Celestial correction for Great Lakes?

[El Pinguino]

Several points.... horizontal parallax of the sun is 0.1'.... the difference caused by being 500 feet further away from the centre of the earth than a navigator in mid Atlantic is somewhere south of very very tiny...

Dip involves the horizon that you can see.. doesn't matter if that horizon is on Lake Michigan, Lake Titicaca or the Dead Sea...... it does not involve the surface of an ocean several thousand miles away from your location.

'Personal Error' should not be random and not varying by 4' or more.

Typically it would be -'frinstance - where two people are using the same sextant... one will measure the index error a consistent 1' on the arc... the other will consistently read it as 1' off.

If it is not an error in principle you are dealing with here the most likely cause would be abnormal refraction... not uncommon near the coast.... but that would not be affecting a sight at a reasonable altitude ( over 20* ) taken using an artificial horizon. It would also vary from day to day.

[bletso]

Are you sure you have recorded or have a correct assumed position?

[Bigjim]

Originally Posted by bletso

Are you sure you have recorded or have a correct assumed position?

Our exact position was N42:21.4 W87:49.5

The assumed position was N42 and the Longitude would be the closest position to deliver a whole number for LHA. Assumed Longitude changes for each LOP based on GHA.

[Bigjim]

Originally Posted by El Pinguino

Several points.... horizontal parallax of the sun is 0.1'.... the difference caused by being 500 feet further away from the centre of the earth than a navigator in mid Atlantic is somewhere south of very very tiny...

Dip involves the horizon that you can see.. doesn't matter if that horizon is on Lake Michigan, Lake Titicaca or the Dead Sea...... it does not involve the surface of an ocean several thousand miles away from your location.

'Personal Error' should not be random and not varying by 4' or more.

Typically it would be -'frinstance - where two people are using the same sextant... one will measure the index error a consistent 1' on the arc... the other will consistently read it as 1' off.

If it is not an error in principle you are dealing with here the most likely cause would be abnormal refraction... not uncommon near the coast.... but that would not be affecting a sight at a reasonable altitude ( over 20* ) taken using an artificial horizon. It would also vary from day to day.

We check for index and other errors on every sighting. If the index error is lower than zero we add it to the reading. If it is higher, we subtract.

Again, we used 6 different sextants to take the sightings, not one.

[Benz]

Jim,

It's puzzling, especially given that the error is consistent.

Here are some personal errors that have made me bungle in the past. Not saying you've made them, but worth checking out and eliminating.
1) Forgetting to add and subtract in 60s.
2) using Deg/min/sec in one place and Deg/min/tenths in another (this usually gets me when I transfer the fix to the chart from the plotting sheet)
3) Bungling the increments and corrections in the H.O. 229. Tricky little devils, those increments and corrections--and I still haven't figured out the 'double-second difference..."
4) reading the wrong minute on the timepiece (easier to do on a dial than an digital display).

Anyhow, it's and interesting conundrum, and I hope you sort it out and tell us.

[Bigjim]

Originally Posted by El Pinguino

...'Personal Error' should not be random and not varying by 4' or more...

If you can say this you've never taken a sighting on a pitching deck, in the rain or in anything but ideal weather and circumstances.

Yes, personal error can be "random" and can vary more than 4'.

Especially, if you are taking shots alone and you are shooting the moon which moves very fast.

In the 70s, I shot sun lines frequently for practice and usually hit within 7 miles of our actual position consistently. But this was in the days of chronometers and Loran.

You can get wild index errors by simply leaving your sextant in the sun or shade on a hot day. Last March, my sextant got 4' off arc just by going from the heated classroom to the deck of the club where the temperature was 40 degrees colder. Had we had time to let the sextant adjust to the temperature the error may have dissipated. Metal sextants are much less susceptible to this issue.

[Adelie]

“Personal error” is not the random effects of temperature change and an unsteady base to shoot from.

