overlay
given two grids that have the same dimensions (number of columns and rows, resolution), and location (min and max X and Y coordinates), overlay allows you to place them on top of each other (as it were), carry out some arithmetic on corresponding grid cells (i.e., cells directly above each other), and place the results in a new, third grid. the arithmetical operetions included are addition, subtraction, multiplication, division and calculation of minimum and maximum. in the value of a cell in the first grid is "nadata", in which case the value of the cell in the second grid is used