Question for Navigators

John Rogers
10th January 2006, 21:37
My question is on charts and navigation devices. Now that GPS is the norm for navigating and getting a fix, what type of datum does the navigator use in going from point A to point B. I have been reading about the different data being used WGS 84,NAD 83,and UTM,and the charts not matching the GPS. What is the standard if there is one.
John.

Ian Harrod
11th January 2006, 02:50
The datum used on a particular chart will be in the notes under the title. Most common currently is WGS84 but there are many others in use. A GPS will have a default datum (sayWGS84) but this can be changed by the user to another datum to suit the chart in use.

John Rogers
11th January 2006, 03:15
Thanks for that info,but what is used internationally for the standard system to use for nautical charts. I have two GPS units,one I use on my laptop and then I have my handheld I use for land navigation. For something to do in my retirement I'm reading all about celestial navigation and trying to learn how to shoot the sun and the stars,it helps me and the gray matter from getting OLD TIMERS disease. Just wondering if there is a standard DATUM. Thanks again for your post.
John.

Ian Harrod
11th January 2006, 03:49
Most charts in use today are WGS84. However there are many other datums in use on older charts and this is only updater when a new edition is published. This is where your problem arises! Charts of very busy areas and areas of high development (North Sea, Channel, port approaches etc) may have a new ed every year or two but the are many areas of the world where the current edition may be 20 to 30 years old. Small corrections are issued for these charts but the datum will not be changed until the new ed.

Australian charts and most other international charts, have the datum printed under the title and a notation in the margin that "WGS84 derived positions may be plotted diectly onto this chart".

John Rogers
11th January 2006, 03:56
Thanks Ian,that cleared thing up for me. On reading my books on navigation I must of read at least four or five different ways to determine ones position,going back to the old way,the present way and the NATO way of dividing up the world in different grids.
John

paul0510
11th January 2006, 10:37
..if you want to shoot the sun & stars, John mylad, you'll need the latest and, in my opinion, the best piece of GPS equipment money can buy....

The General Purpose Sextant

....anything else takes the fun out of navigation !

Chris Field
11th January 2006, 14:07
I completely agree with Paul- the (admittedly wonderful) GPS is no fun whatever- whereas the sextant gives huge satisfaction. One (not really small )point I used to make to new-chum mates is that there are still many parts of the earth's surface such as Papua New Guinea where charts bear only a vague resemblance to actual coasts, except near main ports such as Lae and P. Moresby, where the lats and longs are usually some miles away from those shown on GPS. In these cases, my prime requirement was to get the young guys to ensure that they were happy about the measured distance off dangers rocks/islets etc- using old-fashioned chartwork with or without sextant-and to virtually forget the GPS. We often found that the shapes of coastlines shown on radar were very different from those on the charts- so back to the non-electronic methods!

John Rogers
11th January 2006, 15:57
Chris and Paul, what you suggest is exactly what I plan to do,I have purchased four good books on the subject and I have been shopping around the web for used sextants, since a lot of the owners think they are selling a antique they are asking a lot of money for them. Since I am land locked in the middle of the states its hard to find a nautical shop in my area. Would you beleive it I found the plans on how to build one out of a CD disk, two small mirrors, and three Lego blocks, as I am only tinkering around I just might buy the hard plastic one which people say is fine.
Thanks for your input guys.
John.

lakercapt
11th January 2006, 16:49
On the odd occassion when we left the "lakes" and did a trans Atlantic voyage I played a real villian.
Took the fuses out of the Loran "c" and sat/nav and had the mates do the navigation the old way .By celestrial navigation. At first they did not care too much for that but soon the challenge got to them and a sort of competition took place each day.
Not that I wished to be nasty but that is a skill you should never forget if you pretend to be a navigator.
In the Pacific many charts still to this day use the hydrographic data found by that great navigator Capt James Cook

Ian Harrod
12th January 2006, 02:53
The chart of the Great Australian Bight, BA1052, in use until the early 1990's still had Matthew Flinders 1802 voyage in HMS Investigator as the main contributors to the chart.

John Cassels
12th January 2006, 09:42
G'day Bill,

Hope you put the fuses back during fog , haze or any other time when you
couldn't see the horizon.

