ABC tables

[ChiPete]

I'm making progress with the astro nav, using a combination of Reeds tables and Admiral Cunliffe's book. Things were going well, until I got to the ABC tables for azimuth.

No matter what I do when working through the examples provided, I can't get the numbers to agree with the given answers when using the ABC tables. Versines etc. I've cracked no problem.

Would anyone have any information regarding how best to use the ABC tables or solutions to problems they might have had?

I'm also one of the ''hit 'round the head with a blackboard rubber'' chaps during maths at school, but despite been trying to get my head around this since the weekend, I seem to have hit a wall.

Any assistance would be hugely appreciated.

Cheers,
Pete

 

[Refueler]

which tables are you using ? Nories, Yachtie specials or Burtons ?

Get yourself a copy of Burtons Nautical Tables ....... Nories belong on the Arc ! Burtons have very good explanation section as well as having the 4 quadrants on each ABC page.

As an ex Pro. Ships navigator - I was always amazed that others stuck to Nories ! Burtons actually realised that centurys move on !!

 

[ChiPete]

Thanks Refueler, I'm using Reeds 2010 Astro Navigation Tables by Lt Cdr Baker (Adlard Coles).

I'm just a tad disconcerted as (I'm given to understand) if you can read the tables (that others appear to be able to), it can't be that tricky, why can't I divine the right number??!!

 

[serini]

Can't believe what I have just read.

I am taking a youngster on a 1000 mile qualifier for the OCC in the spring and spent Christmas brushing up on my Astro. Decided to use versines and ABC since that is all that is in Reeds now. Using various astro nav software packages and taking sights off an astronomy prog. All self-consistent with the software packages and calculated heights from the versines. Small intercepts, all looking good. Azimuths from the ABC tables in Reeds a disaster. Average error 30 degrees. I have repeated the calculations many times. I'm not too much of a dunderhead with maths since my background is in physics. I have come to the conclusion something is very wrong. So I searched and found your post. Did you ever make any progress? Please send me a message if you can. My email is serini2@hotmail.com.

 

[VO5]

The use of ABC tables implies you need to have an accurate position....first of all....You work from that accurate position because ABC tables depend upon the three Arguments, which are Latitude, Hour Angle and Declination of the body (being observed) by Azimuth. It does not matter if you use Nories or Burtons. You resolve the relationship that exists between these three arguments in a structured approach to derive the Azimuth, which is a calculated true bearing.
This true bearing is then used to compare the real true bearing of the Celestial Body observed against the Compass Bearing of the same Celestial Body. The difference = the compass error, of course.
You then have to calculate the true Variation, the updated to current variation, that is.
This is done by consulting an Admiralty Chart for the area surrounding your position or if your position is between two magnetic areas then by interpolation between the two, if necessary.
Once you have an accurate Variation value then you apply this to the error (the difference between the magnetic bearing and the azimuth you have observed and calculated) by doing this you now obtain the Deviation.
You can then assess if the deviation you are experiencing is the same as that on your deviation card for that heading. (and a deviation card is obtained by swinging ship and its limitation is that the values are valid only for the Lat and Long in which the swing took place)............therefore......its use is to ascertain how far from the expected deviation is the deviation current in the position you find yourself, and therefore adjusting accordingly.

 

[VO5]

Further to my post above of yesterday, I cannot understand how it is that yachtsmen are expected to be able to take Azimuths since the compasses fitted on steering pedestals nowadays are domed and, as a consequence, are useless for the purpose of taking accurate bearings to check the compass.

The steering compass can only be accurately checked with an Azimuth if it is fitted with an Azimuth Circle, or an Azimuth Ring.

These are not obtainable at all, thus denying the aspirant yacht navigator the opportunity and experience of checking his compass properly and accurately.

The solution is to fit a ring independently to the compass, and for it to be lined up accurately to the fore and aft line of the vessel and the Azimuths taken from there and then applied to the Compass heading at the time.

Edson and Whitlock steering pedestals are fitted with a grab bar above the steering compass. It is to this grab bar that a gimballed Azimuth Ring or Circle can be clamped. This makes for easy reading and comparison for the calculation whose final object is the accurate determination of the deviation specifically for that heading for that Latitude and Longitude.

