When i change the node registered satellite bathy to pixel registered, then, basemap region changes slightly, just like latitude 5 to 4.99999, so i can't use grdmath for grid subtraction

I have Gebco-2014 satellite bathymetry data, from which i have to calculate water effect and find bouguer anomaly. When i change the node registered bathy to pixel registered, then, basemap region changes slightly, just like latitude 5 to 4.99999. Due to which my region is being different from the satellite gravity as it says grid size is not same.

I am using grdsample for converting from node to pixel using -T.

Region does not change, but node positions do, so you cannot subtract two grids that do not align. If the grid you want to subtract is gridline reg, why not do the math before you change teh registration.

BTW, changing registration means you loose the shortest wavelengths in your data.