Gunfleet
New member
Please don't guess. We know that if you add dr after dr the circle that describes your possible position grows. But by how much? How is the multiplier arrived at?
Cheers
Cheers
Question for expert navigators
- Thread starter Gunfleet
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S
Skyva_2
Guest
With DR after DR the error would be substantial, and depend on how much tide has been ignored.
If you mean EP after EP, John Goode has a rule of thumb that says
Good conditions - calm sea, winds force 2-3, neap tides - error 5% per hour.
Poor conditions - high winds, leeway, spring tides - error 15% per hour.
It will also vary with the navigator, boat and the helm on duty, so there is no definitive answer.
If you mean EP after EP, John Goode has a rule of thumb that says
Good conditions - calm sea, winds force 2-3, neap tides - error 5% per hour.
Poor conditions - high winds, leeway, spring tides - error 15% per hour.
It will also vary with the navigator, boat and the helm on duty, so there is no definitive answer.
Searush
New member
Statisticians would say the mean errors often cancel.
Navigators tend to assume they are all additive - towards the nearest hazard!
All depends if you have faith or believe Sod's Laws. Risk assessment needs to take two things into account. The probability of an event and the consequences of that event!
The probability of additive errors may be fairly low, but the consequences can be catastrophic. Mk1 eyeball, nose & lug'oles should help minimise error when close to the coast.
Navigators tend to assume they are all additive - towards the nearest hazard!
All depends if you have faith or believe Sod's Laws. Risk assessment needs to take two things into account. The probability of an event and the consequences of that event!
The probability of additive errors may be fairly low, but the consequences can be catastrophic. Mk1 eyeball, nose & lug'oles should help minimise error when close to the coast.
Gunfleet
New member
Thanks Keith, but that's just passing on Goode's guess (ie his rule of thumb), however accurate he is in practice. Imagine the boat and helm are standardised. I think there must be a statistical answer to the growth of uncertainty and I imagine whether we are referring to EP or DR wouldn't make a difference to it except in magnitude. If I reformulated it as 'for a given size guess in navigation the uncertainty rises at ?' you'll see what I mean. Is the change linear or exponential? Whichever it is, could a number be put on it?
What's the point? Well, we all know, or think we know that the cirle of uncertainty is greater. But unless there's some way we could quantify it it we know nothing of the sort. It just seems right. I'm not suggesting that what is seems right is untrue, rather how do we know what seems right is true?
What's the point? Well, we all know, or think we know that the cirle of uncertainty is greater. But unless there's some way we could quantify it it we know nothing of the sort. It just seems right. I'm not suggesting that what is seems right is untrue, rather how do we know what seems right is true?
S
Skyva_2
Guest
As Searush says, the errors may compensate.
I don't have an answer, but what variables do we have?
Tide set and drift information is not 100% accurate.
Wind borne current may be in play.
Plotting error.
Compass parallax error.
Compass error.
Helming error.
Leeway error.
Log reading error.
You prompted me back to Mary Blewitt, who I reckon carried more navigational wisdom in her litle finger than most 'gurus', and did not have an answer to this. She reckoned a tentative 5%, but assume 8% in fog!
Quote:
' every time you make a landfall check how far from your DR plot you really are and work out the area of uncertainty so that you can build up your own data on which to base your assumptions. It is most important to find out why your position was wrong'.
I don't have an answer, but what variables do we have?
Tide set and drift information is not 100% accurate.
Wind borne current may be in play.
Plotting error.
Compass parallax error.
Compass error.
Helming error.
Leeway error.
Log reading error.
You prompted me back to Mary Blewitt, who I reckon carried more navigational wisdom in her litle finger than most 'gurus', and did not have an answer to this. She reckoned a tentative 5%, but assume 8% in fog!
Quote:
' every time you make a landfall check how far from your DR plot you really are and work out the area of uncertainty so that you can build up your own data on which to base your assumptions. It is most important to find out why your position was wrong'.
Richard10002
Well-known member
he Depth Sounder can play a large part.
Phoenix of Hamble
Active member
I understand what you are asking, but you don't navigate on statistical likelihood in this scenario, you navigate on worst case..... thats why you choose the most dangerous point of a cocked hat, allow an extra bit for calculated tidal depths, and in this case, assume errors on EP calcs are cumulative, not cancelling...
Regardless of that rant above.... the errors certainly wouldn't be linear..... a .5nm error on first EP would compound a 5deg helming error on the second EP etc etc.... so if you really wanted to be serious, you could calculate the first EP, assume JG's rule of 5 to 15% and draw a 'circle of uncertainty'... take the worst case (a lá cocked hat), calculate the second EP from this point, etc etc.... but would be a rather pointless exercise..... and I bet the results would be pretty scary!
