Editing Physical geodesy (section)

==Measurement procedure==
Traditional geodetic instruments such as [[theodolite]]s rely on the gravity field for orienting their vertical axis along the local [[plumb line]] or local [[vertical direction]] with the aid of a [[spirit level]]. After that, vertical [[angle]]s ([[zenith]] angles or, alternatively, [[elevation]] angles) are obtained with respect to this local vertical, and horizontal angles in the plane of the local horizon, perpendicular to the vertical.

[[Levelling]] instruments again are used to obtain [[geopotential]] differences between points on the Earth's surface. These can then be expressed as "height" differences by conversion to metric units.

===Units===

Gravity is commonly measured in units of m·s<sup>−2</sup> ([[metre]]s per [[second]] squared). This also can be expressed (multiplying by the [[gravitational constant]] '''G''' in order to change units) as [[newton (unit)|newton]]s per [[kilogram]] of attracted mass.

Potential is expressed as gravity times distance, m<sup>2</sup>·s<sup>−2</sup>.  Travelling one metre in the direction of a gravity vector of strength 1 m·s<sup>−2</sup> will increase your potential by 1 m<sup>2</sup>·s<sup>−2</sup>. Again employing G as a multiplier, the units can be changed to [[joule]]s per kilogram of attracted mass.

A more convenient unit is the GPU, or geopotential unit: it equals 10 m<sup>2</sup>·s<sup>−2</sup>. This means that travelling one metre in the vertical direction, i.e., the direction of the 9.8 m·s<sup>−2</sup> ambient gravity, will ''approximately'' change your potential by 1 GPU. Which again means that the difference in geopotential, in GPU, of a point with that of sea level can be used as a rough measure of height "above sea level" in metres.