Dead reckoning?

[Reap]

Re: Course to steer

I don't understand what you mean here Mainlystream, that DR is of little practical use I would have thought were clear, given the definitions.
You state that comparison of the DR position with a fix gives actual/current leeway vector etc, but this is also true with the more accurate EP, it should still be compared with the fix.
And so the DR has little practical use.
With all of the variables and all of the possible innaccuracies, when you compare it with a fix, you will not be able to ascertain wether it was your tide allowance or your leeway that was innacurate, or indeed if your log was under reading or your compass that was slightly out or as you rightly say helming error. In fact your next DR/EP will probably be on a different tide vector anyway so you still wont know.

The DR is as stated only a starting poiont for an EP that is its only purpose.

The only practical use for using a DR on the chart is for ocean navigation where you have no idea of tide/current etc and only have course steered and distance run to work with, until that is you get your three sun sights or whatever and use the DR to work up your fix.

<hr width=100% size=1><A target="_blank" HREF=http://www.yacht-links.net>http://www.yacht-links.net</A>

 

[MainlySteam]

Re: Course to steer

The EP is already corrected by a vector for what you thought the current, leeway etc was so comparing that to a fix gives you another vector which indicates the error in your planning. So you have 2 vectors, the one you allowed in your planning and gave you the EP and the one from the EP to the fix. To determine the external influences on the vessel over the track from your last fix you have to add these two vectors together.

If one plots the DR position (which after all is just a speed x time spread of the divider legs along the DR course line from your last fix, without having to worry about anything else) and compare that to your current fix you end up with the complete vector for what the current, leeway, etc actually was.

Lets say for simplicity of example that there is just one external influence, current. For planning the course from the current fix, one can say, for example "I allowed before for a 1 knot NE current, however when I compare my present fix with the DR Position I see it was actually 2 knots E. I do not think anything is going to change for the next hour so I will plan using 2 knots E".

If one does the same thing comparing the EP to the fix one has the vector you planned with plus the vector showing your planning error. They will add to 2 knots E, but neither of them will be that.

Hell of alot more difficult explaining it than doing it, but the key is that a DR Position is simple to plot being just speed x time along the DR course line. A straight comparison of that with the fix immediately gives you a single current, leeway, etc vector to use in planning the next part of the course with.

I would go so far that when navigating with GPS, the DR plot is even more useful because one never needs to work out a EP for the sake of determining where one is as you always have a fix immediately available to you. So one tends to only need to know current, leeway, etc for planning. In which case from the Departure one guesses these influences. At the first GPS fix you compare that fix with the DR position (speed x time divider spread along the DR course line), then use the vector from that comparison, plus any changes you foresee for the next part of the course being planned for, and set your new course, etc, etc from fix to fix.

Whew! - after all this I am going to feel as guilty as hell every time I leave the marina if I don't put all the above into practice! Will have to spend all my time at the chart table now. I think as other threads have shown, most of us, including me, navigate casually most of the time and only tighten up when conditions or situation requires it, so please understand that I am not pushing that we should be pedantic in our practice - just presenting the explanation requested.

John


<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 03/09/2003 23:12 (server time).</FONT></P>

 

[PuffTheMagicDragon]

So you see Olavs....

.....it is one form of mumbo-jumbo technobabble that you would use to impress your companions over a beer (.. several, more likely..) that you can actually work out where you are while secretly hoping that your GPS doesn't pack up! ;>)

It has reminded me of a cartoon in a yachting magazine many, many years ago. It showed a skipper saying "IF we are where I think we are - and I don't think we are - we're somewhere here" while placing his hand over most of the North Sea.....!
Fair Winds.

<hr width=100% size=1>Wally

 

[peterb]

Re: Course to steer

Sorry, I don't agree. I agree that the effects of tidal stream and current are represented by a vector; the direction and distance that the water moves during that leg. But leeway has no such physical embodiment. Leeway is not a movement in that sense, it is the boat moving at an angle through the water. The problem is that our compasses are fixed to the boat, and tell us which way the boat is pointing, not the direction in which it is moving. If I want to represent leeway in my navigation I do so by rotating the vector representing my heading, so that it represents my water course. I don't do it by plotting my heading, then adding an extra little vector marked 'leeway'. So leeway and tidal drift are taken into account quite differently in the calculation.

Of course, you could plot a DR, using heading and log reading, and then compare it with a fix. Assuming that the DR and fix are both accurate, you could say that the difference between them represents the effects of leeway and tidal drift. But to try to give this difference a physical interpretation as the sum of a leeway vector and a drift vector is, in my opinion, quite misleading.

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[MainlySteam]

Re: Course to steer

Peter - I think you are misinterpreting what I am saying.

Look at what I have said in the light of using the DR Position (speed x distance along the DR course line from the last fix, and remembering that the DR Course always points to where you want to go to) and comparing it to a concurrent fix. It gives you the a vector which is the sum of all the external influences upon the vessel's passage since the last fix. Those influences will be from current, wind, helming error, etc. I would have thought that was useful information for planning the next part of the course, and especially if nothing is expected to change, external influence wise for that part.

If, under conditions where some accuracy in navigation is required (and I reiterate, I know most of us don't have to worry most of the time) after an hours sailing from your last fix someone said to you, over the last hour all the external influences on your vessel have caused you to be 1 nm NE of where you would have been if you had just steered straight to your destination at the same speed through the water, would that not be very useful information for planning for the next hour or whatever? That information is exactly what you get if you compare the DR Position (speed x time along the DR course line from the last fix) with a concurrent fix.