Whilst I agree that the old navigational skills are being lost these days ,think
we should all look back at what morning sights really were especially on the
North Atlantic.

2nd.mate came up at around 0830-0845,wound chronometer,made cup of tea,
grabbed sextant and went out to bridge wing , hoping sextant errors were the
same as the last time. If he was lucky , he could see the horizon. If not ,it was
down to the main deck. If that didn't work - end of story.
Assuming he could see horizon from the bridge wing, he then tried to pretend the
ship wasn't rolling 30 degrees ,bouncing and vibrating and took his 3 sights hoping
the 3rd mate could hear him when he shouted time.
Back to the chartroom and out with the very worn and dogeared copy of Norrie's
tables and started work. Height of eye correction ? - well that was anyones guess.
Got his longitude (or intercept) and ran the p/l up to noon.
Back up for latidude at noon ,ran latitude to noon and bingo ...ships position and
between you and the 3rd.mate , if it was within 3 miles of where the ship really was
you were happy. You just hoped that the Mates star sights would be somewhere
near the D/R he ran up using your noon sights.
In my humble opinion it was more luck that anything else in getting the lower limb
on the horizon 3 times after another when the ship was (nearly)upright.

Last year I bought one of these GPS recievers that connects into the laptop. the
thing is no bigger that a packet if ciggies , gives me a position to 3 decimal points
of minutes and continuous speed/course over the ground.

Am not saying the old skills in using a sextant should be forgotten but the accuracy
of any position obtained was dependant on too many factors especially in anything
but perfect weather.

JC

Ron Stringer
12th January 2006, 10:09
John, as someone that didn't get involved in the sight-taking exercises but who was a close observer of both those activities and the (occasionally heated) discussions that followed, I agree that there it often needed a good pinch of salt to be taken with the positions placed on the chart. On one pre-Christmas trip from Montreal to Grangemouth the sky was so consistently overcast and rough that no sights were obtained during the crossing, neither of the sun nor the stars. Approaching the UK the only indication of position came from the D/R assisted by frequent radio D/F bearings of the radiobeacon on the Butt of Lewis. When Cape Wrath eventually appeared on the radar in the right place, were we all relieved? You bet, especially the 2nd Mate (his D/R) and me (my D/F bearings).

Ron

Chris Field
12th January 2006, 22:50
As my earlier note will show, I am and always will be a great believer in those old sextant style methods- just remember to "draw" a circle of possible error around any such position, then you're very unlikely to hit that isolated rock...
Why on earth do marine GPS machines bother with the 3 decimal points of a minute/mile? That indicates an accuracy of six feet or so- whereas the printed lines of coasts etc on charts will be much wider than that. A sounding on a chart occupies an area as big as a soccer pitch - so why bother with the superior accuracy of 3 decimal points?
(I will also maintain, however, that if GPS had existed in 1769 Capt Cook would have demanded one!)

lakercapt
13th January 2006, 00:02
Still have my old sextant and its a vernier one not one of the later models with a micrometer.
Had it out recently and still without reading up on it could correct for index error, error of perpendicularity and side error. Guess the instructors at Leith pounded it in enough so they woulld be rightly be proud(whereever they are)

John Cassels
13th January 2006, 11:00
Chris.

You are correct , of course, 3 meter accuracy is not needed but for me , it's
the sheer facination of a GPS. Every time when I use it on the yacht , I can't
stop thinking back to what we used to have to do to get the same info which
would be hopefully correct to 3 NM.
On the same vein, when I was up for Master's in 1973 one of the other blokes
(ex. P&O passenger division) had the first electronic calculator I had ever seen
It was a sinclair ,it was huge , had a very small red LED readout, could only
add/subtract/multiply and divide and cost him a fortune - in those days.
It wasn't necessary, but he didn't have to get the log tables out to do a simple
calculation.
Most of the ships I sailed on had decca. a few had Loran and none had satnav.
so I also grew up with only sextant navigation but my sextant is now where it
belongs - under the bed.

