  Here we discuss the application of an edge detection filter, the Sobel filter
of GIMP, to the recently discovered motion of some sand dunes on Mars. The
filter allows a good comparison of an image HiRISE of 2007 and an image of 1999
recorded by the Mars Global Surveyor of the dunes in the Nili Patera caldera,
measuring therefore the motion of the dunes on a longer period of time than
that previously investigated.
