To get uniform ASTER data for 2002 and 2003, the method of pseudo-invariant targets present in the image were used (Hill & Sturn, 1991). This procedure is used to eliminate those differences caused by factors which affect the image acquisition. The digital values obtained from these pseudo- invariant targets (clear and dark) at both dates, were adjusted by a linear regression to digital values of the reference image corrected, for clear and dark targets (Mendoza, 2004).