Chart inaccuracies

[peterb]

The latest YM has an article relating to the accuracy of charts. It suggests that the errors may be due to three major factors:

1 Change of chart datum

2 Old surveys using leadlines rather than sonar (particularly sidescan sonar)

3 Old surveys in areas subject to rapid change.

I suspect that factors 1 and 2 are being taken care of already. As each edition is published the datum is being changed to WGS 84, and any new survey is likely to use modern equipment. But I am more worried about factor 3.

There was a time when virtually all survey work was carried out by the navy. Nowadays, though, much of the inshore survey work (i.e. those areas where many small boats sail) is done by harbour authorities. For instance, most of the surveying in the Thames Estuary is done by the PLA. The problem with this is that the main interest of harbour authorities lies in the shipping channels; banks between the channels are of interest only in so far as they act as limits to the channels. But small craft regularly sail across these banks, and sometimes their safety may depend on the accuracy with which they are charted.

Now I would quite enjoy going out sometimes and doing a little survey work. But would the Admiralty Hydrographic Department accept the results of my survey? Having talked to them at various boat shows, the answer apparently is no, because they can't trust the accuracy of my survey. Now they are probably right in that, but at the moment I know several places in the Thames Estuary where the charted depth is wrong by a metre or more, so I can't trust the Admiralty's charts, either.

The real point of this post is to wonder whether other small boat sailors might also have an interest in surveys of their areas. Clearly some do, because of the regular surveys of entrances such as the Deben and Ore. But what might be required for such small boat survey work to be fed into formal charts? Could there be a minimum standard of equipment, for instance, though almost any modern GPS and echo sounder will give the depth to within 0.2 m at a position known to within 10 m? Could there be a standardised format for submitting the results of such surveys? Is there scope for some organisation (clubs? RYA? CA?) to talk to the Admiralty so that a possible cheap source of reasonably accurate data can be made available to a wider audience?

Is anyone interested?

 

[petery]

If a large number of sailors logged the NMEA position, UTC time and depth sentences on to a floppy disk as we sailed and pooled all this disk information, a fairly accurate picture of depth contours could be built up with a computer program - with errors in position and depth from individual boats being averaged out over time. Very similar to the black boxes fitted to some road vehicles to record where they have been (from GPS) and what they have been doing (from sensors on the vehicle) - so the technology is alreay there in principle.

 

[Ohdrat]

There are many chart inaccuracies (read un charted rocks) around Scotland particularly on the West Coast.. quite shocking in this day and age but you can't trust even an Admiralty Chart up here Rather off the point we came across a rare example of a Nav marker light (there aren't many of those up here either) that didn't work.. we notified the Coastguard station.. it took them 2 days to mention it in their nav broadcast!

 

[Twister_Ken]

I fear your suggestion is entirely too sensible for 'them' to accept it.

 

[peterb]

Non-working lights

I'll bet that the reason for the delay was that they wanted confirmation from another source. They won't believe reports from pleasure craft!

 

[NigeCh]

How accurate is accurate?

Does it really matter? Even if you have a depth sounder you can't be sure of how the depth relates to the charts as even LAT is questionable, and depth varies according to barometric pressure, wind direction etc.

Surely part of the fun of sailing with Admiralty charts is a) that you know that they aren't accurate to the n'th degree and b) that it's up to you to work out how to sail around those inaccuracies?

 

[BlackSheep]

Re: Non-working lights

We are more than happy to accept reports of lights extinguished/buoys moved etc from pleasure craft. The procedure once reported is to pass the information on to the relevent authority - for the West coast of Scotland this is usually the Northern Lighthouse board or the local harbour authority & the hydrographer of the Navy in Taunton. After this a navigational warning (WZ) or local nav warning can be issued and broadcast during the MSIB....good to hear that someone listens to more than just the local forecasts!!!

 

[Chris_Stannard]

In using an Admiralty chart you should remember that the soundings are taekn in parallel lines so that there is always a chance of an isolated rock being missed especially if it does not break surface. A friend of mine hit one just outside Chichester harbour in the early seventies and his boat sank.

The other thing you should remember is that the RN teach navigators to give dangers a wide berth. Or at least they did in my day. For a frigate about three miles from a rock was considered standard, which allowed for inaccuracies in the chart and errors in navigation. We were also taught in the passage planning stage to draw a big pencil circle around dangers such as rocks and shallows. This ensured that you were aware of any dangers on the way and drew the attention of the officer of the watch to them.

The answer is of course that charts, like compasses, GPS and so on are an aid to navigation, and should always be treated with as such.

Chris Stannard

 

[peterb]

Rocks

Rocks at least stay still. My problem is more to do with sand or mudbanks. These are always in a state of change, but generally only slowly. So you look at the chart, reckon that you'll be able to cross a bank (or use a swatchway) with a reasonable clearance. Maybe for some years you use that route with no trouble. When a new edition of the chart is published there appears to be no change, but next time you use the channel you find it has shallowed appreciably. What should you do?

I've tried using the forms that used to appear in Notices to Mariners. They appeared to have no effect. I've asked the Hydrographer's Department, and they say that they refer such reports to the body responsible for surveying that area, usually a harbour authority. The trouble appears to be that these authorities are overstretched, and, perhaps understandably, concentrate their efforts on the areas used by big ships. So the small craft information seems to get lost.

As petery points out, much of the technology required for a good survey is already there. But how can we make the relevant authorities take notice?