Chart datum

PabloPicasso

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How can the predicted low tide be below LAT? This morning the predicted low tide at Howth was -0.10m. Now tidal predictions can't account for the effects of weather, wind direction, air pressure etc. so if LAT is accurate how can this be?

Now I know this happens, if rarely, but is it an unforeseen astronomical event, or something else?
 

pvb

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Lowest Astronomical Tide is merely a convenient average prediction, based on historical data. Actual tide levels can be lower in certain meteorological circumstances.
 

AngusMcDoon

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How can the predicted low tide be below LAT? This morning the predicted low tide at Howth was -0.10m. Now tidal predictions can't account for the effects of weather, wind direction, air pressure etc. so if LAT is accurate how can this be?

Now I know this happens, if rarely, but is it an unforeseen astronomical event, or something else?

Tide heights are based on chart datum, which is LAT at the few locations where it has been measured. For Howth the chart datum is LAT at Dublin (which also happens to be the ordnance datum for the Republic of Ireland). At Howth the tides are a bit different and can go a bit lower. This is shown by the secondary port height differences which are 0.0 -0.1 -0.2 -0.2.

While met conditions can affect the actual tide height on the day they have nothing to do with the heights calculated from tables or by plotters.
 
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lpdsn

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How can the predicted low tide be below LAT? This morning the predicted low tide at Howth was -0.10m. Now tidal predictions can't account for the effects of weather, wind direction, air pressure etc. so if LAT is accurate how can this be?

Now I know this happens, if rarely, but is it an unforeseen astronomical event, or something else?

Original data is almost certainly in Fathoms and Feet, so probably just accumulated rounding errors in determining the chart datum and predicted height of tide in metres. I've seen the same thing happen in the Solent.
 

Hydrozoan

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Quite apart from meteorological circumstances, there will be some uncertainty attached to LAT estimates themselves (which are based on analysis of long periods (decades) of past records). For example, a paper I have seen by the Hydrographic Service of the Royal Netherlands Navy stated that "The precision of the LAT level is expected to be in the order of 10cm" - an uncertainty of the same order as the figure which triggered your interest. I guess - but do not have a specific source - that broadly similar levels of uncertainty might apply in the UK and Eire.
 
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shaunksb

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This morning the predicted low tide at Howth was -0.10m. Now tidal predictions can't account for the effects of weather, wind direction, air pressure etc. so if LAT is accurate how can this be?

Blimey Burnham is -0.4 for today and tomorrow. Nearly half a meter but we are approaching the equinox.

_____________________________
 

prv

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Blimey Burnham is -0.4 for today and tomorrow. Nearly half a meter but we are approaching the equinox.

Hence the annual Brambles cricket match round here in the Solent yesterday :)

(Holding it in March would expose more bank, but the sea would be a bit chilly for all the people paddling around in whites :D )

Pete
 

VicS

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How can the predicted low tide be below LAT? This morning the predicted low tide at Howth was -0.10m. Now tidal predictions can't account for the effects of weather, wind direction, air pressure etc. so if LAT is accurate how can this be?

Now I know this happens, if rarely, but is it an unforeseen astronomical event, or something else?

Angus McDoon just about explains it.


Chart datum is a flat horizontal plane equivalent to LAT at the nearest/ most appropriate standard ( or one of a small number of secondary) ports.

Because it is a flat plane , not undulating to match LAT at all locations it is possible for tidal predictions at locations other than the one to which LAT / chart datum is referenced to be negative. ie below that imaginary flat plane.

Tidal predictions are based on the standard atmospheric pressure of 1013 mb
 
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