As was said above.... mk1 ears and eyes are best tools on board!
Regardless of that rant above.... the errors certainly wouldn't be linear..... a .5nm error on first EP would compound a 5deg helming error on the second EP etc etc.... so if you really wanted to be serious, you could calculate the first EP, assume JG's rule of 5 to 15% and draw a 'circle of uncertainty'... take the worst case (a lá cocked hat), calculate the second EP from this point, etc etc.... but would be a rather pointless exercise..... and I bet the results would be pretty scary!
As was said above.... mk1 ears and eyes are best tools on board!
Cruiser2B
Active member
A DR is really the simplest of assumptions - starting from a known position, one assumes course and speed. Once you have two consecutive fixes (all other things being equal) then one knows CMG and SMG, and is able to plot EPs. The farther you get from point A, the reliability of the EP becomes more suspect. Ideally, you fix at an interval determined by proximity to danger - ie more often when coastal than when offshore. If for some reason, you are limited in your ability to fix frequently (ocean crossing, using sextant only for instance), then you apply an "expanding pool of errors". You can look this up - it might be on the web, but the source is the Admiralty Nav Manuals. From your DR you add components for current, leeway, steering and speed errors. Other than current, the other factors are generally specific to you and your craft; usually determined through observation. Obviously, as more time elapses each error adds a larger component, so your DR point becomes a large, odd-shaped area, within which one determines an ellipsoid, known as a PPA (probable position area). Done from memory, so please excuse errors or omissions, but you get the gist.
Gunfleet
New member
John Goode\'s best guess theory
Thanks. I don't mind the rant at all. I know how to navigate and expect I would do it roughly the same way you do. It's the 'how we know the things we hold to be true are true?' part that my question is after. What the theory we all learn says is that as we travel away from the last fix the uncertainty increases. On navigation courses and in navigation books people gaily illustrate this uncertainty with increasing sized circles. But the closest we've got to finding a rule that governs the change in these circles is John Goode's best guess. It feels to me that Goode (and the circle drawers) are right, but what governs it? How do we know it's true that the circle is bigger? If we use his theory does it mean 5-15% of the distance travelled? If it is it means that which connects the circles must be something more like a cone with a circle at each end (or plan of a bicycle chain and two gears) and unlike the lines we draw.
Thanks. I don't mind the rant at all. I know how to navigate and expect I would do it roughly the same way you do. It's the 'how we know the things we hold to be true are true?' part that my question is after. What the theory we all learn says is that as we travel away from the last fix the uncertainty increases. On navigation courses and in navigation books people gaily illustrate this uncertainty with increasing sized circles. But the closest we've got to finding a rule that governs the change in these circles is John Goode's best guess. It feels to me that Goode (and the circle drawers) are right, but what governs it? How do we know it's true that the circle is bigger? If we use his theory does it mean 5-15% of the distance travelled? If it is it means that which connects the circles must be something more like a cone with a circle at each end (or plan of a bicycle chain and two gears) and unlike the lines we draw.
Richard10002
Well-known member
Re: John Goode\'s best guess theory
When I first got interested in sailing at age about 14, my Dad suggested I get a book on DR from the library, so I did.
IIRC The book had a pale blue cover, was about 1000 pages long and was full of statistical calculations and other guff that I didnt understand.
Dont know what it was called, but it probably went some way to answering the question......
if you could be bothered!
In the absence of anything else, I like the circle of uncertainty and keep an eye on the depth, and should probably get a lead and line.
When I first got interested in sailing at age about 14, my Dad suggested I get a book on DR from the library, so I did.
IIRC The book had a pale blue cover, was about 1000 pages long and was full of statistical calculations and other guff that I didnt understand.
Dont know what it was called, but it probably went some way to answering the question......
if you could be bothered!
In the absence of anything else, I like the circle of uncertainty and keep an eye on the depth, and should probably get a lead and line.
Gunfleet
New member
'Expanding pool of errors' which can be derived from observations sounds very promising. The ellipsoid does too. One of the things that always bothered me was that the circle seemed unlikely to express all the probabilities with sufficient weighting to the variables.
G
Guest
Guest
Simple - There is NO multiplier ... there is no error calcualtion possible on this.
DR is course steered and speed by log. In normal circumstances about as much use as a sock on yer wotsits ... Tide / Leeway / etc. are variable and therefore cannot be averaged out to give a "Multiplier" to apply at any time you wish.
EP is really only halfway to correct as it still has influences that cannot be applied to give a really good position - you just hope that it is ...