As I could not find one, I had one made specially by precision enginerrs working in conjunction with an instrument maker.

In this way I can take advantage of Azimuth calculations on the spot to work out the error of the compass using celestial bodies, either by taking bearings or by using the ring as you would a sundial.

When the Azimuth Circle or Ring is used as a sundial the bearing obtained must be reversed and added to the compass heading.

Sometimes the figure obtained is greater than 360 , in which case, by subtraction, it is reduced, for comparison to the true heading.

These Azimuths can be taken when stopped or underway and making way through the water.

As the speed of a sailing yacht is not that great the sighting as taken is considered valid without further adjustment by Traverse.

 

[Skylark]

Azimuth ring

Indeed an azimuth ring is just perfect for taking a bearing of a heavenly body while at sea for the purpose of checking the compass compared to a sight reduction. However, as you rightly say, such things are exceptionally rare on recreational craft. All isn't as bleak as you suggest, is it?

With a little care and attention and perhaps a little ingenuity, it is possible to take the bearing within a degree. With patience and experienced, you can get a reasonable eyeball bearing; I've read but not used a cocktail stick (acting as a sundial) in the top of the dome and you can call upon the portland plotter from the nav station. IMHO and, limited to recreational boats, experience, the accuracy is sufficent to warn of a compass calibration problem which, surely, is the purpose of checking?

 

[Lady Campanula]

Further to my post above of yesterday, I cannot understand how it is that yachtsmen are expected to be able to take Azimuths since the compasses fitted on steering pedestals nowadays are domed and, as a consequence, are useless for the purpose of taking accurate bearings to check the compass.

The steering compass can only be accurately checked with an Azimuth if it is fitted with an Azimuth Circle, or an Azimuth Ring.

These are not obtainable at all, thus denying the aspirant yacht navigator the opportunity and experience of checking his compass properly and accurately.

The solution is to fit a ring independently to the compass, and for it to be lined up accurately to the fore and aft line of the vessel and the Azimuths taken from there and then applied to the Compass heading at the time.

Edson and Whitlock steering pedestals are fitted with a grab bar above the steering compass. It is to this grab bar that a gimballed Azimuth Ring or Circle can be clamped. This makes for easy reading and comparison for the calculation whose final object is the accurate determination of the deviation specifically for that heading for that Latitude and Longitude.

As I could not find one, I had one made specially by precision enginerrs working in conjunction with an instrument maker.

In this way I can take advantage of Azimuth calculations on the spot to work out the error of the compass using celestial bodies, either by taking bearings or by using the ring as you would a sundial.

When the Azimuth Circle or Ring is used as a sundial the bearing obtained must be reversed and added to the compass heading.

Sometimes the figure obtained is greater than 360 , in which case, by subtraction, it is reduced, for comparison to the true heading.

These Azimuths can be taken when stopped or underway and making way through the water.

As the speed of a sailing yacht is not that great the sighting as taken is considered valid without further adjustment by Traverse.

I ( almost ) entirely agree with VO5.

A central part of the reason for this requirement in the YM Ocean Syllabus is to bring to students' attention the problems that can arise for small craft ocean navigators, should the steering compass be damaged or develop significant deviation. While not so significant in coastal sailing, it is an essential part of the ocean navigator's job to know the accuracy or otherwise of the compass(es).

Once convinced of the need for a means of compass-checking while on passage - each day, perhaps - the question becomes 'how', and Amplitude Tables offer an easy and accurate solution. They're readily found in Cdr Harry Baker's 'Astro Navigation Tables', together with lots of other goodies, for little money.

How does one use them on any specific boat? That's where the personal involvement comes in, and most peeps make or adapt a form of 'dumb compass' to take visual relative bearings.

Here's mine:

When quizzed during the 'viva' examination by a gruff old YM Ocean Examiner on this aspect of my practices, I produced the above - cobbled together from a 'dumb compass ring' found in a scrapyard - and my examiner dived into the depths of a cupboard, producing his own version, made up from bits of clear plastic. He was more than pleased to find someone 'echoing' his own approach, and there were no more 'challenging' questions.....

