To clip a grid image to the bounds of another grid image open the Grid Utilities plugin from the Gridding menu and choose the Grid To Grid Clip option from the Utlities button drop down list.
If the grids being applied to this process are not already loaded into the current session of Encom PA click on the Open button (yellow folder) at the top of the Grid Utilities dialog and browse to their file location. Once the grids are loaded they will be listed in the Input grid: and Clip to grid: drop down lists. Nominate from each of these lists which file is the grid to be clipped and which is the grid that the other grid will be clipped to. Click the Save As... button at the bottom of the dialog and specify a name for the resultant grid image file.