jc

paul0510
13th January 2006, 12:32
...the companies I worked for actually provided the sextants, which was just as well seeing as how they were (and still are) pricey pieces of specialised engineering. Although , today, I wish I had one as souvenir and, undoubtedly where I live in landlocked Bavaria, a curiosity. But, John C, it wouldn't be gathering dust under the bed but hanging on the wall in a case with the alhidade (http://www.answers.com/topic/alhidade) set to e.g. to the angle measured by the sun's lower limb on the 22nd July 1969 in Lat. 34 N. Got a great deal of satisfaction every time I picked up a hefty Kelvin Hughes, following in the wake of the ancient mariners and knowing that they used more-or-less the equivalent of our times. It is just amazing that after centuries of tried-and-tested technology how the last two decades of the 20th century have so utterly changed the way we look, see and do things.
Plastic sextants have been around for donkeys, John R, and the first ones had the tendency to buckle in the tropical heat making them absolutely useless!! Still it is the cheap way to go today @ $140 or something and I'm sure the plastic industry hasn't been sleeping for 20/30 years! Good Luck with your project.

norsea
13th January 2006, 12:34
Has anyone ever thought what would happen if the satellitessuddenly crashed and navigators had to revert to sextant, Nautical Almanac and Nories Tables,really doesn`t bear thinking about
Regards to all the Old Salts
Norsea

Chris Field
13th January 2006, 15:54
I just hope that the new European version doesn't crash into its US competitors- my sympathies to all the third mates who would then suffer!

Doxfordman
14th January 2006, 07:20
I new it was all guess work - now it has been confirmed! lol.

Gulpers
14th January 2006, 07:38
I new it was all guess work - now it has been confirmed! lol.

Come on Dox, you're not hiding in the southern hemisphere now, are you? We can easily get a hit man over to Denmark you know! (Jester)

John Cassels
14th January 2006, 10:54
Gentlemen;

Of course Mr.Bush can turn off the GPS satelites at any time and it would
be out with the sextants again and of course I regret that the old skills
have probably been lost due to satnav. But that is not what I am talking
about.

What did the morning sight give you ? -a position line. Noon sight also
gave you a position line so what the noon position actually was was
crossing two position lines taken maybe 3 hours apart. No one can convince
me that this was a particularly accurate form of navigation even in perfect
weather.

Morning and evening star sights gave the possibility of greater accuracy
but even then at best a small cocked hat and at worst a large cocked hat
where the position was so ambiguous as to be just a guess.

Sextants were fine in their own context but only because there was nothing
else in those days.

JC

Allan James
14th January 2006, 13:30
John,

We use to have arguements as to whether navigation was a science or an art, guess we'll never know now 'cos everone seems to have gone electric.

Whatever it was, it got us from A to B (usually without too many panics) and was usually fairly accurate.

Regards

Allan

Doxfordman
14th January 2006, 16:07
I'm on to you Gulpers, got my spies on watch - you won't get me! You'd need a shovel to dig your way in! (Thumb)

Come on Dox, you're not hiding in the southern hemisphere now, are you? We can easily get a hit man over to Denmark you know! (Jester)

John Rogers
14th January 2006, 16:59
Thanks for all of your post,but after three weeks of reading these navigation books I can see why GPS is loved by most navigators, with all the abbreviations of DR,GP,Hs,Ci,Z,H,and those nautical almanacs,hell at my age I am having trouble finding the WC and when I do find it I forgot what I was there for,relieve myself or wind my watch. But I will learn to take a sight and plot a course then go back to my GPS.
John

Gulpers
14th January 2006, 18:04
......... But I will learn to take a sight and plot a course then go back to my GPS........

John,

Good on you!

An ecxellent project which will certainly slow the onset of "Old Timers Disease".

If you are interested, there is an on-line site http://www.oceannavigator.com/ which, although primarily for the offshore yachtsman, you may enjoy. Some of their articles are very interesting and not just on celestial navigation.

The link takes you to their home page and in its welcome section, there are other links to let you "test your celestial navigation skills by solving navigational problems."

I'm not sure if you have to register with them but it may be worth a look! (Thumb)

Jeff Egan
14th January 2006, 18:16
Seem to recall the lecturer in my second mates navigation class saying that if you get a noon position from sun sights and draw a circle with a 5 mile radius around it, if you are somewhere within that circle it was a good sight. In the middle of the ocean it didn't make much difference anyway. I remember on more than one occasion crossing the North Atlantic with no sights whatsoever for the whole crossing, it made life more interesting when you sighted land hours before you expected to.

