aitchw
New member
How do cartographers arrive at a Chart Datum? Is it, as has been suggested to me, the lowest recorded tidal level or is it something else?
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Chart Datum
- Thread starter aitchw
- Start date
BrendanS
Well-known member
<A target="_blank" HREF=http://www.pol.ac.uk/ntslf/tides/datum.html>click here</A>
Tidal levels are quoted relative to chart datum (approximately the lowest level due to astronomical effects and excluding meteorological effects). The heights of chart datum (CD) relative to ordnance datum (OD, at Newlyn) in the UK are listed in the link
If for example, you have a data value for Liverpool of 1m relative to CD and want to convert it to be relative to OD, then this would be -3.93m (i.e., 1m - 4.93m).
<hr width=100% size=1>Utinam logica falsa tuam philisophiam totam suffodiant
Tidal levels are quoted relative to chart datum (approximately the lowest level due to astronomical effects and excluding meteorological effects). The heights of chart datum (CD) relative to ordnance datum (OD, at Newlyn) in the UK are listed in the link
If for example, you have a data value for Liverpool of 1m relative to CD and want to convert it to be relative to OD, then this would be -3.93m (i.e., 1m - 4.93m).
<hr width=100% size=1>Utinam logica falsa tuam philisophiam totam suffodiant
Birdseye
Well-known member
I guess they must do it by recording over a long period and then averaging out. you can adjust tide heights for atmospheric pressure, but I doubt that you can easily adjust for wind since it depends not only on strength but how long its been blowing. so to get a figure unaffected by met issues will only be possible by observation
I always thought that cd also depended on geography in the sense of the shape of the land masses etc. thats why cd for newport s wales is different to cd for plymouth, and why there is little tide in the med. and for this reason also i think that cd would be established by observation.
am going down to ukho on business in a week or two - i'll ask them if i remember.
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I always thought that cd also depended on geography in the sense of the shape of the land masses etc. thats why cd for newport s wales is different to cd for plymouth, and why there is little tide in the med. and for this reason also i think that cd would be established by observation.
am going down to ukho on business in a week or two - i'll ask them if i remember.
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MainlySteam
New member
It is something else but not definitively anything if you know what I mean, because the chart datum is not the same thing from chart to chart, region to region, or chart producer to chart producer ie it is not standardised.
Bowditch "The American Practical Navigator" has an good explanation of chart sounding datum and the various ones that are used and how they are determined.
Bowditch is available in a number of places for reading on the internet eg <A target="_blank" HREF=http://pollux.nss.nima.mil/pubs/>http://pollux.nss.nima.mil/pubs/</A> but a google on {bowditch online} will turn up others. Search in chapters 3 and 9 for "datum".
John
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Bowditch "The American Practical Navigator" has an good explanation of chart sounding datum and the various ones that are used and how they are determined.
Bowditch is available in a number of places for reading on the internet eg <A target="_blank" HREF=http://pollux.nss.nima.mil/pubs/>http://pollux.nss.nima.mil/pubs/</A> but a google on {bowditch online} will turn up others. Search in chapters 3 and 9 for "datum".
John
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graham
Well-known member
CD is also known as LAT which stands for "Lowest Astronomical Tide" ie the lowest the tide will ever go by the influence of gravity .The efects of wind ,storm surges,barometric pressure etc are not allowed for in calculating CD/LAT.So once in a blue moon the tide can fall below CD.
Another point to consider is the "Horizontal"Datum. With GPS giving such accurate results now its important to check your GPS is on the same horizontal datum as the chart.
Most charts are on WGS 84 datum which is usually the default setting for GPS .There are charts still on GRB36 datum which requires a slight correction to position. Especially if your charts are not all the latest edition its worth checking.
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Another point to consider is the "Horizontal"Datum. With GPS giving such accurate results now its important to check your GPS is on the same horizontal datum as the chart.
Most charts are on WGS 84 datum which is usually the default setting for GPS .There are charts still on GRB36 datum which requires a slight correction to position. Especially if your charts are not all the latest edition its worth checking.
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MainlySteam
New member
You might like to read a bit of Bowditch too /forums/images/icons/smile.gif.
