Editing Geodesy (section)

== Measurements ==
{{further|Satellite geodesy|Geodetic astronomy|Surveying|Gravimetry|Levelling}}
{{unsourced section|date=February 2024}}
[[File:GRAIL's gravity map of the moon.jpg|285px|thumb|right|Variations in the gravity field of the [[Moon]], from [[NASA]]]][[File:Gravity measurement devices, pendulum (left) and absolute (right) - National Museum of Nature and Science, Tokyo - DSC07808.JPG|285px|thumb|right|Gravity measurement devices, pendulum (left) and absolute gravimeter (right)]]
[[File:Autograv CG5 P1150838.JPG|85px|thumb|right|A relative gravimeter]]

The reference surface (level) used to determine height differences and height reference systems is known as [[mean sea level]]. The traditional [[spirit level]] directly produces such (for practical purposes most useful) heights above [[sea level]]; the more economical use of GPS instruments for height determination requires precise knowledge of the figure of the [[geoid]], as GPS only gives heights above the [[GRS80]] reference ellipsoid. As geoid determination improves, one may expect that the use of GPS in height determination shall increase, too.

The [[theodolite]] is an instrument used to measure horizontal and vertical (relative to the local vertical) angles to target points. In addition, the [[Tachymeter (survey)|tachymeter]] determines, electronically or [[Electro-optics|electro-optically]], the distance to a target and is highly automated or even robotic in operations. Widely used for the same purpose is the method of free station position.

Commonly for local detail surveys, tachymeters are employed, although the old-fashioned rectangular technique using an angle prism and steel tape is still an inexpensive alternative. As mentioned, also there are quick and relatively accurate real-time kinematic (RTK) GPS techniques. Data collected are tagged and recorded digitally for entry into [[Geographic information system|Geographic Information System]] (GIS) databases.

Geodetic GNSS (most commonly [[Global Positioning System|GPS]]) receivers directly produce 3D coordinates in a [[geocentric]] coordinate frame. One such frame is [[WGS84]], as well as frames by the International Earth Rotation and Reference Systems Service ([[IERS]]). GNSS receivers have almost completely replaced terrestrial instruments for large-scale base network surveys.

To monitor the Earth's rotation irregularities and plate tectonic motions and for planet-wide geodetic surveys, methods of [[very-long-baseline interferometry]] (VLBI) measuring distances to [[quasar]]s, [[lunar laser ranging]] (LLR) measuring distances to prisms on the Moon, and [[satellite laser ranging]] (SLR) measuring distances to prisms on [[artificial satellites]], are employed.

[[Gravity]] is measured using [[gravimeters]], of which there are two kinds. First are ''[[absolute gravimeter]]''s, based on measuring the acceleration of [[free fall]] (e.g., of a reflecting prism in a [[vacuum tube]]). They are used to establish vertical geospatial control or in the field. Second, ''[[relative gravimeter]]''s are spring-based and more common. They are used in gravity surveys over large areas — to establish the figure of the geoid over these areas. The most accurate relative gravimeters are called ''[[superconducting gravimeter]]''s, which are sensitive to one-thousandth of one-billionth of Earth-surface gravity. Twenty-some superconducting gravimeters are used worldwide in studying Earth's [[tide]]s, [[rotation]], interior, [[ocean]]ic and atmospheric loading, as well as in verifying the [[Newtonian constant of gravitation]].

In the future, gravity and altitude might become measurable using the special-relativistic concept of [[time dilation]] as gauged by [[Atomic clock#Research|optical clocks]].