Compass Error by Amplitude - Ships Nostalgia
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Compass Error by Amplitude

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  #1  
Old 19th May 2008, 03:36
NOVICE NOVICE is offline  
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Question Compass Error by Amplitude

Hi All,

This may sound simple to all you experienced Celestial types but to somebody like me who was weaned on GPS/Chart Plotters its all Swahili.

Will be going to school next year to learn the "Art" of Celestial Nav and i can honestly say i am pretty keen to do so, but i would like some idea of what the instructor is talking about before i go into class so have bought a few books on the subject, two of them are the 2008 Nautical Almanac and the latest edition of Nories Nautical Tables.

A friend of mine suggested i tackle a "Compass Error by Amplitude" to say i am stuck is putting it mildly...... while i can get as far as finding the Declination from the Almanac and apply the "d corr" after that im stumped.

I would like to able to solve this two ways, one is using the formula....
Sin Amplitude = Sin Declination/Cos Latitude
...but i have no idea how to input this into a Sci Calculator (which buttons to push??) I realise there is more to it and i have the proforma for this.

The second way is through the Almanac/Norries route. The same friend gave me an worked example of a compleated proforma for a Sun Rise as follows.....

Position: 21 36,S 114 36,E
Time: 2300 GMT
Ships Compass Magnetic: 047
Gyro: 047
Object Sun rise bearing: 065
Local Variation: 1/2 degree East

Instructions:

1. Go to Almanac with date and time and get declination. Dec = 23 26.3
2. Go to Nories and look up True Amplitudes using declination. AMP = 25.6
3. Therefore 90-25.6 = 64.4. Closest 1/2 degree = 64.5
4. True Bearing = 64.5 degrees.

I have no trouble applying the corrections with this, the problem is where do i find the TRUE AMPLITUDES in Nories, what pages??

I should point out that the date on the above example was 20/6/01 so it would have been a 2001 Almanac used.

Well that will do for a start, no doubt i will have more for you later, might be a good idea for a new thread "How to: Celestial Navigation", just a thought.

Any help will be greatly appreciated...
John
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  #2  
Old 19th May 2008, 06:23
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Orbitaman Orbitaman is offline  
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The amplitude pages should be just after the azimuth pages, at least they are in my copy of Nories
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Old 19th May 2008, 06:37
Billyly Billyly is offline  
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Get hold of a book called Principles of Navigation for Second Mates by TG Jones and all will be revealed
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  #4  
Old 19th May 2008, 06:46
oceangoer oceangoer is offline  
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Dear God,
No wonder sales of GPS have gone through the roof.

Learn your times tables first (that's a novelty for a Pom), then come and ask me for a job as a Cadet.
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  #5  
Old 19th May 2008, 07:13
NOVICE NOVICE is offline  
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Originally Posted by Orbitaman View Post
The amplitude pages should be just after the azimuth pages, at least they are in my copy of Nories
Hi, Thanks for the reply, i have two copies of Nories one from 1969 and the new 2007 A4 size and i cannot find any pages with "True Amplitudes" at the top.

As you can probably tell i am starting from scratch here.....
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  #6  
Old 19th May 2008, 07:16
NOVICE NOVICE is offline  
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Originally Posted by Billyly View Post
Get hold of a book called Principles of Navigation for Second Mates by TG Jones and all will be revealed
Thanks for the reply, have tried a search for the book but so far nothing comes up with that author, will keep looking.......
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  #7  
Old 19th May 2008, 07:19
NOVICE NOVICE is offline  
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Originally Posted by Punter View Post
Dear God,
No wonder sales of GPS have gone through the roof.

Learn your times tables first (that's a novelty for a Pom), then come and ask me for a job as a Cadet.
times tables???? pom???? job as a Cadet???? what help are you!!!!!!
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  #8  
Old 19th May 2008, 07:20
non descript non descript is offline
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Originally Posted by NOVICE View Post
I have no trouble applying the corrections with this, the problem is where do i find the TRUE AMPLITUDES in Nories, what pages??