Again, I am not trying to say this is how it has to be done, everyone to their own approach - just saying this is an easy way, and am responding to the questions asked (the foregoing, just me trying to keep out of trouble /forums/images/icons/crazy.gif.

John

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[MainlySteam]

Re: Course to steer

As there seems to be some question/interest in comparing a fix with a DR Position and what the resultant vector means I have sought out a reference. I have chosen Bowditch because it is both authoritative and available for reading on the internet at <A target="_blank" HREF=http://pollux.nss.nima.mil/pubs/>http://pollux.nss.nima.mil/pubs/</A> (select Publication No 9 American Practical Navigator and then select Chapter 7).

Section 707, Chapter 7 (and is the same in the earlier 1995 edition too if one has a paper version of that) says exactly what I have been saying. In reading it one has to be careful because the section immediately flows on to an ending paragraph about plotting an EP and the two bits are not directly related (obviously if you have a fix there is not much point plotting an EP).

In the past, pre GPS, we were (mainly) dependant, in pilotage, on charted features being visible in order to determine a fix. Given that the need to navigate accurately, rather than just looking where one is going, is usually when there is poor visibility (rain, night, fog, etc) it was not easy to get a fix and one had to navigate more by estimation (unfortunately often confusingly referred to as "Dead Reckoning"). Now we can get a fix accurate to within metres any time we wish, and that means that we can at anytime plot the sum of the external influences upon our vessel, using the methodology described, and use that information for planning the next part of ones route, and one would plan that route onwards from the fix just taken.

My electronic charting package does not give me a DR position to fix vector directly (I usually plot on a paper chart) but with simple use of the bearing/distance line one can easily work out the same vector (it maybe that some packages will do this for you, but note that it is not the same thing as XTE). Obviously, if one just lets the boat find its own way to the next waypoint under GPS directed autopilot, then one just forgets about all the above - but it seems many yachtmen, for practical reasons, sail to a compass or wind directed autopilot.

Again, I am not trying to sell the practice, just answer the questions asked of it. I personally find it useful and I appreciate the interest.

John

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[Guest]

Pedantic .... splitting hairs ????

As a Nav cadet - I was always told calculate current drift / set / rate etc. and then make allowance for leeway as a +/- on the course steered ..... based on estimated effect / observation.

The method to arrive at the same point is really immaterial as long as we all arrive at that point !!!


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Guest]

Knowing whats happening to you is good !

If the batterys short out ...if the PC goes down, if the plotter goes PHUT ! ... its nice to know approximately where you'll end up !!!



<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[tome]

Re: Course to steer

Agree with this, having looked at my earlier 1984 paper version. It's a little confusing, as you say, because the DR definition runs into a definition of EP.

In a nutshell, DR is derived from speed and course through the water whereas EP is speed and course over the ground.

So, DR can be derived from log and (corrected) compass and EP directly from GPS. In practice, this makes it easy to estimate tide and leeway, which is what many do instintively. Reading these posts I think that most people are reading off the same hymn sheet without necessarily realising it!

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[MainlySteam]

Re: Course to steer

Yes Tom, I agree about the same hymn sheet. Very hard to explain without a couple of simple diagrams, and even in bibles like Bowditch it is very easy to miss the point as to why they say to keep a DR plot (not only just to give the 2 dozen navigating personnel on the bridge of a navy ship something to do!).

In the end, except when everything disappears into the rain, night or whatever, I think most of us just look where we are going.

John

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[tome]

Re: Course to steer

John

I think the reason for the DR is simply that it's pure observation, ie course and distance based on written-up data from the ship's log. This is why it's plotted. All the other information (leeway, tides) are derived/calculated/interpolated/estimated and therefore more subject to error. It's easier to go back to the DR and correct mistakes if these are plotted separately.

Nowadays we can plot a reliable fix at regular intervals courtesy GPS and the DR assumes a less critical role as witnessed by many postings.

Regards
Tom

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[MainlySteam]

Re: Course to steer

Tom

Yes I have no problem with that - this started with a reference that the DR Position was of little practical use (RYA Yachtmaster notes I think it was). All I am trying to add is that if you have a DR Position and a similtaneous fix (a very accurate GPS one these days) then from the difference between them you have the sum of all of current, wind, helming error, etc effects as a vector for the passage from the last fix to the present one. The knowledge of that quite accurate vector, I find, is of practical use (very) for planning the next part of the course from the present fix.

I think you have some background in navigation systems, so will add - assuming the fix is essentially error free (ie GPS) the only inaccuracy in that vector sum of all of current, wind, etc is the inaccuracy in measuring the speed and the time used to make the DR Plot. One can assume the time is accurate within a fraction of a percent so the only inaccuracy in the current, wind, etc vector, derived from comparing the DR Position and the similtaneous fix, comes from the log.

We are probably on the same merry go round!

John





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[tome]

Re: Course to steer

John

Agreed, this is what I was saying in one of my earlier posts. DR versus fix gives a very complete picture, and the largest error is likely to be in the log.

On the point about timing accuracy, the navigation systems I'm involved with have to be synchronised and accurate within 20 micro-seconds and this is easily achieved with GPS.

Regards
Tom

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