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Monday, 19 March 2012

Atmospheric correction of Landsat imagery

Corrección atmosférica de imágenes Landsat

When an image is downloaded, besides the corresponding bands (which contain the data), you will notice that there are two txt files that accompany the data. Such files contain the Metadata necessary for the calibration of the image. The procedure you should follow is described in several papers, among which Chander et al. (2009) is highly recommended when using Landsat imagery. Next we describe the procedure we recommend to accomplish this task. The following is an example with a Landsat ETM scene, prepared by the ecologist and agricultural engineer C. Buitrón, under the assessment of the geographic engineer J. Fernández. Both are independent consultants in environmental projects.
 Remember that in case you wish to carry this procedure to calibrate TM data, you should follow the procedure suggested in Chander et al. (2009), and generate the equations to convert DN to reflectance (perhaps using ERDAS´ Model Maker).
  • Look for the file with extensión MTL and open it. 
  • On the Tools bar, go to RASTER RADIOMETRIC → LANDSAT 7 REFLECTANCE. There go to Input file and load the image you wish to correct (example: l519971016_stacked_cut) and in output file create a folder for the corrected imagery (e.g, ATMOSPHERIC CORRECTION) and save it as, e.g., l519971016_stacked_cut_corr. 
  • On “ I/O Options” select:
a.       Strech to Unsigned 8 bit
b.      Ignore Zero in Stats 

  • Look for ”Conversion”, and remember that you are now working with the data in Raw DN (select it).
  • Open the MTL metadata in Excel
a. Copy the información under SUN_ELEVATION (look for ” Solar Elevation” in the metadata),
b. On “Technique” select LMAX/LMIN and look for such information on the metadata, for each of the bands you wish to correct.
c.      To calculate the “Solar Distance” you will need the Julian Day corresponding to the acquisition date of the image. You can use some links on the web to do so. Once you get it, go to Chander et al. (2009), table 6: Earth–Sundistance (d) in astronomical units for Day of the Year (DOY), where DOY=289 for, e.g., october 16th. Copy the “d” value (in our case is 0.99690) to the Solar Distance box. Then OK. 
Now the image is corrected for atmospheric effects.
If you found our tutorials useful, remember the material is copyrighted under a Creative Commons Reconocimiento-NoComercial-CompartirIgual 3.0 Unported License. So, please cite it in your work as, e.g.,  Buitron and Fernández, 2012. Evaluation of historical changes with the use of satellite imagery. Annex 3- Tutorial for the atmospheric correction of Landsat imagery - AquaLED online document (http://watertohoku-tutorials.blogspot.com/2012/03/atmospheric-correction-of-landsat.html)

Chander, G., Markham, B.L., Helder, D.L., "Summary of Current Radiometric Calibration Coefficients for Landsat MSS, TM, ETM+, and EO-1 ALI Sensors", Remote Sensing of Environment, 113, 893 - 903, 2009.  (http://landsathandbook.gsfc.nasa.gov/pdfs/Landsat_Calibration_Summary_RSE.pdf)

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