John
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John
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charles_reed
Active member
Bowditch et al
You may be interested but it's available as part of a 2 CD bundle from the Dashews for €69 or so.
There is a lot of useful NOAA stuff, especially for you folk in the Pacific - I'm seriously thinking of getting it and freeing up a lot of bookcase space.
Look on www.setsail.com.
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You may be interested but it's available as part of a 2 CD bundle from the Dashews for €69 or so.
There is a lot of useful NOAA stuff, especially for you folk in the Pacific - I'm seriously thinking of getting it and freeing up a lot of bookcase space.
Look on www.setsail.com.
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MainlySteam
New member
Re: Bowditch et al
I did not realise that Charles.
I have a paper copy of Bowditch (of the previous to current edition) but it takes up a lot of shelf room if on the boat. When I checked to see if the link in my previous post was working I see too that NIMA now have a full downloadable version at 30 something megabytes all in one file (previously only each chapter's pdf file could be downloaded individually) - impractical to download though if one is cruising, but I will put a copy on CD here at home to keep on the boat.
I will have a look at the Dashew's site and see what else is on the CD's. Thanks.
John
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I did not realise that Charles.
I have a paper copy of Bowditch (of the previous to current edition) but it takes up a lot of shelf room if on the boat. When I checked to see if the link in my previous post was working I see too that NIMA now have a full downloadable version at 30 something megabytes all in one file (previously only each chapter's pdf file could be downloaded individually) - impractical to download though if one is cruising, but I will put a copy on CD here at home to keep on the boat.
I will have a look at the Dashew's site and see what else is on the CD's. Thanks.
John
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gjgm
Active member
not sure I understand this. CD is specific to a particular chart- as the LAT for that chart? I had thought all UK charts used the same CD ? CD=OD only at newlyn?
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MainlySteam
New member
Not sure if I understand your question, so if any of the following is what you know already please excuse me. Again, as pointed out before, Bowditch has a very good explanation of all this and is available on the internet.
I do not know the specific practices of the UKHO but in general the tidal datum chosen as the chart datum will vary across regions. Even if the same tidal datum is used, the height of that against a fixed datum such as a pin on land somewhere will differ from place to place simply because of the character of the tides differing from place to place. For example, in locations where the tidal range is less than in another, the same tide datum will be higher than in one of greater range. This is what the table Brendan kindly pointed us to shows - it takes the tide datum Lowest Astronomical Tide and compares it for various locations to a fixed datum (a pin or whatever it is fixed on land) at a single location.
It may be that all UKHO charts use the same tidal datum as the chart datum (or more correctly as the Chart Sounding Datum) but I suspect that is not so when one considers all of their worldwide charts. They may use the same tide datum for charts in a region however.
It is important to realise that the same tide datum is not used for the chart datum on all charts and Bowditch points out that in areas of small tidal effects such as the Baltic, then the tidal datum used as the chart datum may be the mean water level and not one of the low tide datums at all.
John
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I do not know the specific practices of the UKHO but in general the tidal datum chosen as the chart datum will vary across regions. Even if the same tidal datum is used, the height of that against a fixed datum such as a pin on land somewhere will differ from place to place simply because of the character of the tides differing from place to place. For example, in locations where the tidal range is less than in another, the same tide datum will be higher than in one of greater range. This is what the table Brendan kindly pointed us to shows - it takes the tide datum Lowest Astronomical Tide and compares it for various locations to a fixed datum (a pin or whatever it is fixed on land) at a single location.
It may be that all UKHO charts use the same tidal datum as the chart datum (or more correctly as the Chart Sounding Datum) but I suspect that is not so when one considers all of their worldwide charts. They may use the same tide datum for charts in a region however.
It is important to realise that the same tide datum is not used for the chart datum on all charts and Bowditch points out that in areas of small tidal effects such as the Baltic, then the tidal datum used as the chart datum may be the mean water level and not one of the low tide datums at all.
John
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tome
New member
This is broadly correct for the UK.
Chart Datum is referenced to the predicted LATs in the chart area. Sometimes there will be several data (=datums) for one chart. The 'pin' in this case is the Ordnance Survey vertical datum at Newlyn. The differences between tidal levels and the Newlyn pin are tabulated on Admiralty charts for each CD.
