Dear all, I searched the forum, google, and the help files but couldn't find an answer. I have two brief questions; I would appreciate if you could answer them. Q1) I want to use a variable of my own inside idl to run TOPO_DOIT.. Briefly instead of saying: ENVI_DOIT, 'TOPO_DOIT', fid=fid, elevation= 45.00, azimuth=110.00, BPTR = [2], DIMS = dims, POS = [0], R_FID = r_dif, PIXEL_SIZE = pixel_size, IN_MEMORY = 1 I want to use DEM = load(filename) DEM = DEM+500. ENVI_DOIT, 'TOPO_DOIT', DEM , elevation= 45.00, azimuth=110.00, BPTR = [2], DIMS = dims, POS = [0], R_FID = r_dif, PIXEL_SIZE = pixel_size, IN_MEMORY = 1 I want to input the variable of my own (here "DEM") that I obtained previously in idl rather than using "fid=fid".. Can I do it? Q2) TOPO_DOIT assumes the solar-elevation and the solar-azimuth are same for all the input pixels. However, in real life solar elevation and solar azimuth changes from pixel to pixel (for calculation of shade relief). So, ideally I would like to input arrays of "solar elevation", "solar azimuth", and even "pixel_size" (need the accuracy for a scientific calculation). Can I do it? I tried two different ways of shade-relief calculation (below) but they gave me two different answers; a) ENVI_DOIT, 'TOPO_DOIT', fid=fid, elevation= 45.00, azimuth=110.00, BPTR = [2], DIMS = dims, POS = [0], R_FID = r_dif, PIXEL_SIZE = pixel_size, IN_MEMORY = 1 and elev = fltarr( (dims[2]+1), (dims[4]+1) ) elev[*] = 45.00 azim = fltarr( (dims[2]+1), (dims[4]+1) ) azim[*] = 110.00 b) ENVI_DOIT, 'TOPO_DOIT', fid=fid, elevation= elev, azimuth=azim, BPTR = [2], DIMS = dims, POS = [0], R_FID = r_dif, PIXEL_SIZE = pixel_size, IN_MEMORY = 1 I would have expected both "a" and "b" would give the same results (because input elevations and the azimuths are same) but while "a" gave me a reasonable answer, "b" did not give me the same.. In fact "b" gave very unrealistic results.. I can potentially getaway with this by writing the input image to a disk pixel by pixel (~40million pixels), and create a header file for each of them, and then use the "fid=fid" option to calculate the shaded relief, but this is too inefficient and possibly would take too long time to calculate. Thank you very much for your time, Ray