Sets of relative gravity values show spatial patterns, beside a decrease with station elevation, and a poleward increase, also geological effects. The latter are usually obscured by the other influences which must be removed in order to reveal the geological information. This cleaning operation is called “reduction” and the results are called anomalies of various kinds which are the target of interpretation. Proper reductions require knowledge of the station coordinates and of the normal gravity field with which the observations are to be compared. In geodesy, the term “disturbance” is more common, this stems historically from different references used, as described below. In both cases the purpose of the reductions is to relate the observations to a reference, and this is, in the proper sense of the term, not a “correction” of errors or mistakes. The customary term “correction” is therefore avoided in this book. This is also true for the tidal effects, removed by a tidal reduction, while, on the other hand, the removal of the instrumental drift can be considered truly a correction. The ultimate accuracy of the anomaly values is affected by both the observations and the reductions.
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Johannes, W.J., Smilde, P.L. (2009). Gravity Anomalies and Disturbances: Reductions and Analyses. In: Gravity Interpretation. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-85329-9_4
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