...apart from checking my sums in a sample of the Compass Checks I'd logged to determine if I knew my +sses from my -lbow.




ps Hands up all those who have EVER had a compass swing/residual error table done on their boat....

 

[serini]

Hi VO5

I understand your post but the Azimuth was required not just to check a compass bearing (and why did Reeds stop listing amplitudes to do this anyway) but because in 'Reeds Heavenly Bodies' the sight is reduced with versines to find the altitude and ABC tables to find the Azimuth. I have carried out these calculations both with an (within a couple of nm) estimated position and an assumed (within 25nm) position. I have no problem at all with the Versine calculation agreeing within a mile or so with the other (software) methods I am using. The ABC calculations though have all been out by about 30 degrees. This is exactly the same problem as in the original post at the start of this thread. I do not have Norries to check the ABC tables in the Reeds publication but that is my next step. I think that there may have been a misprint in 'signing' A or B or perhaps the tables have been abridged to such an extent that they are useless. My 'control' methods are agreeing with each other, the problem is in the ABC calculation. Thanks for your interest, Mark

The use of ABC tables implies you need to have an accurate position....first of all....You work from that accurate position because ABC tables depend upon the three Arguments, which are Latitude, Hour Angle and Declination of the body (being observed) by Azimuth. It does not matter if you use Nories or Burtons. You resolve the relationship that exists between these three arguments in a structured approach to derive the Azimuth, which is a calculated true bearing.
This true bearing is then used to compare the real true bearing of the Celestial Body observed against the Compass Bearing of the same Celestial Body. The difference = the compass error, of course.
You then have to calculate the true Variation, the updated to current variation, that is.
This is done by consulting an Admiralty Chart for the area surrounding your position or if your position is between two magnetic areas then by interpolation between the two, if necessary.
Once you have an accurate Variation value then you apply this to the error (the difference between the magnetic bearing and the azimuth you have observed and calculated) by doing this you now obtain the Deviation.
You can then assess if the deviation you are experiencing is the same as that on your deviation card for that heading. (and a deviation card is obtained by swinging ship and its limitation is that the values are valid only for the Lat and Long in which the swing took place)............therefore......its use is to ascertain how far from the expected deviation is the deviation current in the position you find yourself, and therefore adjusting accordingly.

 

[VO5]

I ( almost ) entirely agree with VO5.

A central part of the reason for this requirement in the YM Ocean Syllabus is to bring to students' attention the problems that can arise for small craft ocean navigators, should the steering compass be damaged or develop significant deviation. While not so significant in coastal sailing, it is an essential part of the ocean navigator's job to know the accuracy or otherwise of the compass(es).

Once convinced of the need for a means of compass-checking while on passage - each day, perhaps - the question becomes 'how', and Amplitude Tables offer an easy and accurate solution. They're readily found in Cdr Harry Baker's 'Astro Navigation Tables', together with lots of other goodies, for little money.

How does one use them on any specific boat? That's where the personal involvement comes in, and most peeps make or adapt a form of 'dumb compass' to take visual relative bearings.

Here's mine:

When quizzed during the 'viva' examination by a gruff old YM Ocean Examiner on this aspect of my practices, I produced the above - cobbled together from a 'dumb compass ring' found in a scrapyard - and my examiner dived into the depths of a cupboard, producing his own version, made up from bits of clear plastic. He was more than pleased to find someone 'echoing' his own approach, and there were no more 'challenging' questions.....

...apart from checking my sums in a sample of the Compass Checks I'd logged to determine if I knew my +sses from my -lbow.




ps Hands up all those who have EVER had a compass swing/residual error table done on their boat....

That is a handsome bit of kit you have there. I like the colour.I think I will spray paint mine the same.Mine is gimballed. Unfortunately I cannot post a picture as mine is out of reach at the moment.

Hands up ? Me.

I work out all my deviation cards by swinging ship at 10 degree intervals when I can find a quiet anchorage with no wind at all. Then for any heading it is easy to deal with deviation because interpolation in tenths is easier than points. When I have time I work out the deviation for each degree and write it up as a table that I stick to the underside of the lid to my chart table. The two columns are written up in numbers 1/4 " in size making it easy to spot at a glance by just lifting the lid quickly.

