SSC Tide Times

I knew that computers were the world's biggest timewasters. I have recently found that tide tables come a close second. The combination of both is a truly "awesome" timewaster!

As we all know, tides are caused by the gravitational pull of the sun and the moon on the world's oceans. The fact that they rise and fall reflects the rotation of the earth, the rotation of the moon, the rotation of the earth-moon system, the orbit of the moon around the earth and the orbit of the earth-moon system around the sun. If all these things were symmetrical then tides would be relatively simple. However, all these parameters have got wobbles, and each wobble introduces a further level of complexity - known as harmonics.

All this can be calculated mathematically, giving theoretical tide tables for anywhere on earth, assuming that the oceans are all of even depth with smooth floors and there are no land masses! To calculate real tides on a real earth would require the assistance of Marvin, so the rest is done empirically. Most countries with a coast line measure the tides at various points and these measurements are used to refine the mathematical model so actual tides can be predicted. In the US this function is carried out by NOAA and all data is available online. NOAA also have a superb website which carries a fascinating explanation of all this tide stuff. You can ignore the maths and just look at the pictures and text or follow the link to Education / Tides to get a high school version which is still quite good.

In the UK the United Kingdom Hydrographic Office (UKHO) is responsible for measurements dating back hundreds of years. Unfortunately, in 2002 the UKHO became a government cost centre. Despite the fact that the taxpayer has already paid for all these measurements, the data was removed from the public domain and it has to be purchased by any who wish to use it. All publishers of tide tables and tide software were warned of legal action unless they removed UKHO data from their products, or paid for the data. This could have been the end of free tidal predictions in the UK but for the fact that the British Oceanographic Data Centre based at the Proudman Oceanographic Laboratory, Liverpool maintained a limited tidal measurement system in UK waters. This institute gives permission for their data to be freely used with acknowledgement.

Once you have calculated the tables, you have to be aware that other factors can have very large influences on the tides. Wind and atmospheric pressure are different every day and are not taken into account by the model. It surprises me that tide table predictions are given to the minute - my scientific training says that it is a nonsense to calculate a rough approximation to 4 decimal places. SSC tables are rounded to the nearest 10 minute period, which I think is more honest. In any case, the actual moment of high or low tide is of only academic interest (unless you're stuck on the mud at Iken, I suppose!) as there is a period of slack water at each extreme when the tide isn't sure if it's coming or going.

The base tide table is generated using the XTides free software which, for UK waters, uses the data from the British Oceanographic Data Centre, with thanks. Their nearest measurement centre is Lowestoft, so is not ideal as predictions become less accurate with distance from the measurement point. However, I have compared Lowestoft data from the Xtides program with that from the UKHO and come up with an average difference of just 0.5 +- 6.5 minutes; insignificant when compared with other inaccuracies.

Calculating the Slaughden tides from Lowestoft data is a little less accurate. By comparing Xtides with UKHO data for the two locations I find that Slaughden high tides occur 3hr 16 mins after Lowestoft (+- 10 mins), whilst low tides are 3hr 29 mins later (+- 24 mins). The variability here is because the lag between the sites varies with the height of the tide -springs vs neaps. If I'm bored for a week sometime I'll perhaps do some more number crunching to improve on the algorithm! At least I'm honest about the accuracy of the SSC times, which is more than you can say for all the other sources!

Don't forget that SSC members can print out custom tide tables for Slaughden Quay from the members' only area of the website.