How do they figure out how big the tide is?

Question

I look at tide charts and see all the measurements like 0.6 metres, 1.4 meters and so on...

but how do they figure out how big the tide is/will be?

and how do they figure out where 0.00m is??

Answer 1

umm, there is something people like to call "sea level" k?

Answer 2

It's based on the strength of the pull between the moon and the sun.  Since both bodies move, which causes them to change distance from the earth, then the tide height will change accordingly.

http://www.sailingusa.info/tides.htm