Preprocessing
The surface program assumes that the data have been preprocessed to eliminate aliasing, hence we must
ensure that this step is completed prior to gridding. GMT comes with three preprocessors, called blockmean,
blockmedian, and blockmode. The first averages values inside the grid-spacing boxes, the
second returns median values, wile the latter returns modal values. As a rule of thumb, we use means for
most smooth data (such as potential fields) and medians (or modes) for rough, non-Gaussian data (such as
topography). In addition to the required –R and –I switches, these preprocessors all take the same options
(listed in Table 3.4).