Any help will be greatly appreciated...
John
John,

Well done for having the courage to ask - if it helps, I find them on Pages 556 & 557


Mark
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  #9  
Old 19th May 2008, 07:24
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John

Good luck with your studies you will always find help here on SN from the majority
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  #10  
Old 19th May 2008, 08:00
non descript non descript is offline
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A celestial body’s amplitude angle is the complement of its azimuth angle. At the moment that a body rises or sets, the amplitude angle is the arc of the horizon between the body and the East/West point of the horizon where the observer’s prime vertical intersects the horizon (at 90°), which is also the point where the plane of the equator intersects the horizon (at an angle numerically equal to the observer’s co-latitude). In practical navigation, a bearing (psc or pgc) of a body can be observed when it is on either the celestial or the visible horizon. To determine compass error, simply convert the computed amplitude angle to true degrees and compare it with the observed compass bearing.
The angle is computed by the formula: sin A = sin Dec / cos Lat. This formula gives the angle at the instant the body is on the celestial horizon. It does not contain an altitude term because the body’s computed altitude is zero at this instant. The angle is prefixed E if the body is rising and W if it is setting. This is the only angle in celestial navigation referenced FROM East or West, i.e. from the prime vertical. A body with northerly declination will rise and set North of the prime vertical. Likewise, a body with southerly declination will rise and set South of the prime vertical. Therefore, the angle is suffixed N or S to agree with the name of the body’s declination. A body whose declination is zero rises and sets exactly on the prime vertical. The Sun is on the celestial horizon when its lower limb is approximately two thirds of a diameter above the visible horizon. The Moon is on the celestial horizon when its upper limb is on the visible horizon. Stars and planets are on the celestial horizon when they are approximately one Sun diameter above the visible horizon.
When observing a body on the visible horizon, a correction must be applied. This correction accounts for the slight change in bearing as the body moves between the visible and celestial horizons. It reduces the bearing on the visible horizon to the celestial horizon, from which the table is computed.
For the Sun, stars, and planets, apply this correction to the observed bearing in the direction away from the elevated pole. For the moon, apply one half of the correction toward the elevated pole. Note that the algebraic sign of the correction does not depend upon the body’s declination, but only on the observer’s latitude.

If this does not help, then a deeper look at here may set you on the right course. But in any event, please do enjoy the learning curve, never be put off by anyone – after all you can do that yourself… and never be shy to ask; for not only is knowledge worth discovering, but it is also fun. In my case it is huge fun, as I know very little and every day it is not so much a learning curve, as a learning mountain,

Mark

Last edited by non descript : 19th May 2008 at 08:20. Reason: corrected the link
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  #11  
Old 19th May 2008, 08:17
NOVICE NOVICE is offline  
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Originally Posted by gdynia View Post
John

Good luck with your studies you will always find help here on SN from the majority
Cheers, thanks for the support
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  #12  
Old 19th May 2008, 08:27
NOVICE NOVICE is offline  
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Originally Posted by Tonga View Post
A celestial body’s amplitude angle is the complement of its azimuth angle. At the moment that a body rises or sets, the amplitude angle is the arc of the horizon between the body and the East/West point of the horizon where the observer’s prime vertical intersects the horizon (at 90°), which is also the point where the plane of the equator intersects the horizon (at an angle numerically equal to the observer’s co-latitude). In practical navigation, a bearing (psc or pgc) of a body can be observed when it is on either the celestial or the visible horizon. To determine compass error, simply convert the computed amplitude angle to true degrees and compare it with the observed compass bearing.
The angle is computed by the formula: sin A = sin Dec / cos Lat. This formula gives the angle at the instant the body is on the celestial horizon. It does not contain an altitude term because the body’s computed altitude is zero at this instant. The angle is prefixed E if the body is rising and W if it is setting. This is the only angle in celestial navigation referenced FROM East or West, i.e. from the prime vertical. A body with northerly declination will rise and set North of the prime vertical. Likewise, a body with southerly declination will rise and set South of the prime vertical. Therefore, the angle is suffixed N or S to agree with the name of the body’s declination. A body whose declination is zero rises and sets exactly on the prime vertical. The Sun is on the celestial horizon when its lower limb is approximately two thirds of a diameter above the visible horizon. The Moon is on the celestial horizon when its upper limb is on the visible horizon. Stars and planets are on the celestial horizon when they are approximately one Sun diameter above the visible horizon.
When observing a body on the visible horizon, a correction must be applied. This correction accounts for the slight change in bearing as the body moves between the visible and celestial horizons. It reduces the bearing on the visible horizon to the celestial horizon, from which the table is computed.
For the Sun, stars, and planets, apply this correction to the observed bearing in the direction away from the elevated pole. For the moon, apply one half of the correction toward the elevated pole. Note that the algebraic sign of the correction does not depend upon the body’s declination, but only on the observer’s latitude.

