Forgive me if this is well known. I have not seen references to it - but recently had some problems as a result. I have seen some references to the difficulties of getting heights above sea level in some recent posts and though this might be helpful.
OFP appears to have a feature whereby moving a trigger only changes its x and y coordinates. It does not change its z coordinate. So if you create a trigger (called say TRIG) on the sea, then move it to a location on the map, then height of that point above sea level is = -(getPos TRIG select 2)
Note the minus sign, this is needed at the trigger remains at sea level and so is beneath the ground.