“Personal error” is net of the systemic errors that a particular observer has due to the way they hold the sextant, turn the drum, read lines on the drum, whether or not they anticipate the body kissing the horizon, . . .

This has been discussed some at NavList. It’s probably only measureable for advanced practitioners. For beginners and intermediate practitioners it won’t be steady enough to measure.

[Bigjim]

Dip for all sightings was 12 feet or -3.4

IC is noted or zero if not.

Click on thumbnails to see larger images

Here is our Sun sight:




Here is our Moon sight:



Here is the Mars sight:

[Bigjim]

Originally Posted by Adelie

“Personal error” is not the random effects of temperature change and an unsteady base to shoot from.

“Personal error” is net of the systemic errors that a particular observer has due to the way they hold the sextant, turn the drum, read lines on the drum, whether or not they anticipate the body kissing the horizon, . . .

This has been discussed some at NavList. It’s probably only measureable for advanced practitioners. For beginners and intermediate practitioners it won’t be steady enough to measure.

I think "personal error" is whatever a "person" can do incorrectly, including obtaining a bad reading on a pitching deck.

By the nature of human input, it is hard for me to see any type of "error" that would be so constant and consistent that it would happen the same way every time.

In my last class I had 15 students all working through the math together. So, I can say without doubt that we pulled the numbers from the Almanac and HO 249 properly, added and subtracted them properly and achieved a proper result each time and multiple times. I have NEVER experienced a situation where someone (or 15 someones) could make the same error all the time.

In fact, I tell my students to look out for specific types of errors and double-check all numbers. One telltale error is when you obtain an intercept greater than 60 miles. Such an error usually indicates you either chose the wrong Assumed Position or you pulled numbers incorrectly from one or more of the books.

In fact, one of my students had such a result and when we went back through the math and numbers we discovered that he did his GHA and LHA wrong. Once he obtained the proper LHA his math resolved to a more believable LOP.

[Adelie]

Originally Posted by Bigjim

I think "personal error" is whatever a "person" can do incorrectly, including obtaining a bad reading on a pitching deck.

By the nature of human input, it is hard for me to see any type of "error" that would be so constant and consistent that it would happen the same way every time.

In my last class I had 15 students all working through the math together. So, I can say without doubt that we pulled the numbers from the Almanac and HO 249 properly, added and subtracted them properly and achieved a proper result each time and multiple times. I have NEVER experienced a situation where someone (or 15 someones) could make the same error all the time.

In fact, I tell my students to look out for specific types of errors and double-check all numbers. One telltale error is when you obtain an intercept greater than 60 miles. Such an error usually indicates you either chose the wrong Assumed Position or you pulled numbers incorrectly from one or more of the books.

In fact, one of my students had such a result and when we went back through the math and numbers we discovered that he did his GHA and LHA wrong. Once he obtained the proper LHA his math resolved to a more believable LOP.

I think you are missing my point. My point is that the phrase "personal error" has a specific jargon meaning in CelNav and certain other pursuits. You are using it in the general language way and Penguino was using it in the CelNav jargon manner.

Jargon is technical or professional terminology or characteristic idioms of a special activity or group.

The 2002 edition of Bowditch defines "Person Error", "Personal Correction" or "Personal Equation" as "A systematic error in the observation of a quantity due to the personal idiosyncrasies of observer".

The regulars at NavList normally use it in the jargon manner defined above and that was the use I was trying to communicate and that Penguino seemed to be using.

The following link has a good discussion about personal error as defined above. NavList: Personal equation in timed observations (143131)

This is another interesting discussion. NavList: Sextant Observation Errors (000752)

All that said, just because there is jargon meaning to the phrase does not mean that you are required to use it. But perhaps now you can better recognize and understand others when they use the jargon meaning.

[SeanPatrick]

Bigjim, thank you very much for posting your data. I have worked through it and here are my preliminary results:


The good news is: it appears you have not made any [major] errors in your reductions. I got the same results you did ... namely, a fix that is to the northeast of your actual position. Here is what I came up with based on your data:





...and the plot:



...These results are based on a lower limb sight of the Moon (which you did not specify), and I will need some time to try and work out what else might have gone wrong here.