Gulpers
14th January 2006, 18:19
I'm on to you Gulpers, got my spies on watch - you won't get me! You'd need a shovel to dig your way in! (Thumb)

Dox,

What are you talking about? I've seen your picture in the Gallery - the one where you're hiding a "Cat" in a snowdrift! That's only a light dusting, wait till the freeze starts! (Thumb)

P.S. The lads in Hobart are saying the weather is 'brill' down there at the moment ....... shame eh! (Jester)

John Rogers
14th January 2006, 19:29
Thanks for the site Gulpers I will take a look.all help is greatly received.
John

John Cassels
14th January 2006, 20:55
John R;

Good on you, learn to take a sight, but the only use it will be to you is the knowledge
that you have learned how to do it. Don't forget the nautical almanac, norries tables,
paper,pencil.eraser,chronometer and all the other crap you need for this outdated
method of finding a single position line.

Paul ; checked under the bed. Sextant still there. It's a "hezzanith" made by Heaths
complete with test certificate (may 1969).

JC

John Cassels
14th January 2006, 21:09
Since my last #29 just saw Jeff's post'

Jeff . I can remember at least two occasions where reliance on a noon position
based on sun sights nearly led to disaster - in the middle of the ocean (one S.Atlantic
one Indian ocean).

Do I remember correctly that there was an old notice to mariners (or maybe
a statutory instrument ) that said "reliance made on scanty information is dangerous
and should be avoided.

Jeff Egan
15th January 2006, 12:37
The quote was in the Rules of the Road "Scanty information can be dangerous and should be avoided" I was reffering to the North Atlantic, Liverpool to the St. Lawrence where five miles is neither here nore there until you make a landfall this would be different in an ocean full of small islands or shoals. But ships got around quite well before GPS and a lot of early charts plotted with the help of a sextant were later found to be quite accurate.

gdynia
15th January 2006, 12:55
The quote was in the Rules of the Road "Scanty information can be dangerous and should be avoided" I was reffering to the North Atlantic, Liverpool to the St. Lawrence where five miles is neither here nore there until you make a landfall this would be different in an ocean full of small islands or shoals. But ships got around quite well before GPS and a lot of early charts plotted with the help of a sextant were later found to be quite accurate.

Quite correct Jeff we did many crossings from Belle Isle to English Channel without a single sight taken on voyage. All GPS has created is more collisions, strandings etc as people take it as gospel these days (Applause)

Chris Field
16th January 2006, 23:07
A few years after radar became commonplace the phrase "Radar - assisted collision" was introduced by a judge in a collision court-case. Have there been any similar references to "GPS-assisted groundings" yet?

John Cassels
17th January 2006, 09:25
Quite correct Jeff we did many crossings from Belle Isle to English Channel without a single sight taken on voyage. All GPS has created is more collisions, strandings etc as people take it as gospel these days (Applause)


We've all been across the N atlantic without a single sight ( see previous
posts) but even with perfect weather sun sights for the noon position could
be inaccurate. Try getting away with this method when you were up for
2nd.mates chartwork - crossing two position lines taken about 3 hours apart.
Examiner would throw you out.

But your logic , I don't understand. How can GPS create more collisions
and groundings ?. Noon sights used to be taken as gospel which they
seldom should have been.

Jeff, quite correct about the scanty info. phrase but surely same can be
applicable to many shipboard activities.

JC

Ron Stringer
17th January 2006, 09:30
Chris,

Try this link to see the report of a case where the GPS installers and navigators cocked up which led to a GPS-assisted grounding involving a cruise liner, "Royal Majesty". There have been others involving cargo ships where the officer of the watch failed to follow normal operating and basic navigational procedures but only watched the GPS.

Ron

www.ntsb.gov/publictn/1997/MAR9701.pdf

janmike
30th May 2012, 22:06
Joined my first ship with gps in 1974, it was as big as a chest of drawers and as accurate.
Masters instructions to take sun sights at every oppotunity to check the machine!

I came across a formula using the gps position and checking to get a zero P/L
Lat & dec same name:-
sin alt=sin lat x sin dec + cos LHA x cos lat x cos dec
Lat & dec diff. names:-
sin alt=sin lat x sin dec - cosLHA x cos lat x cos dec.

That worked for years until a newer type of gps was fitted. Mike