I am trying to do a bathymetric map for a lake using QGIS. However, I see no option to impose a mask so interpolation does not extending beyond the boundaries of my lake. I have traced around the lake (polygon) to which I would like to restrict the interpolation. Is there a "set mask" function in QGIS that would restrict all subsequent calaculations? I know how to convert the polgon to raster if needed. I have cliped to the lake boundaries but it still runs over. Do I need to go to GRASS to do this? Using version 1.8 on Windows 7.