Further analyses to follow...

[El Pinguino]

Originally Posted by SeanPatrick

Bigjim, thank you very much for posting your data. I have worked through it and here are my preliminary results:


The good news is: it appears you have not made any [major] errors in your reductions. I got the same results you did ... namely, a fix that is to the northeast of your actual position. Here is what I came up with based on your data:





...and the plot:



...These results are based on a lower limb sight of the Moon (which you did not specify), and I will need some time to try and work out what else might have gone wrong here.


Further analyses to follow...

First up.... strange things going on here... t'other day I clicked on a thumbnail and it remained a thumbnail... now all I can see is 3 proformas with no detail.. weird.

Moving right along... looking at your plot above... all bodies are to the SE.... two have 'intercepts' that are away.... one of them is 'towards'.

This is a different set to the 3 in post #10 but - strangely - the position where those three intersect is also NNW in what appears the same position as the one you have established.

So I am left with only two ideas....one remains abnormal refraction... you can check that but need a body higher than 60* to do it.

The other would be an incorrect 'D.R.' position but that has already been checked...

So I am left with refraction...

One other... failure to allow for precession and nutation but I don't see that making that much diff and aren't to sure what epoch the air tables being used are for.... however it can be as much as 4' towards the end of the life of a volume....

You could try reworking the sights longhand using the Haversine formula....

[El Pinguino]

Originally Posted by Adelie

.......

This is another interesting discussion. NavList: Sextant Observation Errors (000752)

All that said, just because there is jargon meaning to the phrase does not mean that you are required to use it. But perhaps now you can better recognize and understand others when they use the jargon meaning.

That is a very interesting paper..... I had never seen the effect of warm air over cold water or V/V quantified like that before.. I only knew it happened... typicalyl close(ish) inshore...

Consider this..

'2) For every ten degree increase in the surface air temperature
above the value of the surface water temperature, the value of the dip is to
be decreased by 1 minute of arc.'

So if Lake Michigan sea temp is 45*F and the air blowing off the land is 75* then the correction to dip is -3'.

So let us say... sextant altitude is 40* 30' .... dip correction from your tables is - 4'.... from above ^^ reduce dip correction by -3' dip to use is - 1'.
Now instead of having a corrected altitude of 40* 26' we have a corrected altitude of 40* 29' .... the altitude is greater... so we are closer to the body's G.P.... 3' closer.....

( I have ignored all the other corrections in the interests of clarity.. and I hope I have this the right way round...)



So.... taking the two sights that had an intercept of maybe 5' away.... we now have intercepts of only 2' away........ looking better all the time....

Also from that paper..

'Random Error
Individual observations by sextant taken on board a rolling, yawing,
pitching vessel often contain large values of random observational error.'

[bletso]

I ran your numbers on mars and got 7.0 A @ Zn 155° from your actual position

I may just do them all and plot them out. I use a 1st surface mirror for an AH and am at 550 ft above sea level. When I take my time and level the mirror perfect it not unusual for me to get 0 to a couple tenths off from my actual position.

Your Ho is fairly low so did you run corrections for temp and pressure? I ran your numbers on mars using 20°C at 30mb

[Bigjim]

Originally Posted by bletso

I ran your numbers on mars and got 7.0 A @ Zn 155° from your actual position

I may just do them all and plot them out. I use a 1st surface mirror for an AH and am at 550 ft above sea level. When I take my time and level the mirror perfect it not unusual for me to get 0 to a couple tenths off from my actual position.

Your Ho is fairly low so did you run corrections for temp and pressure? I ran your numbers on mars using 20°C at 30mb

We ran all the corrections pertinent to the body and the temperature and pressure corrections for that particular day were pretty tiny. For app alt. higher than 20 degrees those corrections are all less than 1'.