# Using MODIS Data after FLAASH

I need to convert modis MOD021km data to reflectance and Brightness temperature.

I know that some plugins for Envi is doing this with modis data but I want to run an atmospheric correction on my data first and for this purpose I am using FLAASH in Envi,

It works fine but The results that I get are a bit strange.For example here is a DN of a pixel

``````before Run flash:129.676651
afrer FLAASH:3144
The Reflactance that is calculated using MCTK plugin(without FLAASH):0.261504
``````

If I want to convert the result of FLASH to reflectance and brightness temperature what algorithms should I use?

I know the algorithms for converting DN's to BT and .. but I think the result of FLASH should not used directly in those algorithms ..

Do you know how to use FLAASH results in case of calculating reflectance and brightness temperature?

the first task you should do after run FLAASH atmospheric correction in ENVI, is that you check the Minimum and Maximum of reflectance image(by use statistics calculation). if your results is not normal and there are negative values, use bellow formula in Band math: (b1 le 0)*0+(b1 ge 10000)*1+(b1 gt 0 and b1 lt 10000)*float(b1)/10000 b1 is a channel. and you should use this for all of the channel in reflectance image.

• Thank you for your help..Can you say why the results of flash are integers and arounf 3000 ?it this normal? Commented Nov 30, 2014 at 15:55

I am not familiar with the tools you mention, but the procedures behind conversion to radiance or brightness temperature are pretty straightforward. These are linear transformations of the pixel values. If you feel comfortable scripting, you should be able to do it with minimum hassle. In this site you will find a useful tutorial for working with MODIS with open source tools. Perhaps it gives you hints on solving your problem.

• Thank you for your help..I know Matlap scripting but I think the result of DN's that retrieve after atmospheric correction is not true.for example the original DN of a pixel was 129.676651 and after atmospheric correction it turned to 3144,Do you think it is normal?I have found Brightness temp algorithm but can not find reflectance .Do you have its algorithm for Modis data? Commented Nov 18, 2014 at 19:32
• Difficult to say from this information. It does not seem normal. I would check the procedures, so that there are no glitches in the calculations. I do not have algorithms for atmospheric correction, unfortunately.
– user21313
Commented Nov 18, 2014 at 19:34
• Thank you very much,I used FLAASH Atmospheric correction that is in ENVI ..Unfortunately I don't know its algorithms..What do you suggest me to do? Commented Nov 18, 2014 at 20:08
• Not sure. You might want to check this page grasswiki.osgeo.org/wiki/Atmospheric_correction
– user21313
Commented Nov 19, 2014 at 8:18
• Thanks very muchbut it is for landsat data..I use MODIS :( Commented Nov 19, 2014 at 11:47