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To study the subsurface features and structures, the gravity effects of the surrounding topography should be reduced from acquired gravity field data. Several methods are available to calculate terrain effects on each gravity station. Some of these methods are tedious and time consuming due to a large number of calculations, long-distance terrain effects, and minimum acceptable errors. The other fast methods do not fulfill the accuracy requirement for local surveys. In rough topographies, using average elevation for sectors of the calculation area leads to overestimation or underestimation of the terrain effect. Since most of the terrain correction methods employ the pre-divided web or mesh for the survey area, the used sectors do not match geographical features with distinct mass centers. An Optimally Selecting Sectors (OSS) algorithm is proposed, which automatically partitions the surrounding area to a set of sectors in a way that separates different mass centers, finds optimum elevation of these sectors, and calculates terrain effect at the gravity stations. This new procedure improves the accuracy of calculated terrain effects. A proper tolerance inside the algorithm controls the accuracy of the method. Defining this tolerance relies on the application of gravity data. The application of this method on synthetic models with different geometrical shapes and real digital elevation data of a mountainous area at the Kurdistan region shows improvement in the accuracy of terrain correction. However, the proposed method OSS introduces extra calculation compared to some of the previous terrain correction methods. [ABSTRACT FROM AUTHOR]