I'm not aware of the specifics on how the USGS is producing the contour plots using the Hydrotherm software. However, IDL does provide a CONTOUR and COLORBAR functions that can be used to generate a similar function. The documentation for these routines is shown below:
http://www.harrisgeospati...om/docs/CONTOUR.html
http://www.harrisgeospati...m/docs/colorbar.html
David
Harris Geospatial Solutions