ENVI FLAASH Atmospheric Correction Module: Getting started with MODIS

I have just read what is possibly the least useful tip since Alan Turing wrote: ” Step 1, build giant computer”. Don’t get me wrong, I think ITTVIS have a great product and am on record for saying so. Their Support service is excellent: the help files might use a little work though. In fact, if you are a FLAASH user you might expect an animated paper clip to pop-up at any moment to tip you over edge you are already teetering on. Enough of the rant and onto some help.

Luckily there is help  out there in the ENVI forums. Here I am distilling the advice from a number of posts and my own experience. Here’s how to correct a MODIS image, easily. It does not use the full potential of FLAASH but provides a starting point.

1. Open your MODIS image using Files>Open External Files>EOS>MODIS

2. Georeference the files using Map>Georeference MODIS (this should be straight forward)

3.Convert from BSQ to BIL or BIP: Basic Tools>Convert Data (this strikes me as a legacy issue and I would expect ITTVIS or their partners to have implemented a more automated, modern function)

4. Open the FLAASH dialogue: Spectral>FLAASH

5. BEFORE you open your Radiance Image, you need to Edit the Header file so that the wavelength of the image bands can be read by FLAASH:>Files>Edit ENVI Header

-select your image file and then choose Edit Attributes

The Edit ENVI header dialogue (Edit Attributes menu)

-Select Wavelengths and enter the correct values by clicking the band number and entering the Wavelength in the “Edit Selected Item:”  box. e.g for Band 1 enter 645.

-Make sure that the Wavelength/FWHM Units: menu is set to Nanometers.

When you have entered the values of all the bands, click OK to update the header file.

6. Now it is time to edit the FLAASH parameters. Select an Input Radiance Image in the FLAASH dialogue. Enter an Output reflectance file and an Output directory for FLAASH files. Add a rootname for your files.

7. Choose the Sensor Type. Edit the Scene Center Location: as far as I am aware this does not need to be accurate the second,  the Flight Date and Flight Time. 

8. Input the Ground Elevation (in KILOMETERS not METERS). Input the pixel size.

9. The Atmospheric Model type and other parameters are dependent on location and prevailing conditions. For these I suggest you at least browser the Atmospheric correction users guide (Pdf). Water retrieval should probably be turned OFF. As should Adjacency Correction in the Advanced settings.

10. Hit Apply, cross your fingers and await the result. Best of luck!