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» Online Classroom   »   » Public Discussion of Cel Nav   » Estimated Position

   
Author Topic: Estimated Position
drlmk


 - posted June 11, 2007 07:31 AM      Profile for drlmk           Edit/Delete Post 
Last weekend, I was working the navigation problem in the back of the May/June issue of Ocean Navigator magazine. (I know, "get a life," but my crew didn't show, so I was stuck on shore.)

The problem involved working a single sun sight. At the end of the problem, they asked for the vessel's "estimated position." It was clear from the answer they gave that what they meant by "estimated position" was the point where the LOP crossed the azmuth line.

What I learned from your course is that all you get from a single shot is a LINE of position and you are somewhere on that line. Emphasis on "somewhere."

So how valid is it to call the point where the LOP crosses the azmuth an estimated position? Or more precisely, what level of uncertainty would be associated with that estimate, even if the sight data were exactly correct?

I would think there is a lot of uncertainty. As a practical matter, though, when I compare my sextant plots with known GPS positions, the point where the LOP crosses the azmuth is pretty darn close to where the GPS says I am.

From: Spokane
David Burch


 - posted June 13, 2007 09:36 AM      Profile for David Burch           Edit/Delete Post 
before answersing this in more detail, can you please confirm this phrase: "call the point where the LOP crosses the azmuth an estimated position?"

i am not sure what this phrase means. Maybe there is a typo?

Note that if it means where the LOP crosses what we call the "azimuth line" then that is simply wrong, as that line depends on the location of the AP, which in turn depends on the time of the sight, unless maybe they are doing their sight reductions from the DR position.

Often one sees the definition of an EP in cel nav as equal to the point on the LOP that is perpendicular to the DR position, ie the one point on the LOP that is closest to the DR position using a calculator, or a special set of tables.

But as you rightfully suspect, neither of these definitions have much significance. Let me come back to this after you fill in the details above.

From: Starpath, Seattle, WA
drlmk


 - posted June 13, 2007 11:36 AM      Profile for drlmk           Edit/Delete Post 
You are correct: I was asking about the validity of calling the point where the LOP crosses the azimuth line an estimated position (EP). The answer they gave for EP coincided with that point on the plot I worked out from the sight data they gave.

But perhaps I shouldn't be asking you what they had in mind. I'll go directly to them for their explanation.

I just hadn't run across the term "estimated position" before.

Thanks.

From: Spokane
David Burch


 - posted June 13, 2007 12:23 PM      Profile for David Burch           Edit/Delete Post 
If that is the case, then they must be doing sight reduction from the DR. and they are calling the closest point on the LOP to the DR equal to the EP. Some software does this. We do not use this term as in sailing vessels this is not a good estimate of your position. It is wishful thinking mostly.

If you are sailing to weather, and you have a point on the LOP that you can get to by rotating your course to leeward by about 10 to 20° then chances are that is your best guess of where you are from the single LOP.

But generally all you really know when your LOP does not intersect your LOP is (1) you are somewhere on that LOP and (2) your DR is wrong.

Here is the Bowditch definition of EP, they do not use this cel nav idea of nearest to DR as EP, but i have seen this before somewhere. It could be the old celestecomp calculator used it?

Estimated position. The most probable position of a craft determined
from incomplete data or data of questionable accuracy. Such a position
might be determined by applying a correction to the dead
reckoning position, as for estimated current; by plotting a line of
soundings; or by plotting lines of position of questionable accuracy.
If no better information is available, a dead reckoning position is an
estimated position, but the expression estimated position is not customarily
used in this case. The distinction between an estimated position
and a fix or running fix is a matter of judgment.

From: Starpath, Seattle, WA
David Burch


 - posted June 13, 2007 12:37 PM      Profile for David Burch           Edit/Delete Post 
Just another subtlety on this question. I note that you are doing sight reductions by calculator using the DR as an assumed position.

I concluded that since you agreed with their answer that they were doing this as well. This, in retrospect, however, is not the case.

the definition they are working under is still EP = closest point on LOP to the DR. They could do their sight reduction from any AP at all and then get the LOP plotted, then they go to the DR and drop a perpendicular to meet that LOP and call that intersection the EP.

When you do your sight reduction from the DR itself, then you interpret the EP as the point where the LOP crosses the azimuth line, but that is simply getting you to the same point... it is not the definition of EP.

In short the definition of the EP is not where the LOP crosses the azimuth line, but rather the closest point on the LOP to the DR--which is a definition independent of the AP used for the SR. It just happens that if you do the SR from the DR it looks like that is the definition.

... none of this, however, is to distract from the fact that this is still not a useful concept.

From: Starpath, Seattle, WA
drlmk


 - posted June 14, 2007 07:50 AM      Profile for drlmk           Edit/Delete Post 
Excellent points. Well stated. Thank you.
From: Spokane


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