms ect that we used to make up and as long as they met the inspectors approval...good enough.
Note: I've in my sea going career have seen Nav lights put on backwards. i.e. green on portside and red on the starboard side. Don't ask why they made this mistake. But it had to have been Murphy's law in action.
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Old 15-03-2011, 12:11   #55
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Re: Deviation Table from GPS

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Originally Posted by hellosailor View Post
"GPS COG is typically accurate to 3,"
I think that relies on assumptions, multiple assumptions, and would appreciate any information to the contrary.


Really only one assumption - the length of the baseline, which is effected by two things - the vessel speed and the time interval between 'fixes'. The later (time interval) is set by the IMO, for any product intended for use on a commercial classed vessel. I don't know if Garmen uses this standard but Furuno does. And Furuno, provides two accuracy numbers for two different speed ranges - the 3 degrees I quotes was for 'below 17kts'.

The point, again, is:

#1 that if you are doing this properly the deviation errors you are trying to correct for will small (if they are large the compensators should be adjusted until they are small) and roughly close in size to the gps COG errors, which makes the whole exercise pretty much a waste of time if you are trying to create a useful/accurate card, but perfectly ok if the OP is just trying to create a nice looking set of numbers.

and #2 it's not so hard to do this properly and accurately, using a bearing to a distant mark rather than COG.


For those of you who for some reason don't believe Furuno's published COG accuracy spec . . . you might consider why neither ships nor grabd prix racers use gps COG as a heading source and why there is a market for both racing high precision gps (this unit uses a special gps chip set and signal processing technology) and ship "gps compasses" (this unit uses multiple gps antenna). Both of these approaches produce 1 degree heading accuracy, which is the IMO requirement for a heading source and also what is needed to provide accurate true wind direction on a grand prix race boat. A 'standard' gps COG just simply will not provide this accuracy.
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Old 15-03-2011, 14:51   #56
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If the accuracy (got the right position) is off, but the precision (repeatable results, but not necessarily accurate) is high, then COG is going to be correct.
CoG repeatability is as poor or even poorer then the underlying CoG accuracy. The error is the error

Quote:
Dave, I think you edited this post a little from the first version that was in the alert email - "ie the boat isnt pointing where the Cog says it is."
- - I think the use of "pointing" is more appropriate as that is what the magnetic compass is actually showing.
- - However, where the boat is "pointing" and where the GPS COG is saying you are going in relation to the mud/bottom underneath your boat will not agree - unless - there is no wind/leeway, no current/set & drift, and no propeller "walk" if you are motoring.
- - Getting all these factors "zeroed out" is not easy so even disregarding GPS system errors, what you see on the magnetic compass and where the boat is actually going in relation to the physical Earth "COG" are rarely the same. That is why I advocate "locking" the boat in a static location/heading with anchors, dock-lines, or whatever has the best chance of getting a reasonably accurate deviation measurement.
I changed the word from pointing as CoG tells you the direction the boat is going rather them where it's pointing which is what the compass is showing ( heading)

The sum total of all these threads is that for example in a fibreglass boat the deviation offsets are likely to be less then the CoG accuracy. So as had been said the GPS deviation method produces pretty numbers that have no meaning.

Dave
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Old 15-03-2011, 15:04   #57
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Re: Deviation Table from GPS

It all boils down to this.... Heading is not the same thing as Course Over Ground., although at times the numbers might coincidentally be the same.

Compasses give you heading, (corrected or uncorrected), and GPS's give you course over ground...period. A single GPS is not capable of giving you an accurate heading that you can absolutely depend on as being accurate although it might coincidentally be accurate.

If you want a rough estimate of your deviation then use a GPS to create a deviation card. If you think you may actually need a deviation card one day that is accurate, then swing your boat using your compass to create an accurate deviation card.
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Old 15-03-2011, 15:10   #58
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Re: Deviation Table from GPS

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Right on,David M!
All these posts should inform the OP that using the gps is a lot more difficult than just swinging the boat per the simple directions included with a cheap compass.Use a handbearing compass held up high away from any ferrous or electrics. etc. By the way,watch out for untwisted dc wiring near the compass.
"Steering" is altogether different than GPS.
Thanks. I need to make it clear that you do not swing a boat by comparing one compass to another...that's like letting the blind lead the blind. You swing a boat by using a range of known true bearing and comparing what your compass claims the range is. The difference after correcting for variation is your correction East or West. You then plot what ya got, draw out a smooth sine wave between your points and then you write out your corrections for every ten degrees or how may degrees you want., 15, 20 or whatever.
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Old 15-03-2011, 15:12   #59
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Re: Deviation Table from GPS

barnakiel-
"As I have said in my post - I do not think that in a situation when something is required by the law the item can be self-made."
Some things are "required" by law, as in required to conform to a standard or required to be carried. Other things must be CERTIFIED as being in compliance. It is only these latter things that you cannot make yourself. Or rather, you certainly can make them yourself--but then you'd have to submit them for certification, and government certifications are typically charged mid five-figure testing fees.
So by all means make your own life vest. In the US, you certainly can, and you don't have to submit it for certification--as long as you still carry the proper number and type of CERTIFIED ones properly on the boat.
Nav lights? Fog horns? Don't have to be certified.
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Old 15-03-2011, 17:34   #60
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Re: Deviation Table from GPS

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Thanks. I need to make it clear that you do not swing a boat by comparing one compass to another...that's like letting the blind lead the blind. You swing a boat by using a range of known true bearing and comparing what your compass claims the range is. The difference after correcting for variation is your correction East or West. You then plot what ya got, draw out a smooth sine wave between your points and then you write out your corrections for every ten degrees or how may degrees you want., 15, 20 or whatever.
And you are right-good description too.Sorry,I'm wandering from the "Deviation Card" of OP...But I would still go the high up handbearing compass route before using the gps while moving along a random crosstide range as I figured OP was setting out to do....
Gee,I hope the lubber line is right...It can be "off".Not that that'd matter to straight steering or "Inspectors" but it'd shift the whole deviation altogether port or starboard...
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