DED Reckoning Question

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Flutterby

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When learning boat navigation, I was taught how to DED reckon and even was given the opportunity to try it a bit. But I've never really had to use it in real life.

While watching a TV show about bush pilots in Alaska, the weather issues have come up. Each time, a pilot will refer to the ability to navigate by visual observance of landmarks [rivers, mountain passes, etc] as "DED reckoning". In my training, "DED reckoning" was done by using the vector of two radio signals. The boat's position was where these two signals crossed.

What is the true definition of "DED reckoning"? Or is it different for boats then for planes?

EDIT: to correct spelling
 
While you can find numerous definitions, dead reckoning is simply the method of estimating a future position by using your present position along with travel time, direction and velocity, to predict your future position. From this future position you use the same method to predict the next future position. It is a problematic method since each new position incorporates the cumulative errors of all previous positions. It does not involve the use or radio beacon intersection to determine position, just time and direction determination (compass) to calculate a future position.
 
deduced reckoning. any deduction of your present estimated position from observations that depend on a uncertain measurement of history such as speed. EPs or estimated positions are the calculated points along the estimated line of position.

A fix is a more accurate position. Two bearings create a fix. The position is much more certain. Passing close to a fixed mark is a fix as well.

Typically a dr line of position would be plotted using course speed and other factors such as estimated current.

When bearings could be taken on objects with a known location and those bearings crossed that point was considered accurate and a fix. Three bearings even better, especially if the other two bearings were nearly on the same line. The fix then became the new starting point for the DR plot, regardless of how close it was to the last previous EP.
 
Both are correct, you start with a known position, either a landmark or radio fix and estimate your position from there.
 
I always thought of it a determining position using unproven data.

That is: If your bow is touching the dock, then your position of "at the dock" is "proven". If you leave the dock, travel for 1 hr at 10 kts on a course of 180deg in an area with slack current and no wind, then your DR position is 10nm due south of the dock. An educated guess.
 
And I think the process is rightly called Ded for deduced reconining, vs Dead.
 
If you can't use dead reckoning to navigate you should not be out on the water, it is a compass, a chart, and a watch, and some way to estimate speed of the boat.
 
It's using common, sense of where you are and by using your senses to determine where you want to go.

If you find yourself in fog so thick you can't see the bow, it really makes it hard. If you can see a difference in the light in the sky you have a sense of where the sun is. A compass will only tell you the general direction you need to go. It's fine for the open ocean but not so much in a bay or running along the coast. turning off the engine and listening can give you more info. Judging speed and distance(which includes tide and current) is one of the hardest for me. If you have it, Relying on a GPS can really get you in trouble because of the slow update rate which can have you going in a totally wrong direction...especially at dead slow. Increasing speed and holding a straight course will let the update rate catch up with the direction you are moving. This is where the compass helps the GPS, it responds quicker.

Everything becomes more critical the more confined the area such as an inlet or bay.

If you have radar, that's the best reason to use it especially on clear days so you familiarize yourself with your surroundings and can recognize targets.
 
Charlie: Never notied the slow update problem. Any more detail??

In fog a fix may be established when you are right under the GG bridge and the horn lets off. :D
 
I think Charlie is talking about the slow heading update of a GPS. Most GPS units don't have a compass per se and only show the direction you are moving, not your actual heading. If you're stopped you get no direction at all, and if moving slowly it's not accurate. Steering by one can be a real hassle without an actual compass.
 
must be very slow because at 8 kts I havent noticed it. Of course I use a compass all the time anyway
 
The "error" when drifting may be course over ground which may be very different than heading.
 
In the Air the ship is moving in only One medium. Atmosphere.
On the water the Ship is moving in two mediums. Air and Water. Both moving.

Trying to move a boat in a straight line where there are differing tidal currents is a lot more difficult than flying in or around mountains and Valleys.

Flying, in what I thought was a straight line toward a target, I alwys looked down between the Wheel and ship at the ground. If the Ground was slewing off to the left or right, I was being moved sideways by the medium I was flying in.
Dead Reckoning is sorta like a Moving map in your head.
So you need to observe the 'other' factors to make sure your flying where you think you should be flying.
Same with boating.
 
ded recogning is simply the process of guessing where you think you are based on whatever information you are using. A fix is where you actually are based on observation of something with certain, or fixed, position.
 
Gee guys, didn't I just say that?:) Of course my explanations even confuse me. It's all Brucies fault.
 
It's when you wake up and find that Elvis and Marilyn are your new neighbors, you then reckon that you are dead.
 
Hmmm, I think maybe what was demonstrated to me is a different animal, now obsolete. This guy who had 30 yrs as a Navy Navigator, first ships then airplanes, brought a special radio receiver onboard. As we were sitting off of Half Moon Bay on a foggy day, unable to see anything, he used the "radio direction finder" [his words] to locate two different frequencies and plotted them on the chart. Where these two signals intersected on the chart was our location. Then he charted a course to a buoy and we followed that course using a compass, constant speed and a stop watch. When he said we were at the buoy, we looked off the bow and there it was! The boat damn near hit the buoy, we were so close. I was impressed. No radar, Loran or GPS onboard. This was in the early 1980s.
 
you were dead reckoning once you left the "fix" point located by the radio signals.
 
Getting a RDF "fix" "proved" your location, but as getakey states, once you turned off the RDF, you were DR.

--

RDF was a useful tool. While it had it's problems, sometimes it was real easy to use. Case in point: We fished way offshore from South Pass of the Mississippi River. There was a RDF beacon located 2 mi upriver from the entrance to the pass. You basically kept the beacon off your bow, and you were headed in the right direction. When far out, you could get a sighting ( fix ) on the lightship located at the entrance to the MRGO. The combination told you where home was, and about how far out you were.

The errors arose as you neared the pass. Since the beacon was not at the mouth of the pass, and the pass amounted to two long mud bars on either side, RDF was fairly useless once within a few miles. You could easily be heading for the beacon yet be on either side of the pass ( extremely shallow clay/glue )
 
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