| 23 Nov 2009 05:14 PM |
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Dear Friends,
I am new user of ENVI so please help me
I have downloaded Landsat 5 TM data from Earth Explorer avaialable free of cost for Change detection study. When I see header file, there is no information related to Gain or Bias.
When I see the product description on USGS website, it says that the data has already been normalized. I am really confused what to do. I am doing vegetation phenology study and have four season data for three consecutive years. I want to know which season data is the best for vegetation mapping and how it changes season wise.
a) Could you please help me how to do that---I mean which method I should follow
b) Is it required to convert DN values to ground refelectance values.....if yes then what would be the output data range and how to scale it. Or how to normalize other season data with base data (say January Data)
c) I tried image differencing technique but donot know how to fix the threshold values.....
Could you please guide me how to solve these problems, my studies are getting affected ...
Many Thanks in advance.
PK
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MariM
Veteran Member
Posts:2396  
| 24 Nov 2009 10:07 AM |
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The USGS distributes Landsat data as separate geotiff files with a metadata file (_met.txt). It is this text file that contains the calibration information which can be used with the Landsat calibration tool in ENVI 4.7. You would need to select the metadata file to open the dataset, then use the Landsat Calibration tool to convert to radiance or at-sensor reflectance. While it is not required to convert data to reflectance, it is desirable that the data be as close as possible to true reflectance when performing a change detection. Converting to at-sensor reflectance will take into account changes in reflectance due to time of year but it does not take into account atmospheric effects. You would need to also perform an atmospheric correction using something like FLAASH, QUaC, empirical line calibration, dark subtraction or similar method.
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Deleted User
New Member
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| 24 Nov 2009 04:36 PM |
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Many Thanks for your reply.
I have converted All four season data to At-sensor reflectance value. All data are showing nearly the same range. I donot know how to interpret it. When I downloaded the data and unzipped it, I found all the bands to be as a separate file. First I staked all the bands to one file and then I performed the normalization process. Is it ok or I need to do normalization individually and then stacking them all to one. Does it make any difference?
Secondly, when I perform radiometric normalization from Basic Tools, it asks for sun angle in degrees, I am not sure wheteher it is Azimuth angle or sun angle. I am not able to differentiate between these two. Is Zenith angle and Azimuth angle the same.
PK
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MariM
Veteran Member
Posts:2396
| 25 Nov 2009 07:23 AM |
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What version of ENVI are you using? In ENVI 4.7, you can open the _mtl.txt file which opens all the separate TIFF files into a single meta file which can be calibrated based on information in the _mtl.txt metadata. This is the best option to get the correct calibration.
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Deleted User
New Member
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| 25 Nov 2009 03:25 PM |
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I am using ENVI 4.5
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