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Chart Datum is referenced to the predicted LATs in the chart area. Sometimes there will be several data (=datums) for one chart. The 'pin' in this case is the Ordnance Survey vertical datum at Newlyn. The differences between tidal levels and the Newlyn pin are tabulated on Admiralty charts for each CD.
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gjeffery
New member
Newlyn Datum is the Ordnance Datum and is a consistent across the OSGB grid.
Chart datums are local to each survey. This begs the question of what happens on small scale charts, where surveys meet? More importantly, it does raise a concern that quoted tide heights may not always be related to the datum of the chart in use. Does anyone know if there is an issue when using, say French Charts and UKHO tide data, or conversely if UK charts are used with French tidal predictions?
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Chart datums are local to each survey. This begs the question of what happens on small scale charts, where surveys meet? More importantly, it does raise a concern that quoted tide heights may not always be related to the datum of the chart in use. Does anyone know if there is an issue when using, say French Charts and UKHO tide data, or conversely if UK charts are used with French tidal predictions?
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gjgm
Active member
ok..think I ve got there, thanks to all. I had thought CD was constant but as Brendans table shows (now i understand), different loactions actually have different LATs-against OD.
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whisper
New member
I think you will find that all UK charts are based on the sea-level datum point of Newlyn. It tells you where the datum point is, on the chart itself. This is not the same as the lowest astronomic tide but is an average level that has been decided upon after many years of recording tidal data at that point.
Well that is what my RYA course notes say anyway./forums/images/icons/smile.gif
We had another thread on this a month or so ago, maybe worth a search.
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Well that is what my RYA course notes say anyway./forums/images/icons/smile.gif
We had another thread on this a month or so ago, maybe worth a search.
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Birdseye
Well-known member
I was wrong - it isnt based on observation but on prediction. chart datum in the uk = lowest astronomic tide = lowest low tide predicted in a 19 year period. so you predict the tides, find the lowest, call that zero and re-calculate all the others based on that one being zero.
Presumably they use predictions rather than observations to avoid any met effects.
Clearly it was the failure of the UKHO to properly explain the true meaning of CD that caused me to go aground last season! My wife was wrong (a very rare event). It wasnt cr*p navigation on my part after all.
<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
Presumably they use predictions rather than observations to avoid any met effects.
Clearly it was the failure of the UKHO to properly explain the true meaning of CD that caused me to go aground last season! My wife was wrong (a very rare event). It wasnt cr*p navigation on my part after all.
<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
graham
Well-known member
The Ornance Datum at Newlyn gives a common datum which ties all the Admiralty charts and maps together ,The Chart Datum of each individual chart is LAT (Lowest Astronomical Tide)for the area it covers.
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MainlySteam
New member
I assume that Newlyn is the Newlyn in Cornwall - is there any particular historic reason that the vertical datum is based there?
John
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John
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roger
New member
Tidal Observatory
Newlyn is referred to as a tidal observatory - so presumably they do just that.
In a puzzled sort of way I would imagine you would need more than one tidal observatory. For one thing the UK is sinking in the south east and rising in Scotland. I'm unsure how else you would find out.
<hr width=100% size=1>Roger
Newlyn is referred to as a tidal observatory - so presumably they do just that.
In a puzzled sort of way I would imagine you would need more than one tidal observatory. For one thing the UK is sinking in the south east and rising in Scotland. I'm unsure how else you would find out.
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BrendanS
Well-known member
Re: Tidal Observatory
See my original link. These are the folks who have numerous tide observatories around the UK, and provide the information for a whole bunch of organisations in the UK
<hr width=100% size=1>Utinam logica falsa tuam philisophiam totam suffodiant
See my original link. These are the folks who have numerous tide observatories around the UK, and provide the information for a whole bunch of organisations in the UK
<hr width=100% size=1>Utinam logica falsa tuam philisophiam totam suffodiant
whisper
New member
Yes, it's due to the fact that there is little else to do down there other than watch the tide rise and fall. Once the tin mines closed there was a ready supply of workers willing to sit with their toes in the water all day. It was a very good place to get the surveys done as cheaply as possible. Unfortunately, drownings were common.
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