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daddle · 01-02-2011, 07:43

Quote:

Originally Posted by barnakiel

And (IMHO) using GPS fix for our Assumed Pos (while talkin accuracy) is cheating....if we take GPS fix as our Assumed Pos then the error obtained in our Astro fix will be smaller than if we based it all on our cumulative DR / Astro positions.

The assumed position does not affect the LOP accuracy, does it? It can be anywhere. It's helpful if it's close enough to be plotted on the current sheet. But it's choice does not affect accuracy of the final LOPs in any way. Or I've misunderstood (or forgotten) something. Any random position will do. So it might as well come from the GPS in a practice session.

There was some trick to choosing the AP so the arithmetic was simpler....but obviously my memory is shot thru.

**nigel1** · 01-02-2011, 07:51

Quote:

Originally Posted by daddle

The assumed position does not affect the LOP accuracy, does it? It can be anywhere. It's helpful if it's close enough to be plotted on the current sheet. But it's choice does not affect accuracy of the final LOPs in any way. Or I've misunderstood (or forgotten) something. Any random position will do. So it might as well come from the GPS in a practice session.

There was some trick to choosing the AP so the arithmetic was simpler....but obviously my memory is shot thru.

If you are using sight reduction tables, pick the nearest tabulated latitude as your DR Lat, and use a nearest longitude figure so that when combined with the GHA, the resulant LHA is a whole number of degrees, and no minutes.

goboatingnow · 01-02-2011, 12:27

Quote:

If you are using sight reduction tables, pick the nearest tabulated latitude as your DR Lat, and use a nearest longitude figure so that when combined with the GHA, the resulant LHA is a whole number of degrees, and no minutes.

This really refers to the Air navigation reduction tables.

Dave

Lodesman · 01-02-2011, 15:53

Quote:

Originally Posted by goboatingnow

This really refers to the Air navigation reduction tables.

Dave

And the marine tables (pub 229)

**atoll** · 01-02-2011, 16:29

6-10 miles with a davis plastic then start looking for birds and clouds,though on hot days can vary more,lop better than noon,am+pm stars the best(selected stars)

North26West80 · 01-02-2011, 16:31

I sometimes do morning trips to the beach for sextant practice and I've finally gotten similar results to some of the previous posts. The nice thing about beach practice is that it is consistent and you can weed-out the little errors that have been turning 1 mile accuracy into 5 mile accuracy.

barnakiel · 01-02-2011, 17:55

Quote:

Originally Posted by daddle

The assumed position does not affect the LOP accuracy, does it? It can be anywhere. It's helpful if it's close enough to be plotted on the current sheet. But it's choice does not affect accuracy of the final LOPs in any way. Or I've misunderstood (or forgotten) something. Any random position will do. So it might as well come from the GPS in a practice session.

There was some trick to choosing the AP so the arithmetic was simpler....but obviously my memory is shot thru.

Nobody said it does. (And yet it may).

I said: " ... if we take GPS fix as our Assumed Pos then the error obtained in our Astro fix will be smaller than if we based it all on our cumulative DR / Astro positions ..."

Using GPS to get our Assumed Position will invariably give us an intercept relatively close to our actual position. If the LOP is a huge circle (read it is relatively well represented by a straight line) then no issues.

But imagine a day when the LOP is a small circle and you use the intercept method from a completely wrong Assumed Possition ...

(Sure thing - in such a case we will forego the intercept and just draw the circle! But frankly how many of us will know such tricks?)

Hehehe.

barnie

David_Old_Jersey · 02-02-2011, 02:32

A little bit of thread drift, but seeing as the Astroheads are all in one place (give or take

)...........

.........how long does it take to get a sight you are happy with (time? / attempts?). Appreciate that conditions will affect - but all things being equal blah blah.........

........and how long to crunch the numbers into a location?

Astro on my list. but it's a longggg list

But I still recall the satisfaction of making landfall (or simply any marks) using simply DR - the skill is never looking surprised

daddle · 02-02-2011, 08:10

Quote:

Originally Posted by barnakiel

Nobody said it does. (And yet it may).

I said: " ... if we take GPS fix as our Assumed Pos then the error obtained in our Astro fix will be smaller...

I thought you were saying the error in the sight would be less. But I see by error you meant the distance from the AP the the LOP. I meant that using ones actual position as the AP does nothing to improve the crooked hat.

Lodesman · 02-02-2011, 08:42

Quote:

Originally Posted by barnakiel

But imagine a day when the LOP is a small circle and you use the intercept method from a completely wrong Assumed Possition ...

(Sure thing - in such a case we will forego the intercept and just draw the circle! But frankly how many of us will know such tricks?)

AP isn't the centre of the circle - GP is. Unless you're taking stars that are right above you (which I believe we can all agree is fraught with inaccuracy and frustration) that is going to be one gigantic circle.

barnakiel · 02-02-2011, 17:59

Quote:

Originally Posted by Lodesman

AP isn't the centre of the circle - GP is. Unless you're taking stars that are right above you (which I believe we can all agree is fraught with inaccuracy and frustration) that is going to be one gigantic circle.

Again - you are correct some of the time. And, for most sailors, most of the time.

But I have sailed N to S of Equator with the sun doing just the opposite and for sights taken close to noon (my favourite method is to make one LOP before noon and then transfer it to my noon sight, if possible), as well as for the noon sight, a line DOES NOT represent the circle - exactly because the circle is too small - its center very close to your actual position. In fact the situation mentioned will be similar (so I guess) to any sight where the body is close to zenith. But I am not too much into other sights than of the Sun, hence my example.

Making some statements in astro navigation (also in other kinds of navigation, why not) we have either to avoid generalisations or else make them but use the infamous BUT, EXCEPT etc. statements on the side so that someone who is at the learning stage does not jump to false conclusions thinking what we state is the whole truth rather than a particular (even if the most common) case.

barnie

barnakiel · 02-02-2011, 18:18

Quote:

Originally Posted by David_Old_Jersey

.........how long does it take to get a sight you are happy with (time? / attempts?).

........and how long to crunch the numbers into a location?

(I only take sights of the Sun). Anyway:

I think it takes me from 5 to 20 minutes to get a sight, but more often 5 than 20. Alas, I take a couple of sights (and reject the extremes while averaging the 'good' ones), so normally it will be like 10 minutes.

I think calculating the intercept with formulas (and a plain Casio) takes me 15 to 20 minutes. Less, with a smart good Casio ;-)

I think it also takes me 20 minutes (often more) to calculate the intercept with tables. I do not do this often. But I have the tables onboard - just in case the calculator goes the way of the GPS.

In any case it would boil down to about one hour of sighting/calculating/plotting to get me a fix.

I think it takes less time if someone does their astro day in day out. I don't.

b.

Bash · 02-02-2011, 18:28

In the days of yore, an accurate-enough sight was anything close enough to the DR plot that you figured you could live with splitting the difference.

Kinda spoils the fun, once you've run the numbers on a noon sight, to have someone tell you that according to the GPS you're 8nm off.

My response: "Wow, those gizmos are getting more accurate all the time. Last one couldn't get within 12nm of my position."