Do you trust your plotter?

[Quandary]

This year I have been using Seatrak based on Admiralty charts. The package includes everything from Western Europe to very large scale harbour charts. With the large scale versions of the charts and a 17" screen the displacement of the datum is more obvious but I have been surprised at how much out the old data on these charts is. In Mallaig harbour on the pontoons in the inner harbour we were shown sitting about 0.2 miles north, right outside the harbour and beyond the reef. In Inverie we were a similar distance ashore and well up the hill. Similar in Canna. The error is not obvious on Imray or the medium 1/75,000 1/25000 scale admiralty charts but must still be there. Since this summer has (exceptionally for the west of Scotland) been very thick with fog not burning off until well into the afternoon the temptation was to try to make our way into anchorages using the screen, not a good idea, though fortunately as you close the shore visibility did seem to improve to at least 10 boat lengths or so.
I presume the displacement of the various charts is consistent within a given area, in our case we mainly seemed to be shown well north of our actual position, though trying to cheat the tide in the Sound of Luing we were well into the heather, is there a simple way to correct it?

 

[nigelmercier]

Once in Greece I anchored my boat 500m inland.

 

[Bru]

It's unlikely that there is a consistent and widespread displacement error with the charts (not impossible I suppose, but unlikely)

Far more likely is that either

a> you have the wrong datum selected for the chart(s) in question - most Admiralty charts are now WGS84 but there are some exceptions and IIRC several such exceptions are charts of the Western Isles

or

b> The charts are incorrectly calibrated in the software and/or there is a calibration or configuration issue

I'd take a very close and detailed look at the setup of the GPS and software were I your good self!

HTH

 

[Tony Cross]

Once in Greece I anchored my boat 500m inland.

Only once? You were lucky!

 

[nigelmercier]

Only once? You were lucky!

Yes, normally I was only 400m inland

 

[NorthUp]

I had a similar problem with early Navionics charts on a Raymarine plotter.
The error varied from chart to chart, biggest error was in the Sound of Harris, 60metres north, only about 15m in Barra, and perfect in Stornoway.

 

[westhinder]

This year I have been using Seatrak based on Admiralty charts. The package includes everything from Western Europe to very large scale harbour charts. With the large scale versions of the charts and a 17" screen the displacement of the datum is more obvious but I have been surprised at how much out the old data on these charts is. In Mallaig harbour on the pontoons in the inner harbour we were shown sitting about 0.2 miles north, right outside the harbour and beyond the reef. In Inverie we were a similar distance ashore and well up the hill. Similar in Canna. The error is not obvious on Imray or the medium 1/75,000 1/25000 scale admiralty charts but must still be there. Since this summer has (exceptionally for the west of Scotland) been very thick with fog not burning off until well into the afternoon the temptation was to try to make our way into anchorages using the screen, not a good idea, though fortunately as you close the shore visibility did seem to improve to at least 10 boat lengths or so.
I presume the displacement of the various charts is consistent within a given area, in our case we mainly seemed to be shown well north of our actual position, though trying to cheat the tide in the Sound of Luing we were well into the heather, is there a simple way to correct it?

I didn't experience any problems in Mallaig, Inverie or Canna. I use Navionics charts, which must be based on the same admiralty data. Are you sure your settings are ok?
I seem to remember there was a slight discrepancy when entering Gometra Harbour, but nothing more than a few metres.

 

[Bosun Higgs]

Your comments make me wonder about the age of the chart data. When uk charts were converted to wgs 84 or the euro equivalent, ukho took the opportunity to make major changes to scottish charts and some others where locations were out by up tp a quarter mile. The problem was that the land locations on which the immediate coastal positions were fixed were wrong and it was when satellite land mapping became readily available that this could be corrected. So if your chart data is the old osgb stuff converted to wgs with a simple displacement, then I wouldnt be surprised. But if it is true wgs 84 then theres something wrong.

The above explanation is a simplified version of what the ukho told me when i was a member of a consultation committee they had with rya, ca etc Or to be really accurate, its what I think I can remember of what they said

 

[onesea]

0.2 miles is less than 500m, if I am that close to navigational hazards and relying solely on one source of navigation it would not be by choice.

