The correction needed when a survey is conducted on a mobile vehicle to account for centrifugal effects, especially in an East-West traverse, is called what?

• A. Eötvös correction
• B. Free-air correction
• C. Bouguer correction
• D. Instrument drift correction

Answer: (A)

When you measure gravity from a moving vehicle, the rotation of the Earth creates a centrifugal acceleration that changes with your motion and latitude. Moving east or west changes how much centrifugal force you experience because you’re effectively riding around the Earth's axis at a different tangential speed. This alters the gravity reading, so you must correct for it to get true gravity.

This adjustment is known as the Eötvös correction. It’s typically summarized as roughly 2 times the Earth's rotation rate times your transport speed, times the cosine of your latitude, reflecting how the effect is strongest for east-west motion and at mid-latitudes. So, moving east tends to decrease the observed gravity and moving west tends to increase it.

Other corrections address different factors: the Free-air correction accounts for height above a reference surface, the Bouguer correction accounts for the mass between the measurement point and that surface, and instrument drift corrects for the gravimeter’s own bias. They don’t pad in the motion-related centrifugal effect that the Eötvös correction handles.