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Reference surface

Introduction

A reference surface is a datum that provide a reference point to the Earth's surface. There are horizontal datums, that describe the position on the Earth's surface and vertical datums that describe elevation. Two main reference surfaces have been established to approximate the shape of the Earth: one is called the Geoid, the other the Ellipsoid.

The surface of the Earth is far from uniform. Its oceans can be treated as reasonably uniform, but the surface or topography of its land masses exhibits large vertical variations between mountains and valleys. These variations make it impossible to approximate the shape of the Earth with any reasonably simple mathematical model.

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Figure: The Earth’s surface and two reference surfaces used to approximate it: the Geoid; and a reference ellipsoid. The Geoid separation (N) is the deviation between the Geoid and the reference ellipsoid.

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