Hi sherehan, You can calibrate DN to radiance by "Apply gain and offset". Basic tools->preprocessing->general purpose utilities->Apply gain and offset ENVI mulitiply the selected band by inpput gain and add offset, so here gain value should be the inverse of gain found in header file. Hope it helps Rong

hello RONG Thank you for your help.Actually,you put me on the right first step.but i'd like to ask about : from where can i get the values of gain and offset. i search alot but i failed to get an answer??? cheers, sherehan

Hi Sherehan, Gain and offset value are included in the header file, which format of SPOT image you are using? If in CAP format, gain and offset are located in LEAD.DAT file, see the discription from the following website: http://www.crisp.nus.edu.sg/spot/cap.html If your image has metadata file with DIM extention, I think you can use "SPOT calibrate" under calibration utilities. Rong

hello RONG i do the calibration with this method.Actually, i need the calibration to reach the step i need which is atmospheric correction.after i did the calibration and convert data to BIL format i begin to do atmospheric correction with flaash and it gives me the following error"no valid data encountered in this file" . so i don't know what is problem that i make to get this error. cheers, sherehan

Hello Sherehan, I did atmosphric correction for SPOT using FLAASH, following the procedures described in ENVI online tutorial: calibrate to radiance, convert the interleave to BIL, then apply atmosphric correction in FLAASH. Sorry to tell you I do not know the reason for that error. Rong

hello RONG sorry for distirbance. the distribution of each parameter of spot's image depend on the the band or for each pixel??? cheers, sherehan