So do I trust my Plotter (well phone and Handheld GPS) only if I have to....

 

[markdj]

We used to use seatrak extensively and never had any accuracy issues and used it in closed quarters. Check that your GPS is set to the correct datum.

 

[Quandary]

That is similar to my own interpretation, Bosun.
Chart 2635, West Coast of Scotland WGS84 has an inset with key listing the source of the data. Category 'e' which covers a lot of coastal areas including some of those I referred to, is described as 'British Government Surveys (leadline) 1828- 1933. So surely it is not surprising if there is some displacement. Meridian, who publish the software, offer two packages, the one I had previously was based on Imray charts which being smaller scale did not make the errors obvious and they are not apparent on the 1/75000 and 1/25000 Admiralty passage charts either. However the Admiralty harbour charts are very large scale and indicate features with precision relative to each other but perhaps not to WGS84. I had a brief career as a surveyor in the Ordnance Survey and have experienced the priority given to populated areas including those in which the Government had an interest. I suspect that is why N. Uist and St. Kilda charts are more recent and probably more accurate relative to datum than W. Mull or Mallaig to W. Skye where there has been less recent military activity. Interestingly I have two separate GPS receivers with displays at the chart table, they never show exactly the same position but I put that down to different rates of processing and updating or perhaps using different satellites, the difference is not enough to worry me.
I am baffled as to how Navionics can manage to produce more up to date data than the Admiralty, I did not know that they carried out surveys.

 

[Elessar]

This year I have been using Seatrak based on Admiralty charts. The package includes everything from Western Europe to very large scale harbour charts. With the large scale versions of the charts and a 17" screen the displacement of the datum is more obvious but I have been surprised at how much out the old data on these charts is. In Mallaig harbour on the pontoons in the inner harbour we were shown sitting about 0.2 miles north, right outside the harbour and beyond the reef. In Inverie we were a similar distance ashore and well up the hill. Similar in Canna. The error is not obvious on Imray or the medium 1/75,000 1/25000 scale admiralty charts but must still be there. Since this summer has (exceptionally for the west of Scotland) been very thick with fog not burning off until well into the afternoon the temptation was to try to make our way into anchorages using the screen, not a good idea, though fortunately as you close the shore visibility did seem to improve to at least 10 boat lengths or so.
I presume the displacement of the various charts is consistent within a given area, in our case we mainly seemed to be shown well north of our actual position, though trying to cheat the tide in the Sound of Luing we were well into the heather, is there a simple way to correct it?

You can trust a plotter these days.

But remember:

Take some data compiled in the first decade of the last century using a leadline, send it to Italy and give it to a woman in a factory who earns minimum wage and has never seen a boat, put it on a chip costing 300 quid than can be displayed on a screen costing 10 times that or more. The bloke with the leadline would be bemused don't you think?

So don't trust the data.

But overlay radar and you have a powerful tool which can help keep you safe if used with care.

 

[dunedin]

I didn't experience any problems in Mallaig, Inverie or Canna. I use Navionics charts, which must be based on the same admiralty data. Are you sure your settings are ok?
I seem to remember there was a slight discrepancy when entering Gometra Harbour, but nothing more than a few metres.

+1. Fine in Mallaig, and Canna, with our Raymarine and NT+ charts, datum wise.

Clearly some places in Scotland have base charting errors - such as the rocks in the Bay on Muck. And going into Loch Spelve our track always goes the wrong side of the perch, that we are the correct side of by eye. But that is a different issue.

PS. Been convinced for some time that the maps in the BMW Satnav have a datum error, not for the roads (which are accurate) but the things like rivers - which I think the must have got from a different source as typically about 25 metres out

 

[jwilson]

I trust the plotter well once I've been there in good visibility and seen the plotter confirm what I know. Around Falmouth now the accuracy is superb: shows you which pontoon you are on, but I have once sailed a plotter track across a hilly island - and that was only about 15 miles from one of the busiest commercial ports in the world - the container terminal in Hong Kong. A few years ago, and an early and very expensive green CRT screen plotter.