Really thick question about GPS

[Guest]

Whose error .....

It is not the GPS .... With Autoroute, and other programs I have my position plotted correctly ....

It is the cartographer ... who produced the SHOM / Fugawi bsb's .... having run the GPS group - in fact I started the group - I have had contact with various charts suppliers and the point was raised and confirmed. There is a 'glitch' in the SHOM / fugawi bsb which is not apparent to other BSB reading plotters .... anyway words to that effect .......

I have tried out various set-ups for others as I am known locally for this work ... interesting to see the differences .... wish I had the money to buy what I found to be best !!

/forums/images/graemlins/tongue.gif

 

[tome]

[ QUOTE ]
every banana republic invented its own mapping datum

[/ QUOTE ]

This is one of the most ill informed posts I've seen on the subject. It was nothing to do with national pride and everything to do with local topography.

The fact is that the earth is far from being a spheroid, and different datums arose to try to get a 'best fit' for accurate local survey work.

WGS84 (and strictly WGS72, though no longer in use) are the only whole-world datums. Because of this, they are a poor fit in many parts of the world and you will see some very erratic elevations if you travel the world at sea level.

So before you spout on about wonky datums or joined up charts I suggest you do at least a little learning on the subject of geodetics and in the meantime refrain from spouting rubbish

rant over

 

[bilbobaggins]

Rude and intemperate. Although you list 'Pontification' as one of your hobbies, you should know better.

I refer you to 'The Volpe Report', to professional guidance issued by the Nautical Institute, to similar guidance issued by the Royal Institute of Navigation, to the latest edition of 'Bowditch - American Practical Navigator', to Vol 1 of The Admiralty Manual of Seamanship and - for the benefit of amateur sailors who use these forums - the excellent RYA Navigation Handbook, pages 10-13, where the issues and their significance are clearly illustrated.

Those who know the area of the example given in the above, Round Island in The Scillies, know full well that a 200 metre error there will have them - on a dark and stormy night - missing the entrance to e.g. Tean Sound and connecting firmly with Pednbean Rock or Rough Ledge instead - or, on a thick foggy morning, missing the entrance posts into The Hamble and running aground. It matters.

( BTW, professional navigation instructor (air ) 15 years+, RYA Ocean Yachtmaster-Commercial, RYA Cruising Instructor, over 35 years sailing in all seas from Lofotens to Cape Agulhas, photogrammetric survey specialist 11 years+, recent member of the Royal Institute of Navigation's 'Small Craft Committee'....)

Even you, who has a vested commercial interest in the 'act of faith' discussed, will concede there is *some* relevant background to inform my views. /forums/images/graemlins/ooo.gif

 

[Ships_Cat]

Until the satellite era, every banana republic invented its own mapping datum 'startpoint'. Including France. Most of these *did not* link together seamlessly. Sometimes hundreds of metres of error existed. It didn't matter much, 'cos mariners allowed enough margin and then 'converted to visual' for the tight, intricate bits - until satellite navigation.... and yotties wanting to blind-nav within a couple of metres. Why? Only the GPS salesmen know why.


Cannot see anything in Bowditch (or any other authoritative reference for that matter) that says anything like that. Perhaps you can refer us all to where Bowditch says that, as you claim it does, so that we can satisfy ourselves that what you say is not a load of rubbish.

As Tome says the differing datums arose because of genuine local survey needs. You are the one who is pontificating and what you are saying is totally ill informed.

John

 

[tome]

Nobody disputes that datum errors can be 200m or more (actually constant, not just on a dark and stormy night) but listing your memberships and experience and quoting a load of publications doesn't advance your understanding nor that of anyone reading this thread. Most of the publications to which you refer are on my shelf and they are irrelevant to my point and yours.

The fact remains- there are very good reasons why different datums exist and also why different mapping projections are in use in different regions. Even a basic understanding of geodesy would explain this. Your posts show that you totally misunderstand the subject and I stand by my earlier comments.

Whilst I list pontification in my bio, I see that yours is empty. And you shouldn't assume that others hereabouts are less qualified or don't hold higher memberships of the institutions you boast about.

 

[duncan]

Now Tome you ssaid last time you weren't going to get involved again..... /forums/images/graemlins/smile.gif...........why this has got personal is difficult to understand but it certainly has.

Back to the keys for those reading the thread as a source of information (and it certainly is that)..........

You would both seem to be saying the following -

1. Chart datums, through which lat and long figures are related to the charted features, vary from charted area to charted area.
2. These are usually 'common' to a geographical area - reasons later.
2. Any differences between datums will be common across a charted area (pretty much) so where charts meet, having been complied on different datums, those differences will exist and not be 'smoothed' towards their edges.
3. If you are using the wrong datum for a chart you could be as much as 200metres out; and that could be important to navigation so getting the right datum is important where this sort of error would be critical.
4. Because of GPS most navigational charting is gradually being converted to a world standard, WGS 84, and most GPS plotters default to that standard.

The only difference I can see are that on the one hand 'Banana Republics' are being given as the source of the different datums historically - and on the other it is being explained that local conditions and the shape of the Earth were good reasons for these different datums. Additionally conversion may work OK on the surface but you will find altitude errors creeping in (if I read you right Tome?).

Personally I support the Banana Republic theory because I like the blame culture and don't understand the science of the other theory well enough - but no doubt other readers will form their own conclusions.

 

[Oldhand]

Why on earth would you have a chart of Malabo? Perhaps its improved from the days just after the Russians left when the place was a derelict shambles. We had to get trig point co-ordinates dug out of Spanish archives in Madrid in order to set up a Maxiran ranging network to survey the waters and sub-sea strata for the early oil exploration. The survey data could have been used to update the commercial chart but it rarely happens.