I sort of abandoned points years ago although I was made to strictly learn headings relative for night sailing using points if theoretically encountering a fully rigged vessel. It was part of the Syllabus for the Ocean Ticket at the time and all Merchant Navy Deck Officers were obligated to learn it too. I found it very difficult and unfamiliar at first until I got used to it. Nowadays I am aware of points and use them sort of subliminally but not concretely for conceptually dealing with wind direction only.

 

[VO5]

Hi VO5

I understand your post but the Azimuth was required not just to check a compass bearing (and why did Reeds stop listing amplitudes to do this anyway) but because in 'Reeds Heavenly Bodies' the sight is reduced with versines to find the altitude and ABC tables to find the Azimuth. I have carried out these calculations both with an (within a couple of nm) estimated position and an assumed (within 25nm) position. I have no problem at all with the Versine calculation agreeing within a mile or so with the other (software) methods I am using. The ABC calculations though have all been out by about 30 degrees. This is exactly the same problem as in the original post at the start of this thread. I do not have Norries to check the ABC tables in the Reeds publication but that is my next step. I think that there may have been a misprint in 'signing' A or B or perhaps the tables have been abridged to such an extent that they are useless. My 'control' methods are agreeing with each other, the problem is in the ABC calculation. Thanks for your interest, Mark

Hello Mark, I understand your problem.

Let's see...

I use both Burtons and Nories.

When I was a deck officer in the Merchant Marine the view then was that Burtons were sort of Royal Navy stuff and Nories more Merchant Navy. I suppose this was due to the fact that the colleges teaching the syllabuses habitually were staffed by MN Masters and Extra Masters with a preference for Nories, although in the teaching cadre at the time was included a retired RN Commander. I rarely use Burtons nowadays. My preference is for Nories for the simple reason that they are tabulated with heavier print that makes the more readable. What you are experiencing (from what I gather of your current difficulties) is that there is a misprint and the values have been inadvertently printed the wrong way round or there are intermittent printers devils present embedded in the tabulation.

Brown Son and Furguson of Glasgow are publishers of nautical textbooks of many years of experience with a very high reputation who can supply you with a copy of Nories. I think you ought to acquire these tables and abandon your current ones. I think you will find the print and layout easier. It will become second nature to you I promise if you persist in familiarising yourself fully by careful and diligent practice.

Do let me know how you get on.

 

[Lady Campanula]

You'll find what you need in the current online version of Bowditch ( American Practical Navigator )

here


Simply scroll down, select, then download. The relevant chapter wanted is No. 17 and the relevant tables are in Nos. 22 and 23.

Enjoi!

 

[JayBee]

I haven’t got Reed’s ABC tables, but with a simple scientific calculator, the table entries can be spot checked using the following formulae:

A = tan Lat/tan LHA

B = tan dec/sin LHA

C = 1/(tan Az cos Lat)

A is:
+ve in the range of LHA 0-90, 270-360
-ve in the range of LHA 90-270

B is :
+ve when Lat and dec are of contrary names
-ve when Lat and dec are of the same names

Entering the tables with C produces an azimuth angle in each quadrant.
The correct quadrant will usually be obvious, but the rule for North Latitudes is:

Body rising:
+C SE quadrant
-C NE quadrant

Body setting:
+C SW quadrant
-C NW quadrant

Another vote for Burton's Tables, BTW

 

[serini]

Thanks for the last very helpful posts! I have Bowditch but it is on the boat (not with me here) so the web listing is very helpful as is the formula. I will report back!

 

[timbartlett]

It's a bit tangential to the way this thread has developed, but most of the better yacht compasses with domed tops have a vertical pin sticking up from the centre of the card. It's called a shadow pin ... and the shadow it casts gives a very good (reciprocal) bearing.

Another way to do it, if you haven't got a shadow pin, is to stand with your back to the sun, between the sun and the compass, and line up the reflection of the sun on the dome with the centre of the compass, then read off the (reciprocal) bearing from the far side of the compass card.

