Hello, I think you are misunderstanding the way that GMT works with projections. When you specify the projection -JM10c that means that you are going to provide data as latitude and longitude in degrees and want GMT to project that to a Mercator projection. That is why it is wrapping the values that are greater than 360 degrees. If you want to make a map where you have data already in kilometers, then you need to use one of the “Cartesian” projections -JX10c or -Jx. You should read the GMT manual on projections for more details.