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Chart Datum query


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Ok, time to show off my lack of knowledge to the world now:

On another thread, Perry stated "Tide table predictions for 22nd/23rd is 0.61 Gorleston which is under chart datum so provided no North or Easterly Gales or heavy rain you have a good chance of keeping your Radar Gelcoat intact Mark ".

Now my understanding was that Chart Datum was based on the "lowest anticipated astronomical tide", and the depth figures on charts are measured at this tide level. I also thought, that the tide heights on tide tables were given from this same level, so that you simply added the tide height to the charted depth to get the actual depth of water.

From what I have said above, I would therefore have concluded that on the dates quoted above, actual depths would be 0.61m more than the charted depths at low tide. What Perry has said has now confused me completely, as I can't see how a tide height of 0.61m could be below chart datum?

I'm obviously misunderstanding something fundamental about tide heights, but for the life of my can't work out what. HHEEEEEELLLLLLPPPPPPP! :cry

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Chart datum is just that Mark, the datum that all measurements vary from, it is also based on LAT as you say, so if you take as an example a datum of 1m then a 5m high water would give you 6m depth at that charted place, conversely a 0.1m low water would give you 1.1m in that charted place and if you had a -1m low water would see that area drying.

It is vital that you accept that all numbers are predicted and the soundings may be inaccurate or out of date so always allow lots of depth.

This will be a useful read.

http://www.sailingissues.com/navcourse6.html

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As far as I am aware you are correct Mark Chart Datum is normally Lowest Astronomical Tide or LAT, that is not to say that it is impossible for the water to go lower than this given extraordinary events. Given Perry's information I would expect the 0.61 to be added to the Chart Height expecting the tidal prediction to be 0 at LAT.

Having said that am a mere baby when it comes to this sea stuff and always eager too learn from those with more experience so I too would be interested in the answer.

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That's what I thought too Ian. A Datum is a nominal point where all things are measured from, so assuming the tide heights are measured from the same datum as the charts, how can a tide height of 0.61 be below datum? Surley the datum is zero? That would mean if a tide height was given as 0.00m, then that would be AT datum.

Also, where do you find out what LAT the chart datum is based on then? I have been looking at Imray chart C28 which states "Depths are reduced to chart datum, which is approximately the level of Lowest Astronomical Tide (L.A.T.)" It doesn't state what this level actually is?

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Sorry bit slow but how did both of you get there before me

As far as I can work out David saying the same as me and Mark so does that mean that there is a negative tide prediction for the day and Perry has already applied the correction to Chart Datum?

Confused, you soon will be.

this thread now seems a very good argument for checking your own tidal charts rather than possibly confusing what others have said.

Ian

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You are right Mark that you add the tide prediction to chart datum and as far as I know all tide predictions are positive. 21st of March is the equinox but I think that increases the hight not the range. You don't want to run aground a few days after the equinox or it will be September before you float again. :naughty:

Jonathan :Stinky

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Mark,

I will try an explain it as I see it and then when someone who really knows comes along they can sort us both out (Where is Rod when we need him?).

Yes the chart datum is "approximately the level of Lowest Astronomical Tide" but it is for the area of the sounding and is not a constant plane.

The UK Hydrogtaphers Office Website http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/Support/faq.aspx quotes

Chart Datum is the plane below which all depths are published on a navigational chart. It is also the plane to which all tidal heights are referred, so by adding the tidal height to the charted depth, the true depth of water is determined. By international agreement Chart Datum is defined as a level so low that the tide will not frequently fall below it. In the United Kingdom, this level is normally approximately the level of Lowest Astronomical Tide.

In order to determine the manner in which tidal levels vary along any given stretch of coastline, it is necessary to refer all levels to a common horizontal plane. Chart Datum, being dependent on the range of the tide, which varies from place to place, is not a suitable reference plane.

Ordnance Datum (Newlyn) can be regarded as a horizontal plane and, where comparisons of absolute heights are required on the mainland of England, Scotland and Wales, this should be used.

Ordnance Datum (Newlyn) is the datum of the land levelling system on the mainland of England, Scotland and Wales, and on some of the closer islands offshore; this datum was established at the level of the average value of Mean Sea Level at Newlyn for the six-year period 1915-21. Table III in Admiralty Tide Tables Volume 1 (NP201) gives the connection between Chart Datum and Ordnance Datum (Newlyn or Local) for all Standard Ports and many Secondary Ports in the UK

If you use the tidal heights in your Almanac then this should be the case, as they are adjusted to show height above the chart datum used for that standard port. Below? Yes it does happen, the chart datum for an area will be based on the LAT at a specific point (I believe this is the LAT at the standard port for the area) and hence somewhere else in the area may actually get tides that fall lower then the local datum. I haven't updated my almanac for 2010, as I haven't got any sea going trips planned yet but looking at last year's tide tables it uses Lowestoft as the standard port for Great Yarmouth and quotes "Chart Datum is 1.5 metres below Ordnance Datum (Newlyn)". Walton on the Naze (Essex) however gives "Chart Datum is 2.16 metres below Ordnance Datum (Newlyn)".

