The first thing to remember about a tide table is that it is only a prediction. It forecasts the time of high tide or low tide (sometimes called high water or low water) at a particular location, together with the height of the tide, usually these days in metres. These predictions are based on actual measurements over many years, together with knowledge of astronomical movements - the positions of the sun and moon relative to our planet. What the predictions cannot take account of is the weather. For the Thames, both the tide times and their heights can be affected by these meteorological factors:
Fortunately we do have the Thames Barrier to protect us from the worst effects. The Barrier is closed either to protect London against exceptional tidal surges coming up the estuary or, under more normal tidal conditions, if there is not enough room for the rising tide because of a full river after heavy rainfall. The Barrier is supplemented by flood defences, mostly by embankment walls. But not everywhere! Roads such as The Embankment at Putney or Chiswick Mall regularly flood because they are on the "wrong" side of the flood defences, planned that way in order to preserve the character of such places.
The height of a tide at any location is supposed to be the height of high water (or low water for a low tide) above the "chart datum" for that location. Chart datum is roughly the lowest water level that can theoretically occur, taking account of astronomical factors but ignoring weather and river flow effects. The problem is that local height predictions and/or measurements are only available for a few locations on the Thames. I've therefore decided to stick to London Bridge heights which are readily available, and which people may already be familiar with, as they are quoted in newspapers and printed tide tables. The "London Bridge" data provided by the PLA in fact relates to the nearby Tower Pier, where they have a tide gauge.
The use of London Bridge heights in my tables means that they will not correspond to the heights shown on any local tide gauge poles such as the one in the photo on the left (near Chiswick Pier). But the heights in the tables do give a good indication of the relative height of the tide.
The PLA publish "tidal differences" which give the average time difference between high tide at London Bridge and at other locations further upstream, and similarly for low tides. Until 2011, I added these on to the predicted London Bridge times to give the local tide times. But the published differences are only averages, and in practice the tides move up the river at different speeds.
What I have done for the last few years is to estimate the speed of each individual high tide from the predicted times for Chelsea Bridge and Richmond Lock, and to calculate the local tide times based on that. This should give more accurate times than the averages used previously. The 2014 tables use the same technique to estimate the low tide times too.
If you have any other queries about tides or this website, have a look at the FAQ page to see if you can find the answer there.