Knowledge Bank

A practical scheme of approximately equal area blocks in various sizes is considered for the representation of the global mean gravity anomaly field. It is assumed that the starting data is mean anomalies over 1° x 1° equiangular blocks, instead of point anomalies. This leads to the constraint that the latitudinal and longitudinal limits of all blocks should be in integer degrees, so that the calculation of proportional anomalies over fractional 1° x 1° blocks is avoided. Similarly, a second condition is imposed that the external limits of smaller size blocks should match exactly with the limits of a large size block, e.g. a 5° equal area block should encompass a whole number of 1° equal area blocks. This necessitates the subdivision of a larger block into component blocks of smaller size. It is found that different block limits result when we consider a 10°, 15°, or 30° equal area block and its subdivisions. It is proposed that the largest block should be taken as a 10° equal area block, and 5°, 2°.5, 2° and 1° equal area blocks should be obtained by the subdivision of 10° blocks. This will allow the representation of the global gravity field by anomalies in blocks of different sizes with matching boundaries. The block limits of 10° equal area blocks, and the component 5° and 1° blocks are given. The areas have been tabulated on unit sphere, as well as on unit ellipsoid, with semi-major axis as unity.