There two different definitions of a global vertical datum:
Given the definition of the Geoid, global vertical datums can be defined by gravity satellites. These satellites measure the earth's gravity. The variations in gravity are converted to heights that can be compared globally all over the world (so mountain ranges in America can be measured against those in Europe). The local datum systems can be linked together which simplifies height-related issues.
global height datum
Explain the relevance of reference surfaces, coordinate systems, and coordi-nate transformations in mapping (level 1 and 2).