DR is course steered and speed by log. In normal circumstances about as much use as a sock on yer wotsits ... Tide / Leeway / etc. are variable and therefore cannot be averaged out to give a "Multiplier" to apply at any time you wish.
EP is really only halfway to correct as it still has influences that cannot be applied to give a really good position - you just hope that it is ...
Gunfleet
New member
So if there's no calculation possible, how do you know the uncertainty about your position has changed?
G
Guest
Guest
Simple answer again ....
As a Navigator who gains experience of the vessel - you "apply" a factor of your own determination .... saying to yourself - the max error I reckon with this could be ..... Note I say COULD be ...
DR is DR .... why try to make it something more than what it is ..... a position by speed and course steered alone taking NO account of any errors / influences on vessel. If you start to apply "error" possibilities etc. - it is no longer a DR is it ...
Nav is simple ... finding out where you were ... and then the art of extrapolating to say where you probably will be in x time ... That's it. Plain and simple.
As a Navigator who gains experience of the vessel - you "apply" a factor of your own determination .... saying to yourself - the max error I reckon with this could be ..... Note I say COULD be ...
DR is DR .... why try to make it something more than what it is ..... a position by speed and course steered alone taking NO account of any errors / influences on vessel. If you start to apply "error" possibilities etc. - it is no longer a DR is it ...
Nav is simple ... finding out where you were ... and then the art of extrapolating to say where you probably will be in x time ... That's it. Plain and simple.
Phoenix of Hamble
Active member
Re: Simple answer again ....
in that Nigel I disagree completely...
Calculated where you probably will be is easy..... being smart enough to understand where else you could be isn't so easy.... and is something any good navigator does IMHO...... if nothing else to assess the localised dangers etc etc... noone knows for certain what leeway they are making, how accurate the tidal predictions are, or whether the log has under read becuase of a bit of weed etc etc
in that Nigel I disagree completely...
Calculated where you probably will be is easy..... being smart enough to understand where else you could be isn't so easy.... and is something any good navigator does IMHO...... if nothing else to assess the localised dangers etc etc... noone knows for certain what leeway they are making, how accurate the tidal predictions are, or whether the log has under read becuase of a bit of weed etc etc
G
Guest
Guest
It definitely seems you didn\'t read properly ...
DR is a position by Dead Reckoning ... a plain position that has no account of any influences. Normally only used in times of short distance such as sun-sights / running fix's etc.
EP is a slightly more "accurate" postion using tides / currents / estimated leeway if known ... to try and derive a position more near true.
A Fix is a position bu observation of electronic or visual means to determine a position that is as close as you can determine to true.
Now go back and read my previous posts based on the original question - a DR position ... and possible "multiplier" for error. A possibility that really does not exist by definition - never mind reality ....
Ok - onto practical matters ... as I already said in different words ... the xperienced navigator will apply his own estimated error factor in his head NOT on chart ... a DR is a DR is a DR ...
So all the years and examinations / tickets etc. I gained in The Merchant Navy are wrong ???
And in fact if you compare properly what you wrote and what I did - we are not so different - but you haven't read mine properly .... QED
/forums/images/graemlins/frown.gif
DR is a position by Dead Reckoning ... a plain position that has no account of any influences. Normally only used in times of short distance such as sun-sights / running fix's etc.
EP is a slightly more "accurate" postion using tides / currents / estimated leeway if known ... to try and derive a position more near true.
A Fix is a position bu observation of electronic or visual means to determine a position that is as close as you can determine to true.
Now go back and read my previous posts based on the original question - a DR position ... and possible "multiplier" for error. A possibility that really does not exist by definition - never mind reality ....
Ok - onto practical matters ... as I already said in different words ... the xperienced navigator will apply his own estimated error factor in his head NOT on chart ... a DR is a DR is a DR ...
So all the years and examinations / tickets etc. I gained in The Merchant Navy are wrong ???
And in fact if you compare properly what you wrote and what I did - we are not so different - but you haven't read mine properly .... QED
/forums/images/graemlins/frown.gif
Searush
New member
Re: Simple answer again ....
I'm with SBC. DR is a way of getting an EP, as in ESTIMATED Position. Doesn't that say it all? How much accuracy do you expect? The more you expect, the more likely you are to be dissapointed (err, like stuck on a reef?)
Use the other information to help inform you - birds, planes, ferries, depth, bottom type, surf sounds. smell of grass, dogs or seals barking - whatever!
