Tide lower than LAT - how come?

[TheBomber]

According to my regular tide app (Boatie) the low water tidal height at London Bridge tomorrow (Thursday 21 Feb 2019) is -0.3m. How can this be?

 

[Findhorn]

High pressure ?

 

[Fr J Hackett]

Super full moon.

 

[LittleSister]

Super full moon.

But isn't that astronomically big/close?

 

[RobF]

I asked the same question a few years ago. Got some sensible answers here
http://www.ybw.com/forums/showthread.php?451585-Lower-than-LAT

 

[alan_d]

High pressure ?

I would doubt that the tidal prediction app predicts barometric pressure as well.

 

[TheBomber]

Rob - your previous thread seems to have it well covered, thank you. To my mind now it goes something like: because chart datum is determined for only for a limited number of ports (presumably standard ports) tidal heights at other locations (secondary ports) can be lower than CD.

For me this was only out of interest but it could have implications for moored vessels that don’t want to dry out and had done their calculations based on a LAT of CD? Presumably HAT is also similarly affected which could be more significant for craft on the Thames passing under all those bridges.

 

[ProDave]

This happened a few years ago. The conclusion is they got the chart datum wrong and 0 should really be a little bit lower than it is.

I think this is only the second year since I have been a boat owner that this has happened.

It also means high tide is higher than normal, today and tomorrow being very high. Some low lying areas will be worried and praying for still weather, i.e small waves.

 

[penberth3]

...For me this was only out of interest but it could have implications for moored vessels that don’t want to dry out and had done their calculations based on a LAT of CD? Presumably HAT is also similarly affected which could be more significant for craft on the Thames passing under all those bridges.

In theory, yes. But we're talking about a few tenths of a metre, less than a typical wave height. I hope you'd allow more than that as a safety margin?

 

[AntarcticPilot]

Chart Datums can only be established at locations where there is a permanent tide-gauge; the locations are available at https://www.bodc.ac.uk/data/hosted_data_systems/sea_level/uk_tide_gauge_network/#map, as are the data from which tidal constants can be derived.

However, the important thing to recall is that a datum (either horizontal or vertical) is an arbitrary fixed reference frame. For convenience the vertical datum of charts is usually said to be LAT - but it can only be LAT at the site of a tide-gauge. Elsewhere, the lowest tide may well drop below the chart datum because of local effects - estuaries and embayments are particularly liable to these, as they can amplify the tidal amplitude, resulting in tidal extrema that are above or below the datum defined at a point elsewhere.

It is not inconceivable (though unlikely) that two overlapping charts that happen to take their datum from different sets of tide-gauges COULD quite correctly show different charted depths. Of course, hydrographers will avoid this situation, and I certainly don't know of an example, but it is possible! I do know of maps in the Antarctic where the horizontal datum differs from place to place within a map; the map-makers (not us!) lost the errors in large glaciers!

 

[Lucky Duck]

According to the link below, a stepped datum is used for the Thames

http://www.pla.co.uk/Safety/Hydrographic-Survey-Notes

 

[dunedin]

Rob - your previous thread seems to have it well covered, thank you. To my mind now it goes something like: because chart datum is determined for only for a limited number of ports (presumably standard ports) tidal heights at other locations (secondary ports) can be lower than CD.

For me this was only out of interest but it could have implications for moored vessels that don’t want to dry out and had done their calculations based on a LAT of CD? Presumably HAT is also similarly affected which could be more significant for craft on the Thames passing under all those bridges.

It’s not just Secondary Ports - Greenock looks to be slightly minus the next couple of evenings.
And it is not particularly uncommon - happens a number of times every year, as far as I can recall.

Indeed, it is not the most extreme tide of the year. The French sensibly use tidal coefficients regularly, as an easy way to assess tide strength and ranges. I used to have to refer to La Rochelle or similar, but Tides4Fishing has helpfully started showing these (page well down)
https://tides4fishing.com/tides/tidal-coefficient

This shows yesterday was a 112 coefficient tide, a slightly bigger 113 in a month’s time, and a 115 peak spring tide in September.
I used to use these to work out when my boat was at risk of grounding in the shallow La Rochelle marina

 

[LittleSister]

It is not inconceivable (though unlikely) that two overlapping charts that happen to take their datum from different sets of tide-gauges COULD quite correctly show different charted depths. Of course, hydrographers will avoid this situation, and I certainly don't know of an example, but it is possible! I do know of maps in the Antarctic where the horizontal datum differs from place to place within a map; the map-makers (not us!) lost the errors in large glaciers!

I think I read somewhere that some UK charts have more than one datum on the same chart. An example was, I think, along the course of a river, where the datum at the river mouth is inconvenient further inland.

 

[JumbleDuck]

I'm in London at the moment and took a Thames Clipper trip down the river to North Greenwich and back. This afternoon's high tide was VERY high; the lions along the Embankment were half submerged and the main exit gangway from Westminster Pier was under water.

 

[AntarcticPilot]

I think I read somewhere that some UK charts have more than one datum on the same chart. An example was, I think, along the course of a river, where the datum at the river mouth is inconvenient further inland.

Yes, a chart of the Thames will have that - as mentioned above, they use a "stepped" datum, with abrupt changes as you move upstream. It's an interesting solution to the problem that a river (naturally) has a slope. It would be appropriate for the Severn and the Humber as well.