| 18 | | 1. Click "Select" next to "ATMOSPHERIC FILE". Select an atmosphere model as appropriate to the flight location and altitude. |
| 19 | | 1. Click "VISIB. ESTIMATE". Enter the given visibility value in the "Visibility" box. |
| | 18 | 1. Click "Select" next to "ATMOSPHERIC FILE". Select 'rural' and select the atmosphere with a water vapour value of 1.0 (*wv10*) with the altitude closest to that of the flight above the ground (note, not absolute altitude). |
| | 19 | 1. Click "AEROSOL TYPE". Wait for the tool to run. When it completes it will present you with two values for each atmosphere type. The second is the visibility, the former is a measure of how good the agreement is with the data - a lower number is better. Check that "rura" (rural) is the best value, if it's not then change the atmospheric file to a different atmosphere model as appropriate depending on which one is best. Enter the appropriate visibility number in the "Visibility" box |
| | 20 | 1. Click "VISIB. ESTIMATE". Check the number agrees with the previous step - if not don't worry but it may need to be changed later. |
| | 21 | 1. Click "WATER VAPOR". Click on "Calculate interpolated atmospheric file(s)" (if you don't get this option, go back and re-select the same atmospheric file). Once it's finished, click OK. Leave all the options on the next screen as default and click Done. |
| | 22 | 1. Click "Calculate Water Vapor Map". Wait for it to complete. |
| | 23 | 1. Open the water vapour map calculated in the previous step in ENVI. Go to Basic Tools > Calculate Statistics, select the WV map, leave all settings as default and check the mean value. If the mean value is closer to 400 than to 1000, go back and change your atmospheric file in ATCOR to use a water vapour value of 0.4 (*wv04*) instead of 1 (similarly if it's higher). |