Sorry – been meaning to reply to this earlier and hopefully clarify any confusion from the earlier explanation. Apologies also for a rather long, detailed post, especially if I’m describing stuff that people already know well.
As I said before, our anyTide app uses two types of tidal predictions:
1. Based on known ports around the UK & Ireland
2. Models generated from a grid of triangles with 1.8km sides anywhere along the coast on the NW European shelf
The predictions at known ports are based on “real” data. i.e like other tide prediction engines, they’re based on a physical tide gauge sitting at a known location. The historical measurements from this are analysed and used to generate future predictions (via tidal harmonic constants). Since the tide gauge is sitting at a known location, the chart datum at that point is also known/can be calculated. Hence, the predictions at known ports are based on chart datum.
This is also true for our anyTide app – all the known/listed ports (shown as purple pins on the map) are referenced to chart datum by correlating the known mean sea level (MSL) with the known chart datum for that tide gauge. (MSL is a measurement of the sea level when high and low tides and wave peaks/troughs are all averaged out and is available directly from a tide gauge or derived from MHWS, MLWS etc).
However, as a general point, it’s really important to note that there can still be differences between the tide prediction for a tide gauge and the actual recorded tide at that point, e.g. for the tide gauge at Portsmouth (located on the south west corner of Victory jetty), here’s a chart showing the differences for yesterday and today:
Real-time/near real-time data display for Portsmouth | National Tidal and Sea Level Facility
The tide predictions are influenced by the number of harmonic constituents you choose to use for your calculation and are solely based on astronomical effects. I’d be extremely cautious about assuming any tidal prediction is accurate to the odd few cms and then being surprised when the “real” tide is sometimes considerably different. Meteorological effects can cause significant differences – e.g. all tide predictions assume an average atmospheric pressure (1016mb for the UK south coast). A 1mb difference in atmospheric pressure can cause a change in water height of around 1cm. So, yesterday in Southampton, the average barometric pressure was 1027.8mb, causing a possible lowering of water level by about 10cm. This will not be shown on a tide chart. The max meteorological effect is usually around 30cm, but can be worse for storms, etc. There’s a good summary at:
I think the confusion about datums has arisen because of the 2nd type of prediction in anyTide – using models to predict the tide at any point around the coastline. In this case, the model is using a complex algorithm for a specific triangle of sea area without a tide gauge to generate a tide prediction and doesn’t have a known reference point with a record of historical tides or an associated chart datum. In this case we have access to detailed mean sea level (MSL) records and use these to generate a datum for each triangle. You can use the MSL to centre the tide predictions and then estimate a Lowest Astronomical Tide (LAT) based on a series of spring tide calculations. Since LAT approximates to chart datum, it gives a reference point for the modelled tides – with the important proviso that LAT is usually calculated over a very long period of time – years – to account for long term (and often more extreme) astronomical events. So the calculated datum is by no means perfect, but it's the best effort to produce a useful tide chart in these areas. We’re looking at adding precise chart datum to the modelled tides in a future app update, but it’s a non-trivial task since we’ll need to add reference data for every 1.8km coastal grid on the NW European shelf.
As regards Littlehampton specifically, I’m not sure why there’s a difference. I’ve correlated anyTide with our professional tide prediction engine, POLTIPS3 (sold to marine organisations and not cheap!) and it looks good. Littlehampton is a port with a tide gauge, so isn’t using the modelling side (to be precise the chart datum at the Littlehampton gauge is 2.74m below Newlyn Ordnance Datum, in case anyone’s interested).
e.g. today I’ve compared easy Tide online, anytide and POLTIPS and don’t see a great deal of difference. To quote the FAQ section on the UKHO easytide webpage about why different sources may estimate different tides:
“...the two sets of predictions are simply different, and neither is necessarily wrong. The best way to quantify the accuracy of any predictions would be to carry out a rigorous statistical analysis of the observed tide against the predicted tide.”
As regards the yachtsman running aground – always err on the side of caution. If they thought low tide was at 08.25 instead of 08.57, they should have actually had more water under the hull than they thought. However, the bottom of the tide curve is not going to show much change in water over about 30 minutes.
Best to use the tide prediction that you’re most comfortable with – but do so knowing what the inherent limitations are.
P.S. NOC is based at Liverpool & Southampton and we run sea going research ships and coastal vessels. Lots of the people who work here enjoy boating and understand the practicalities...