The earth’s spherical shape imposes an effect that is similar to that caused by the earth’s rotation. As an object travels across the surface of the earth, its path becomes an arc due to the shape of the earth. Consequently, the gyros of a stabilized platform measure a rotational rate, because traveling in a curved path always involves rotation. This rate does not describe the rotational motion with respect to the earth’s surface and therefore must be compensated for best performance of the gyro-stabilized platform. Latitude and heading must be known precisely for Earth Rate Compensation.

Gravity measurements made in a move (e.g. on a moving ship) get induced by centrifugal acceleration. This is because the defined measurement mass’s weight is influenced by centrifugal acceleration. As earth is rotating to the east the weight of an object is reduced by the centrifugal force depending on latitude (different rotating speed on different latitude). On the equator rotation is 465 m/s, resulting in approx. 0.34% of weight reduction on a static object there. If the object is moving eastwards the motion adds to the earth’s rotation, resulting in an additional reduction of weight and therefore less measured gravity. If the object is moving westwards, some of the earth’s rotation is cancelled out and therefore more gravity is measured. The effect can be huge and up to 2.5mGal per km/h. Speed, latitude and heading must be known precisely for Eötvös Corrections.

Gravity measurements on the same position, but in different heights differ because with more distance to the earth centre gravity must decrease. This effect is approximately 0.3086 mGal/m in free air. Even a ship on sea therefore perceives significant measurement differences compared to mean sea level (respectively seafloor), which is the reference level when assuming that earth is a perfect sphere. The height above or below mean sea level (respectively sea depth) must be precisely known for Free-Air Correction. The Free-Air Anomaly can be calculated by incorporating the Free-Air Correction on the measured gravity.

With the Free-Air Correction in mind, in reality there is not air between the measuring point and sea level but some sort of denser material, for marine gravity measurements this is water. The density of the material must be known in addition to the height for the Bouguer Correction. The Bouguer Anomaly can be calculated by incorporating the Free-Air-Correction and the Bouguer Correction on the measured gravity.

Tide induces acceleration on a swimming object like a ship resulting in a gravity measurement error similar to that one induces by a moving object. Incorporating an advanced tidal this effect can be compensated. Position must be known for Tide Compensation.

Grid-measurement incorporates the ship to turn for the next straight run on the grid. The centrifugal acceleration of turns does not allow for accurate measurements. Accelerated measurement-readiness is provided by the Turn Compensation saving time and costs.