low tide - negative value please help me understand!

[Burnham Bob]

Such an obvious question I'm sure someone will take me to task and say the answer has been posted x million times, but my Bellfield tide plotter says to expect a tide of -0.2 today at Burnham on Crouch. I always thought that chart datum was LAT - but this is below that. Can somebody explain please?

 

[maby]

Various factors can push the tide lower - mainly air pressure and wind direction. Pressure is quite high at the moment and that will drop the tide level an inch or two. Not much wind right now but think about it - if you are in a fairly narrow channel and the wind is blowing directly along it in one direction or the other, it can also make a difference.

LAT is a theoretical value - unusual for the actual tide to be lower, but by no means unknown.

 

[MoodySabre]

This is the definition of Chart Datum from UKHO website:


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.

The highest and lowest predicted tides that can occur are deemed Highest Astronomical Tide (HAT) and Lowest Astronomical Tide (LAT) respectively. These levels are the highest and lowest levels which can be predicted to occur under average meteorological conditions, and under any combination of astronomical conditions; these levels will not be reached every year. HAT and LAT are not the extreme levels which can be reached, as Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur) and pressure effects can significantly alter the times and / or heights of the observed tide.

When full moon occurs at HW (around here after lunch or just after midnight) then the astronomical conditions are not "average" and HW and LW often exceed HAT and LAT - according to Belfield. The Oceanographic tide tables never show a negative value. All these are predictions of course and metreological conditions affect even the best of them.

 

[David2452]

Coincidence, we are visiting Burnham at the moment and my Garmin plotter tide graph shows a predicted -0.3 for today, just means its that much less depth than shown on charts, which is LAT.

 

[AngusMcDoon]

But it does say any combination of astronomical conditions. I thought it was because the datum was often fixed many years ago not very accurately and since that was done more measurements have been made and accuracy improved.

 

[Burnham Bob]

okay - understand the air pressure can make a difference but tidal software can't take that into account........and as quoted above "it does say any combination of astronomical conditions". maybe angus is right that datum was fixed a while ago not so accurately as it might have been. mind you for all practical purposes this is irrelevant to the sailing that i do unless i'm trying to get into somewhere like Yokesfleet Creek at low water which i wouldn't...........

 

[VicS]

okay - understand the air pressure can make a difference but tidal software can't take that into account........and as quoted above "it does say any combination of astronomical conditions". maybe angus is right that datum was fixed a while ago not so accurately as it might have been. mind you for all practical purposes this is irrelevant to the sailing that i do unless i'm trying to get into somewhere like Yokesfleet Creek at low water which i wouldn't...........

The negative tidal predictions occur because the chart datum is based on LAT at certain specific locations I think you will find, for example, that the chart datum applicable to Burnham is the level of LAT at Sheerness.

Locally predicted tide heights can fall below chart datum but they wont normally at the port on which chart datum is based.

 

[Burnham Bob]

ah - thanks VicS - that does make sense!

 

[Elessar]

Such an obvious question I'm sure someone will take me to task and say the answer has been posted x million times, but my Bellfield tide plotter says to expect a tide of -0.2 today at Burnham on Crouch. I always thought that chart datum was LAT - but this is below that. Can somebody explain please?

Vic's right of course. Add the high pressure and its a good time to take some pics of things you won't normally see.
And to watch the "stay in the blue bits" brigade run aground