If this does not help, then a deeper look at here may set you on the right course. But in any event, please do enjoy the learning curve, never be put off by anyone – after all you can do that yourself… and never be shy to ask; for not only is knowledge worth discovering, but it is also fun. In my case it is huge fun, as I know very little and every day it is not so much a learning curve, as a learning mountain,

Mark
Thanks for that Mark it will certainly help, also thanks for the Amplitude pages, i found them in the 69 Edition right where you said, i had not looked in the old one at the time , still can not find them in the new edition though

John
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  #13  
Old 19th May 2008, 08:40
MM² MM² is offline
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It's a long time since I took an Amplitude...

...but here goes.

The table is at p556 in my Nories and p243 in Burtons. You enter with your (DR or Obs)) at the side and the declination (from Almanac) at the top. Both these values to the nearest degree. You then get the amplitude angle which is applied to East or West (body rising or setting). This gives you a bearing to compare with the one that you observed thus yielding the compass error.

If this sounds like gibberish it's because either I don't know what I'm talking about (always a possibility after all it is a long time ago) or you should do a little reading on the basic principles. Us old timers did this in most cases by reading Principles for Second Mates by T G Jones or Nicholls Concise Guide.

Anyway best of luck; at least you can use a calculator these days!
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  #14  
Old 19th May 2008, 08:59
non descript non descript is offline
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Originally Posted by NOVICE View Post
Thanks for that Mark it will certainly help, also thanks for the Amplitude pages, i found them in the 69 Edition right where you said, i had not looked in the old one at the time , still can not find them in the new edition though

John
Old is sometimes best... mind you not always
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  #15  
Old 19th May 2008, 09:13
NOVICE NOVICE is offline  
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Originally Posted by MM² View Post
...but here goes.

The table is at p556 in my Nories and p243 in Burtons. You enter with your (DR or Obs)) at the side and the declination (from Almanac) at the top. Both these values to the nearest degree. You then get the amplitude angle which is applied to East or West (body rising or setting). This gives you a bearing to compare with the one that you observed thus yielding the compass error.

If this sounds like gibberish it's because either I don't know what I'm talking about (always a possibility after all it is a long time ago) or you should do a little reading on the basic principles. Us old timers did this in most cases by reading Principles for Second Mates by T G Jones or Nicholls Concise Guide.

Anyway best of luck; at least you can use a calculator these days!
Thank you very much for that, p556 in Nories in my 69 edition . Have done a couple of exercises and they are working out ok. I was trying to find the True Amplitudes in my new 2007 Nories to have a go but could not find them, will not be throwing out the old 69 Ed now, just have to sew/tape it back together.

Cheers, John
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  #16  
Old 19th May 2008, 10:20
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Just checked in my copy of Nories page 556 and found "Spherical Traverse Tables Part One" Oops, sorry. It's dated 1923. Seriously, enjoy your trip down the Celestial Navigation path and always remember, what happens when the Satellites are knocked out??? Good Luck Mate.....................pete
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  #17  
Old 2nd July 2008, 09:53
John Beaton John Beaton is offline  
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Tonga's exposition is spot on - read it and inwardly digest. Like myself he must have been a lecturer in "Principles of Navigation for Second Mates.
The formula "Sin Amp = Sin Dec Sec Lat" must be your start point in order to get to understand the maths behind it all. To one of my generation I am disturbed to feel that you are inclined to go immediately to the calculator without gripping the fundamentals first. In my days on the bridge this amplitude calculation was a ritual strictly observed, as was the "Azimuth" for compass error. (Even when we had gyros!) Good Luck John Beaton.
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  #18  
Old 31st January 2009, 19:54
sidsal sidsal is offline  
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Fascinated by the messages on this thread !. Establishing the compass error was an integral part of every watch when I was at sea - many moons ago. There was no gyro and the compass was not 0 to 360 but the old notation N to E, S to E, S to W and N to W. We learnt to "box the compass" in 1/4 points too. Nicholls Concise Guides Vol 1 and 2 and Norries were our Bibles.
A few years ago a mad adventurer planned to raft across the Atlantic (raft made of gas pipes) He advetised for 3 geriatrics to accompany him and I was one of those selected. However he failed to get sponsorship so it didn't happen. However I brushed up on my astronav and found it enjoyable.
No caculators in my day. I once had a slide rule and the master used to refer to it as my "guess- stick".
Incidentally in the D Telegraph each day there are 3 little maths problems and at first I had forgotten how to do square roots. The daughter of the people next door is Head of Maths in a very well known fee paying school so I asked her to remind me how to do it. She didn't know and suggested a calculator !!
I eventually remembered - dead easy !
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  #19  
Old 1st February 2009, 10:58
Pilot mac Pilot mac is offline  
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sidsal,

got me worried now, but I think to find a square root you divide the log of the number by two?. or maybe not.... please put me out of my misery!