 

[serini]

ABC Tables

OK, I have the answer.

The tables in Reeds do not title what is in the rows. So the collumns are titled but the rows are not. The rows all have the same values to the left hand side (0 to 66) but in table A it is lat, table B it is Dec and table C back to lat all made clear by JayBee's post on the formula.

So I screwed up (consistently I might add!) using Dec in table C.

It also helps to know that 'C' is applied FROM South to West or East or FROM North to West or East IMHO the notes and labels could be much improved.

Is the balance of opinion that Burtons is the best set to buy?

Thanks for your help people,

Mark

 

[VO5]

It's a bit tangential to the way this thread has developed, but most of the better yacht compasses with domed tops have a vertical pin sticking up from the centre of the card. It's called a shadow pin ... and the shadow it casts gives a very good (reciprocal) bearing.

Another way to do it, if you haven't got a shadow pin, is to stand with your back to the sun, between the sun and the compass, and line up the reflection of the sun on the dome with the centre of the compass, then read off the (reciprocal) bearing from the far side of the compass card.

I have read your post and I am puzzled as to why you would think this thread has developed a bit tangental. I think it has remained perfectly on topic and very very interesting.

But here are a couple of tangents if you like.

I have yet to see a steering compass with a pin stuck in it. I have a Plastimo on my Whitlock pedestal. It is several years old, has not leaked, the card is perfectly dead beat and for me, easy to adjust magnets if sailing in high latitudes. The light has a dimmer. It has a snap shut / open cover that I find very useful at night. But it would not be suitable for taking azimuths as the batons on the card are ideal for steering but not for deadly accurate Azimuth Observation.

There again some of us on this site are devoted to maintaining the Science and Art of Classical Navigation and for this reason burden ourselves with Sextants, Chronometers, Stopwatches, Azimuth Circles, Nautical Almanacs and Nories Tables. Are we forbidden to mention these or to post pictures out of common interest and relevant to the topic ?

Not everyone has full flag sets, pillot ladders, bells, semaphore flags, Aldis, full storm gear, aqualungs, hand leads, striking clocks etc., arcane though you might think these are.....we are all entitled to choice.

If some of us choose to run our yachts as a replicas of windjammers in miniature for example, we are surely free to do so and to derive immense pleasure from it I shouldn't wonder..

And a Happy New Year to you.

And my vote is for NORIES, by the way.

 

[JayBee]

Shadow Pins

Hi VO5
I think you might be pleased to find room on board for a Sestrel Moore compass. They come up on ebay from time to time. There is provision to mount an external shadow pin and an alidade, also a set of corrector magnets to play with if you have any significant deviation.

 

[VO5]

Hi VO5
I think you might be pleased to find room on board for a Sestrel Moore compass. They come up on ebay from time to time. There is provision to mount an external shadow pin and an alidade, also a set of corrector magnets to play with if you have any significant deviation.

I have one !
Its in a cardboard box in the garage, you have just reminded me.
I may decide to rig it inboard as a second compass.
Finding it is the thing...its like HM Dockyard Stores in there..

 

[Lady Campanula]

There again some of us on this site are devoted to maintaining the Science and Art of Classical Navigation and for this reason burden ourselves with Sextants, Chronometers, Stopwatches, Azimuth Circles, Nautical Almanacs and Nories Tables. Are we forbidden to mention these or to post pictures out of common interest and relevant to the topic ?

Not everyone has full flag sets, pillot ladders, bells, semaphore flags, Aldis, full storm gear, aqualungs, hand leads, striking clocks etc., arcane though you might think these are.....we are all entitled to choice.

That's the stuff! A true enthusiast....

I'll bet you even have a selection of Residual Deviation Cards!!


Now here's a thought. Did you know that the Master Compass on Vulcan V-bombers was 'swung', reading the precisely-calibrated theodolites to 1/100th of a degree?

And can you imagine rowing out, of a winter's morning, onto Southampton Water near Calshot, or Plymouth Sound near Mountbatten, to swing the Main and Standby Compasses on a Sunderland flying boat?

A navigator's job is never done...