Next, we show how to plot a data grid that is distributed in projected form. The gravity and predicted bathymetry grids produced by David Sandwell and Walter H. F. Smith are not geographical grids but instead given on a spherical Mercator grid. The GMT supplement imgsrc has tools to extract subsets of these large grids. If you need to make a non-Mercator map then you must extract a geographic grid using img2grd and then plot it using your desired map projection. However, if you want to make a Mercator map then you can save time and preserve data quality by avoiding to re-project the data set twice since it is already in a Mercator projection. This example shows how this is accomplished. We use the -M option in img2grd [1] to pull out the grid in Mercator units (i.e., do not invert the Mercator projection) and then simply plot the grid using a linear projection with a suitable scale (here 0.25 inches per degrees of longitude). To overlay basemaps and features that has geographic longitude/latitude coordinates we must remember two key issues: This map of the Tasman Sea shows the marine gravity anomalies with land painted black. A color scale bar was then added to complete the illustration. Source.