Our wonderful BA however in their website quote "The predicted heights refer to the Datum of Yarmouth Bar, which is 0.99 metres below Ordnance Datum Newlyn." so Yarmouth bar should show on a large scale Admiralty chart as drying by -0.5m...

I dont actually have any charts for the Norfolk coast (I haven't got north of Harwich on the North Sea yet!) so I can't actually look this up for real. This month's BA tables show low water height at Gorsleston to go down to -0.45m on the 31st (which would still be positive if based on the Lowestoft Chart Datum!)

Confused? Well I sure am! :?:?

I do have a good book that should answer this, “How to Read A Nautical Chart, by Nigel Calderâ€, but I lent it to someone for their theory course studies and haven't got it back yet...

What he has to say about chart soundings etc in that book makes very worrying reading, like "when was the area last surveyed and how?"

Some of his examples of charting "whoopsies" are particularly educational.

e.g. many "modern" charts are actually still using soundings, taken in the 19th century with a lead line, and their positions are actually accurate when refered to the landmarks used by the surveyors but the position of those landmarks may get corrected (moved) by today's surveys (e.g. using photos from GPS equipped aircraft ) but the soundings are left in situ...

The online course that Perry has pointed you to is very detailed and useful.

(Personally I also like Deiederik's cruising guide section of that web-site , where he uses several of my photos of the harbours in the North Ionian!)

There is someone you can contact for a real explanation, the Easytide (UK Hydrographer.s Office Site) says:

"If you have any further detailed question that you would like to ask, you can contact us at tides@ukho.gov.uk"

Martin

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"his thread now seems a very good argument for checking your own tidal charts rather than possibly confusing what others have said."

There never has been, is not and never will be any other way to do it. :naughty:

There is no need in practice to over complicate it though, otherwise you get involved in all sorts of bollocks you dont really need. It is a very simple subrtaction or addition to the datum and use of rule of twefths for in between states, always checking more than on source of data, in this case tide tables and allowing for margin of error. Don't get too involved in over egging the pudding.

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Wow, what a post Martin, thank you!

I think Perry has confirmed the little error he posed, just to see if anyone was paying attention, was the source of my confusion. I'm quite relieved that I haven't got to re-learn all that I thought I basically understood :lol:

What does surprise me, though, is that there are so many datumseses! I would have thought all the charts for the British Isles should be based on one location, as the Newlyn example seems to suggest, but from what you have quoted for the Almanaces, it seems that isn't the case. Surely that's just silly, when sea areas can be included on more than one chart but the tide tables could be based on different datums (eg Lowestoft and Walton appear on just one chart).

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And just to reinforce that things change and you shouldnt take soundings for granted there should have been >1m over the place where the waves are breaking in this photo and it's not even rough.

post-74-136713665151_thumb.jpg

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What does surprise me, though, is that there are so many datums! I would have thought all the charts for the British Isles should be based on one location, as the Newlyn example seems to suggest, but from what you have quoted for the Almanaces, it seems that isn't the case. Surely that's just silly, when sea areas can be included on more than one chart but the tide tables could be based on different datums (eg Lowestoft and Walton appear on just one chart).

Wide area charts dont give much more than approximate contours close inshore, and offshore soundings are usually deep enough not to bother us "pleasure boaters". Personnally I use the Admiralty Small Craft Folios when I venture onto the salty stuff. They give a mix of wide area "planning" charts and detailed local harbour charts. I think that using local datums for the charts in a specific area just gives a easier idea of what to expect without having to continually add or subtract the difference beetween there and Newlyn.

Take a look at the charts for the Walton Backwaters (just south of Harwich) and you will see a series of channels, islands, and drying mud flats, when drawn using the local LAT. Move the datum up 2 meters to use Newlyn and almost the whole area would show as being under water. As it is the chart gives you a good idea of where the water should be at low tide, and a bar that shows as 0.5 sort of brings home the point that you really need to take care.

I think that is where that photo from Antares was taken...

(Give me a nice steep stable rocky shoreline anyday when I am navigating, like Western Greece, where the echo sounder often only finds the bottom when you are about a few boat lengths off the islands and hence reads 0.0 most of the time),

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I don't think I'd want to be navigating that channel at that particular moment in time :lol:

If it is the entrance to the Walton Backwaters then use the local chartlet published on East Coast Rivers, when we went in last year the buoys were not where they were on the chart but the channel is well buoyed and easy to follow with the right information.

There are lots of shallow bits though where as David's picture shows there is a lot less water than the chart is showing.

The good news is you can get a 310 up from Stone Point to Titchmarsh even at low water springs and there is nothing that says you can do that on any chart :naughty:

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The Deben Bar, that showed shallower than expected on my sounder in August.

I was told by a local yachtsman that had had the misfortune to run aground there that the shale bank moves with almost the same speed as the water, flowing in and out with every tide. He found this out when he waded out to put out a ketch anchor and by the time he refloated he had 4' mound of shale one side of the boat and a 4' hollow the other.

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