A pilot sets his altimeter to the ASL of the destination airfield so as to avoid “landing early”. Tides rise and fall at different rates and by different amounts around the globe, so how is this mitigated? is there an average sea level that is consistent across the globe?
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The altitudes usead are above “mean” sea level, or the average sea level. Tides have no effect on the altimeter indication at all, since the altimeters use air pressure to calculate altitude.
While pilots do set the altimeter to correct for local barometric pressure differences, the error in indicated altitude can be significant, far more so than the variations in tides. To account for this, flight rules, minimum altitudes, etc., take into account this error to keep airplanes sufficiently separated from terrain, obstacles, and each other.
The altitudes that are flown in reference to an altimeter are above Mean Sea Level (MSL). So, the sea level is an average. I don’t know how they figured out the average sea level, but that’s what is used.
Unless you are flying a seaplane or landing on an aircraft carrier, most planes land on solid ground, which only moves a few inches with the tides.
Who cares? Go design an altimiter that corrects for tides and waves too and go test it by flying 5 feet ASL. Look out for boats.