First please note that none of our data should ever be used for navigation or passage planning or for making decisions where there is a potential for harm to anyone. It should go without saying that we are not going to be responsible for any decisions you make!
A tidal prediction is just that, a well educated guess based on years of observed data. Many factors will affect the tide including local winds and pressure systems. A margin of error of at least 10 minutes is to be expected with any forecast and its this level of accuracy we aim to achieve. Data quality varies by port and country: large western commercial ports will be very accurate, small Asian fishing villages are probably going to be somewhat less accurate. If you spot any that are wildly inaccurate please let us know.
Tidal predictions were primarily of interest to mariners so tidal observation data has traditionally been collected at major ports. Minor ports are then deduced from these. As surfers we don't spend a lot of time in ports or harbours but as a rule a tide prediction for a nearby beach will only vary by a few minutes from the nearest port, but check that the port listed is actually close to your chosen break!. We automatically select the nearest port for which we have data based on distance. Occasionally this produces undesirable results, as a further away port might have a better correlation. Again, let us know if you spot one that is right out.
The tide chart can be a little daunting at first, but it's really very simple and once you get used to it a lot easier to read than a sheet of numbers.
The greys at the top represent day and night and twilight, the greens at the bottom tell you what the tide is doing. The rising and falling tides are different colours only to make it easier to see where the highs and lows are.