The YoYoMan wrote:
Ep!

where is the zero point of "sea level" (gps coordinates) ????


That depends on how you set the datum. Look in the menus of your GPS
machine and see which datum sets are available, then Google up the
references to see which is most appropriate for your usage & area.

in what sea/ocean ????


Varies widely, "mean sea level" for specific areas is defined by
averaging out the tidal variation over a 19 year cycle.



how is measured altitude of mountains ?

With a very very long piece of string.

DSK