# Glossary#

- Co-located grids#
One or more grids that have grid points are at exactly the same locations. This property is important when performing operations like adding and subtracting grid values since they must be located at the same geographical locations.

- Geocentric gravitational constant#
The multiplication of the total mass of a reference ellipsoid and the gravitational constant. Often represented as \(GM\).

- Gravity disturbance#
The gravity disturbance is the difference between the magnitude of observed gravity and the magnitude of normal gravity evaluated at the same point [HofmannWellenhofMoritz2006]: \(\delta g(\lambda, \phi, h) = \|\vec{g}(\lambda, \phi, h)\| - \|\vec{\gamma}(\lambda, \phi, h)\|\)

- Gravity potential#
The combined gravitational and centrifugal potential \(U = V + \Phi\) [HofmannWellenhofMoritz2006].

- Normal gravity#
The magnitude of the gradient of the gravity potential of a reference ellipsoid.

- Reference ellipsoid#
An ellipsoid with geometric and physical properties used to best approximate a given planetary body (the Earth or others). See Wikipedia.