We once got caught by a dodgy compass, SWMBO insisted that we were off course because of the plane must have been going to Ronaldsway & I thought it might be going to Cork. As usual, she was right and we had a 20deg error on that course that I hadn't spotted. That's what happens when you don't keep checking readings against assumptions! EP was miles out due to unexpectedly wrong compass reading. Factor that in if you can.
Basically this is a purely academic discussion. In practice it is utterly pointless to try to "accurately" estimate aggregated errors. But it's still fun to argue!!! /forums/images/graemlins/cool.gif
I'm with SBC. DR is a way of getting an EP, as in ESTIMATED Position. Doesn't that say it all? How much accuracy do you expect? The more you expect, the more likely you are to be dissapointed (err, like stuck on a reef?)
Use the other information to help inform you - birds, planes, ferries, depth, bottom type, surf sounds. smell of grass, dogs or seals barking - whatever!
We once got caught by a dodgy compass, SWMBO insisted that we were off course because of the plane must have been going to Ronaldsway & I thought it might be going to Cork. As usual, she was right and we had a 20deg error on that course that I hadn't spotted. That's what happens when you don't keep checking readings against assumptions! EP was miles out due to unexpectedly wrong compass reading. Factor that in if you can.
Basically this is a purely academic discussion. In practice it is utterly pointless to try to "accurately" estimate aggregated errors. But it's still fun to argue!!! /forums/images/graemlins/cool.gif
G
Guest
Guest
Ok - lets go one step further ....
DR .... is a position based on log reading and compass course steered.
Let us introduce a few errors that navigator on board is unaware of ... eg - a small magnet near his compass and re-calibrate his log to show x high or low.
Question to Magnacarter ... now how does Nivigator plot a DR ?
MY answer is by definition of DR ... by still reading compass and log.
Bo**cks isn't it.
But EP might start to show up an error POSSIBLY but unlikely as we are still talking Log / Compass + Tide / current / Leeway influences ....
So where's your navigators assesment now ????
DR .... is a position based on log reading and compass course steered.
Let us introduce a few errors that navigator on board is unaware of ... eg - a small magnet near his compass and re-calibrate his log to show x high or low.
Question to Magnacarter ... now how does Nivigator plot a DR ?
MY answer is by definition of DR ... by still reading compass and log.
Bo**cks isn't it.
But EP might start to show up an error POSSIBLY but unlikely as we are still talking Log / Compass + Tide / current / Leeway influences ....
So where's your navigators assesment now ????
G
Guest
Guest
Re: Ok - lets go one step further ....
We of course assume that he doesn't take an azimuth / compass error during the exercise !!!! That he still applies Var'n and DEv'n as per Chart / Dev'n card ... which lets be honest how often do people check their compass on board ... (a merchant ship will do it nearly every watch ... ie about every 4 hrs ...)
We of course assume that he doesn't take an azimuth / compass error during the exercise !!!! That he still applies Var'n and DEv'n as per Chart / Dev'n card ... which lets be honest how often do people check their compass on board ... (a merchant ship will do it nearly every watch ... ie about every 4 hrs ...)
David_Jersey
New member
Re: Simple answer again ....
[ QUOTE ]
Basically this is a purely academic discussion. In practice it is utterly pointless to try to "accurately" estimate aggregated errors.
[/ QUOTE ]
Not being an "Expert" I didn't jump straight in, but I am glad someone else has said this, as well as the theoretical adjustments you would also need to factor in the human element to the equation - of getting calculations wrong / inconsistent. The "secret" is just to accept that you are going to be wrong and therefore not to be overly aggressive with your navigation, "Just in case".
DR & EP <u>are</u> perfectly acceptable ways of getting from A to B (I am still here!), the more you gain experiance of a specific boat and the area of travel the better your estimates / guesstimates become............. but out of choice I prefer to use a GPS, cos' it is easier /forums/images/graemlins/smirk.gif But is nice not to have to call out the lifeboat just cos' the GPS stops working.
[ QUOTE ]
Basically this is a purely academic discussion. In practice it is utterly pointless to try to "accurately" estimate aggregated errors.
[/ QUOTE ]
Not being an "Expert" I didn't jump straight in, but I am glad someone else has said this, as well as the theoretical adjustments you would also need to factor in the human element to the equation - of getting calculations wrong / inconsistent. The "secret" is just to accept that you are going to be wrong and therefore not to be overly aggressive with your navigation, "Just in case".
DR & EP <u>are</u> perfectly acceptable ways of getting from A to B (I am still here!), the more you gain experiance of a specific boat and the area of travel the better your estimates / guesstimates become............. but out of choice I prefer to use a GPS, cos' it is easier /forums/images/graemlins/smirk.gif But is nice not to have to call out the lifeboat just cos' the GPS stops working.