MSL

It's a chicken and egg problem: one often speaks of - makes use of -
the notion of 'at sea level' with respect to things like barometric pressure
and so forth. And sure enough various countries have observation posts in use
since the 19th century making repeated measurements to have a reliable measure
of the mean sea level for navigation and other uses. So where does all this talk about
rising sea levels come into the picture. If sea level is the standard, how would one
know it had changed?

Turns out we have a reliable solution to this and it comes from space observation,
with a radar altimeter. To this we have added global measurements of water temperature
and salinity, and satellite readings of the earth's changing mass. Fascinating reading, below:

https://en.wikipedia.org/wiki/Sea_level

https://www.nae.edu/Publications/Bridge/195218/195285.aspx?CFID=3f687960-14a3-471f-8491-874ca33ce554&CFTOKEN=0