Export Overlay The Export Overlay command displays the standard Windows Save As dialog box. Here you specify the name of a text file that will hold the overlay values (z) in a tab delimited format. The first two lines of the file contain the number of columns and rows, respectively, of the data in the text file. The file can then be used outside of Graphic Groundwater with any program capable of reading a text file. Type in the name of the file. Graphic Groundwater will add the txt extension. Consult Chapter 4 for more information on exporting data.