Measuring the elevation of a mountain means measuring variation in , whereas doing the same on a mid-ocean ridge would be measuring variation in bathymetry; topography bathymetry; isostasy topography; isostasy topography; bathymetry

Measuring the elevation of a mountain means measuring variation in topography, whereas doing the same on a mid-ocean ridge would be measuring variation in bathymetry. 
Topography refers to the physical appearance of natural features and includes the mountains, hills, creeks, and other bumps and lumps.
Bathymetry is the term used to define the depth of the ocean, sea or lake relative to sea level.