Really thick question about GPS

S

scottsail

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Hi, this is probably sooooo obvious, however...
In order for my fixed and handheld GPS's to accurately reflect a position that relates to my nautical "paper" charts, the Map Datum that I should use is WGS84 ?

Pls advise

Thanks

Steve.
 
yes.

there *may* be a note on the chart that ihndicates the teeniest correction to apply. But generlly, provided you just use gps to get "generally" where you want to go it will be ok.
 
The chart will say on it which datum it is using. If the chart says it is using WGS84, then yes, you should set the GPS to use the WGS84 datum too. You need to do this for each chart you use, as they can vary
 
Map datums are normally written on the chart/map. However, if the chart is a new UKHO product it almost certainly is WGS 84. Whereas the land maps from Ordnance Survey are more likely to be OSGB36, and french charts are to their own standards. i.e. check before using. Some charts may not have a datum description, and although you could line up a point of land and then adjust datums until you get the best match, there may be a slant problem that you are not aware off so be careful with unknown datums, and always check the datum that you are using.
 
Most british charts were OSGB36 but are gradually being replaced with WGS84. French charts will generally be ED50 but may also be migrating to WGS84 (which is the 'native' GPS datum)
 
Not a thick question, but some thick answers.....!

GPS now offers some unprecedented accuracy - but not every day and not on every occasion. See 'The RYA Navigation Handbook' by Bartlett, or the Royal Institute of Navigation website. To make the most of it, your GPS and your chart need to be 'singing from the same hymnsheet' or related to the same datum start-point.

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.

Today, most charting/mapping authorities are changing the charts they issue to conform to the earth-model that the US GPS system uses -'cos it's 'joined up'. Nevertheless there are thousands of marine charts still in use - and still being sold - which are drawn up based on some other wonky datum.

The easy answer- and the one recommended by all authorities and experts - for the next decade or so, is simply to read on your chart what datum it's drawn to, then reset your GPS to match. Then do that for each chart you start working on. That means that the pencil mark you put on your chart will be accurate - on 95% of occasions - to within about 8 metres. What happens on t'other 1 in 20 fixes? Well, you need to read the book!

That's quite simple - it tells you how in the GPS manual. It takes only a second or two - once you've read up and learned how. If that seems complex, do an RYA evening class. It's all in there.

Be wary of b******t in this thread. It could lose you your boat.
 
would you care to point out exactly where the " b******t in this thread." occurs that might result in someone losing their boat. All were in the same ballpark, though maybe a bit less detailed than your post
 
That's a question I cannot answer, as chart datums vary worldwide, and I've only personally used a few of them. Here in the UK, you might be up to 10's of metres out.

It's much easier to actually use the correct datum for the chart you are using. Or use a chart plotter, then it will use the correct datum for the chart it's using

If you really want to know from a theoretical point of view, can do a search tomorrow evening for accurate data if it's available, or maybe Tome will jump in as I'm sure he has this sort of information at his finger tips
 
it does in fact vary from area to area, as was said before charts are slowly being ammended to reflect the wgs84 and the corrections needed placed within the text on the chart, I have noticed errors of up to about 1/4 a mile(e.g. the chart of malabo in equitorial guinea), though more often only a hundred meters or so is more likely.
most well used charts are spot on and its nice to watch the track of a vessel correspond to the actualitie
 
[ QUOTE ]
I have noticed errors of up to about 1/4 a mile(e.g. the chart of malabo in equitorial guinea),

[/ QUOTE ]

I'll remember that, next time I do a passage plan from the Solent. /forums/images/graemlins/laugh.gif
 
Thanks very much to everyone who took the time to reply to my question... I have in my further investigations found this info, and here is the URL - http://www.ukho.gov.uk/attachments/2005/annual_nms/ANM19%202005.pdf

Programme to transfer charts of the British Isles to a WGS84 compatible datum
17. A programme to transfer Admiralty charts of the British Isles from OSGB36 Datum (or OSI Datum) to a WGS84 compatible datum commenced in June 2000. Positions referred to WGS84 Datum can differ from those referred to OSGB36 Datum by more than 130 metres. During the period of conversion, it is inevitable that some adjoining or larger scale or smaller scale charts will be referred to different datums for a limited time. Caution must be exercised when transferring positions between charts that are referred to different datums (see paragraph 12 for further details).
18. During the conversion programme, the following procedures should be used to transfer positions between charts referred to different datums:
a. To transfer a position from a chart referred to a WGS84 compatible datum to charts and documents referred to OSGB36 Datum refer to the charted National Horizontal Datum note.
b. When transferring positions from charts referred to OSGB36 Datum to charts referred to a WGS84 compatible datum, the lat/long adjustments to be applied to the positions are in the reverse direction to those shown in the Satellite–Derived Positions notes on the OSGB36 Datum charts. For example, if the adjustments shown in the Satellite–Derived Positions note on the chart referred to OSGB36 Datum are 0´·03S. and 0´·12E., then the adjustments to be applied when transferring a position from that chart to a chart referred to a WGS84 compatible datum are 0´·03N. and 0´·12W:
OSGB36 Position 51°55´·50N. 001°40´·50E.
lat/long adjustments 0´·03N. 0´·12W. WGS84 Position 51° 55´·53N. 001°40´·38E.
c. The method shown in the above example should also be used when transferring positions from charts referred to Ordnance Survey of Ireland Datum, Ireland (1965) Datum, or European Datum (1950), to charts referred to a WGS84 compatible datum.
 
couldn't get your link to work Scott so apologies if it already does this however
1. either just set your gps to the datum of the chart you are working with at the time, as already said - both succinctly and with bells and whisles........
2. if you want to convert waypoints from one datum to another for storage in your GPS then use this whish will save a whole lot of time and rule out arithmetic errors.

If you have old decca cordinates I can let you have a conversion for those too!
 
error ...

I have a BSB / KAP chart of my local area in Baltic. It says WGS84 on the chart when diosplayed on screen. Put it in plotter program and it puts me 0.3nm offshore about 0.5nm NW of Ventspils ......... my real position is 2nm SE of Ventspils harbour ... that is about 3 nm se of displayed position. All GPS / Plotter are set to WGS84

My Uk boat base in solent .... same co's BSB / KAP charts ... they were previously SHOM / Fugawi .... displaces my position about 400 m N from her berthed position ... again all WGS84 etc. etc.

So - its not only what they say - its also what actually happens - and that may not be same !!

As I use a plotter program with user correrection allowed to above ... I can correct this out and get spot on ... but thats another story ...

/forums/images/graemlins/cool.gif
 
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