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The three things that make tide sites disagree
1. Reference port
A tide prediction belongs to a specific point on the coast. Cork Harbour is the clearest example in Ireland. The flood reaches Roches Point at the mouth first, then Ringaskiddy, then Cobh, then Cork City at the top of the river. Over that distance the time of high water moves by the best part of half an hour. A site that says "Cork" without telling you which one has hidden the most important detail on the page.
Every location page here names its station and gives the coordinates.
2. Datum
Height has to be measured from something. Admiralty tide tables use chart datum, set near the lowest astronomical tide, so their numbers are nearly always positive and a "4.5 m" high water is normal. The Marine Institute publishes against Ordnance Datum Malin, which sits near mean sea level, so the same tide reads as about plus 1.8 m at high water and minus 1.9 m at low water.
Same water, different ruler. If you are comparing this site to a printed almanac, compare the range and the times, not the raw height.
3. Prediction versus observation
These are astronomical predictions. They assume average air pressure and no wind. A deep low pressure system can lift the real water level well above prediction, and a hard offshore blow can hold it down. In a storm surge the gap can be significant. That is why no tide table is a safety device.
Spring and neap tides
Twice a month, around the new and full moon, the sun and moon pull together and you get a spring tide: the biggest range of the fortnight, highest highs and lowest lows, and the fastest flow between them. A week later, at the quarter moons, they pull against each other and you get a neap tide with a much smaller range.
Nothing to do with the season. "Spring" here means to spring up. Every location page labels the day as spring, neap or mid range and gives the range in metres.
Questions
- Why do tide websites show different times for the same place?
- Because they use different reference ports, different datums and different prediction models. A site quoting Cork City will be later than one quoting Ringaskiddy, because the tide takes time to travel up the harbour. Neither is wrong.
- What is chart datum?
- Chart datum is the level charted depths are measured from, set at roughly the lowest astronomical tide. Admiralty tide tables give heights above it, so they are almost always positive. This site uses Ordnance Datum Malin instead, the Irish land survey datum, so our heights swing above and below zero.
- Why are your heights negative at low water?
- Because they are measured from Ordnance Datum Malin, which sits near mean sea level, not from chart datum at the bottom of the range. A low water of minus 1.8 m here is the same physical water level another site might call 0.4 m.
- Are these times in Irish local time?
- Yes. The Marine Institute publishes in UTC and we convert to Europe/Dublin, so the times shown already account for Irish Summer Time.