brgds
Dave
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  #20  
Old 1st February 2009, 11:32
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Nick Balls Nick Balls is offline  
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Navigation is an Art NOT a science! Keep it simple ...This is a routine job .
For practical reasons don't worry about the "fine" corrections such as "d"
Take the bearing then get the table of "True Amp" and using Latitude and declination determine how to apply it by considering if it is a rising or setting body (E or W) This is how it is "named"
Using the scientific formula is OK if you have a calculator. BUT you don't need one! Declinations do not change quickly and Latitude can be approximate.So dont get "lost" in calculation. Seafarers did this long before we all got hooked on GPS!
Do this a few times to get your confidence (This is how all navigation is perfected)
Remember the whole point of the exercise is to find out if your compass is OK.
Getting bogged down in the finer points makes it seem complicated. Its NOT!!
A nice thing to calculate to a fraction of a degree on a supertanker, on a small coaster in a heavy sea another matter . However at the end of the day its the result (better still a series) that counts. Are you going the right way!
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Old 1st February 2009, 11:44
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All,
Of course there is in addition to "Principles for Second Mates", "The American Practical Navigator 1936 By Bowditch Chapter XIV", I am not sure if there was a dissertation on Amplitudes in "Lecky's Wrinkles in Practical Navigation (I am still getting to grips with "Rosser's Arrangements, Sumner Lines etc.")
The Admiralty Manual of Navigation Vol 2/3?,also "Companion to Modern Navigation by Macilwaine and Morniment -1954"
Finally in my recall there were Amplitude Tables in Myerscough and Hamilton Rapid navigation Tables, the list is endless.
On a similar vein I always preferred Weirs Star Diagram for Azimuths.
Novice, please keep going and never stop asking questions as there will always be someone to help you.
Yours aye,
Slick

PS. F76 Nato "B" to those in the know.
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Old 1st February 2009, 11:46
Bill Davies Bill Davies is offline  
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All,
On a similar vein I always preferred Weirs Star Diagram for Azimuths.
Would agree.
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Old 1st February 2009, 13:00
roddy roddy is offline  
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This has probably been addressed in a previous thread but can someone remind me, and save me doing the research.

(1) Is it permitted to operate a Merchant Vessel that has no GPS

(2) Is Astro-navigation still part of the syllabus for unlimited certification.

Can remember one voyage with an extended period out of site of land, which included periods drifting, awaiting orders, and numerous course and speed alterations as these orders changed. we got the cadets to navigate (under discreet supervision) they got the land fall spot on. Would the same apply now? The gyro also threw a terminal wobbler early in the voyage so amplitudes and various other methods of monitoring compass error were also very much part of the equation.
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Old 1st February 2009, 13:06
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Originally Posted by Pilot mac View Post
sidsal,

got me worried now, but I think to find a square root you divide the log of the number by two?. or maybe not.... please put me out of my misery!

brgds
Dave
Yep, that's one way of doing it. I used to enjoy using log tables...strange I know but they always fascinated me.
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  #25  
Old 1st February 2009, 16:37
sidsal sidsal is offline  
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Dave: - Square roots. No tables involved. I am not much good at explaining things but this is how it works.
For example to get the square root of 676 .
From the decimal point put a line down between every second number - in this case - between the first 6 and the 7.
Draw lines around 676 as if you were doing long division.
Now take the 6 - what is the nearest square that fits below it. It's 2X2=4.
Put 4 under the first 6 and take it away - leaves 2.
Put 2 on the top of the line for records.
Now you've got the 2 and you bring down the 76 making - 276
alongside this to the left write down double the 2 which is 4.
Now you need a figure 4 something, which will divide into 276 .
You will find that it is 6 making it 46, so 6 times 46 = 276.
Put that 6 which you've found alongside the 2 above the top line ,so 26 is the square root of 676.~
QED
I myself couldn't follow the above if it was sent to me but it works. If it is a bigger number - say 676.857 then you do just the same, putting a line after each 2 numbers from the decimal point.
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