My boat on top of the Marina Housing!

[Gludy]

Ok- having just installed my new chartplotter that linked to myexisting philips GPS, the plotter show the marina pontoons etc but placed my boat about 120 metres away right in the middle of the marina housing.

The chart datum was obviuosly wrong (WS84) and so experimenting I chose O.S. Great Britain for the datum and the boat was correctly placed within 20 foot of its true position. Looking at my paper chart it indicates I should apply a very small south and east correction when plotting WS84 GPS positions on it, but this correction is very small and nothing like the 120 metres that my boat position was out by.

Question - Have I appled the right chart datum i.e. OS GB?
Question - what chart datum positions do you apply to your plotter?




Paul

 

[kimhollamby]

What plotter and what cartography? This subject handled in different ways by manufacturers.

Wrote quite a huge missive about this a year or so back - potentially a bit of a riddle of a subject when it comes to plotters. But in a nutshell difference between WGS84 and OSGB36 is in the region 100m-140m, so that would explain why the displayed position changed by that much.

What you also have to look out for is lifting waypoints off a paper chart (which on the British south coast would tradionally have been OSGB36 but on most recent charts will possibly be ETRS89, to all intents and purposes the same as WGS84) and applying them to electronics set to global or local datum.

If you want to read a lot more try MBM Aug 2000 p80, or if there is enough interest perhaps we'll publish the piece on www.mbmclub.com

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[Gludy]

Thank you for the response.
Its the latest Raytheon colour chart plotter using C-map NT cartidges.
Is there an official datum that I should apply?

Paul

 

[kimhollamby]

All C-Map CF-95 charts for home and near continental waters (NT chips use this format) are based on WGS84 datum.

You need to check that your separate GPS and your Raymarine plotter are both set to WGS84 as well (there are menu options on both units). If after that you get the error again then you may have established a datum error on the chart cartridge. But fundamentally you will be safer leaving everything set to WGS84 in this case.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[ArthurWood]

My first reaction was HURRICANE in S. Wales!

 

[jfm]

Re: The phillips is bust or set wrong

Gludy the prob is with the Phillips. As said above, the CMaps are WGS84 and the Raytheon is expecting a GPS position to be given to it in WGS84, so sounds like someone with non WGS84 paper charts has set the phillips to give its output in another datum. So bash around with the phillips to get it to output in WGS 84

When it's working you sould be able to see your boat on its pontoon. With DGPS you should be able to tell if the boat is parked stern to or bows to, assuming the DGPS antenna is at one end of the boat, and with non DGPS you should get almost that, I mean it should show the boat on the right pontoon.

If still no good get the Raytheon GPS and connect it to the plotter or have it fitted internally

 

[hlb]

Re: Ah But.

All thats ok if the man put the pontoon in the right place in the first place. Or if chart is right to the odd foot or two.
Largly depends on the thickness of pencil.

I dont think I'd worry to much about the problem to a foot or two. Mines some times bang on and some where else it might be a little bit out. Err keep looking out of the window!!

Haydn

 

[Gludy]

Thankss - I am going to check out the Philips GPS to check its datum setttings when next on the boat. This must be the problem.

The current setting is showing me about 20 foot out but it picks up the boat swinging ok as it swings and tightens on the warps, so does the plotter image.

I have ordered a sea talk Raytheon WAAS ariel, so that will solve it anyway - I ordered this after being advised it was needed, although now that I have beocme involved in the datums, the advice appears ill founded.

Thank for all your help.

Paul

 

[Deleted User YDKXO]

My boat in the Marina car park

Kim, I've experienced similar problems. My boat has a Philips GPS driving an Autohelm Navcentre using CF cartridges. I have checked that the datum settings are WGS84 but the plotter still gives a position error of about 200m. I have corrected this error in my marina using the correction facility on the Navcentre but on a trip to Brittany last year, I found the plotter giving the same sort of magnitude of error despite the original correction.
Any ideas?

 

[kimhollamby]

Re: My boat in the Marina car park

Is your Navcenter one of the original ones using CF85 (the bigger) cartridges? In which case the datum used may well be local, hence the errors (ED50 over the other side of the Channel can be as much as 200m).

It was only in the switch to the smaller NT cartridges (as used on the Raymarine plotters) that C-Map universally switched to WGS84, at least for all charts in the immediate locality.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[Deleted User YDKXO]

Re: My boat in the Marina car park

Kim, my Navcentre is about 9yrs old and uses what C-Map term 'mini CF85' charts. The charts I actually used in Brittany were purchased in 2001
Are you saying that if I upgraded to a plotter using NT charts, this error is eradicated?
Whats ED50?

 

[Gludy]

Re: My boat in the Marina car park

I have found the advice on this thread helpful and I think the subject is worthy of an article in Motor Boat Monthly. W ell researched article covering msot folks combinations could be helpfulk to a lot of folks.

I will not know the result until I get down to the boat and mess about with the Philips - in any evebt I have the Raytheon antenna coming this week, but I still like to know and understand what is happening.

I would also like to see an article testing lifrafts to see if it is a 50/50 chance of upside down inflation!

Paul

 

[kimhollamby]

Re: My boat in the Marina car park

ED50 is the common datum used on the near continental coast and, often, for Channel Island charts as well.

CF85 charts (the ones your Navcenter use) are not as accurate as C-Map NT (the ones all of the latest C-Map driven plotters use), not least because as stated before, CF85 were often drawn from local datum charts but there is no indication to tell you so.

Arguably, although it is not absolutely certain that all CF85 charts are local datum, you might be better off setting your GPS to OSGB36 when using the Navcenter on this side of the Channel and ED50 on the other side.

Would emphasise for others reading this that with C-Map NT, everything throughout the mix (GPS and plotter if separate) is best left on WGS84.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[Deleted User YDKXO]

Thanks Kim (NM)

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