Radiometric Corrections

1. Radiometric Corrections Atmospheric Corrections Atmospheric Effects on EMR

3. Kodiak Island, AK – Volcanic ash and clouds (MODIS image)

6. Learning Objectives • What are the main ways that the atmosphere affects light traveling through it? • What are the different types of scattering, and what causes them? • How can we correct satellite data to remove the effects of scattering? • What causes differences in atmospheric transmittance? • How can we correct satellite data for differences in transmittance?

7. Learning Objectives (cont.) • When do we have to atmospherically correct imagery, and when does it not necessarily matter? • What is temporal compositing for cloud removal?

8. Atmospheric Effects • EMR from the sun passes through the atmosphere TWICE before it reaches a satellite • The atmosphere is made up of molecules that interact with EMR • Many (not all) atmospheric effects are wavelength dependent!

9. Atmospheric Effects • What are the 2 primary effects of the atmosphere on EMR? • EMR can also be refracted (bent) by the atmosphere (relevant for target location applications)

10. The Green Flash!

11. What factors affect the amount of radiance the satellite measures? • Hint: What is the equation for at-satellite radiance??

12. Scattering • Amount of atmospheric scattering is affected by: • Wavelength of EMR • Size of atmospheric particles • Density of atmospheric particles • Length of travel path (optical depth) Diagram from U. of Illinois Dept. of Atmos. Sci.

13. Atmospheric Scattering Lλ = (Etmr/𝜋) + Lp Where Lp is called the path radiance (atmospheric scattering TOWARDS the satellite that increases measured radiance)

14. Path Radiance Radiance Top of Atmosphere Irradiance Surface Irradiance

15. Types of Scattering • Rayleigh • Affects blue wavelengths most strongly • Blue sky during day • Contributes to red sunsets • Mie • Caused by larger particles with diameter comparable to or larger than light wavelengths. • Not very wavelength dependent – e.g., white clouds • Nonselective • Not wavelength dependent

16. What are atmospheric corrections? • Atmospheric “corrections” are methods used to convert satellite DNs to numbers that represent radiance leaving the earth’s surface. • Required if you want to calculate surface reflectance.

17. Atmospheric Corrections • So…if you want to remove path radiance from the satellite DNs, how would you do it? (Again, what’s the equation for satellite radiance?)

18. Atmospheric Corrections (cont.) • Each band must be corrected separately! • Green radiation is scattered by atmosphere 4x more than near-infrared • In general, atmospheric effects are much stronger in visible part of the spectrum than in the IR

19. Correcting for Path Radiance (Scattering) • Techniques include • Dark pixel subtraction (a.k.a. the histogram minimum method) • Regression of short wavelength band against long (unscattered) wavelength bands.

20. Dark Pixel Subtraction • Assume that the darkest objects in the image (the minimum value in the histogram) should have a DN of 0 (little or no reflectance) • Not always a correct assumption! • Find the true minimum pixel value from each band (using histograms or dark areas) • Subtract that value from all of the pixels in the band

21. Histogram

22. Lλ (satellite radiance) - Lp= (Etmr/ + Lp) - Lp = Ground radiance corrected for scattering Does not account for absorption (transmittance) by the atmosphere!

23. Regression Technique • Assumes that long-wavelength bands are not scattered • Plot the DNs from a shorter wavelength band on the x-axis against long wavelength DNs on the y-axis. • y-intercept should be at 0. If not, the difference is Lp.

24. Band 7 (for example) DN Path radiance (Lp) Band 1 (for example) DN

25. Atmospheric Absorption (or transmittance) • The atmosphere absorbs some light for all wavelengths • The atmosphere absorbs some wavelengths more than others due to specific atmospheric constituents (e.g., water vapor, CO2, ozone)

26. Absorption Ozone Hole Thermal IR – Greenhouse Effect

27. Correcting for Atmospheric Transmittance • Must correct separately for each wavelength (band) • Must either measure or make assumptions about optical depth, atmospheric density of various constituents, etc. • Transmittance can vary spatially • Often not done because it is difficult.

28. Atmospheric Measurement and Modeling • Requires measurement of many atmospheric characteristics at different heights above the earth at same time as satellite overpass • There are “canned” atmospheric models that work fairly well. • Lowtran • Modtran • ACORN

29. Reasons for Atmospheric Correction • Atmospheric Correction is not always necessary! • Single scene studies • Atmospheric differences can be reduced by ratio based spectral indices • Often necessary when comparing multiple scenes • Scene matching (mosaics) • Change detection studies (sometimes) • Applying classification statistics to multiple scenes • Always necessary if you need to calculate ground reflectance or compare satellite radiance to ground measurements

30. Clouds! • Most EMR wavelengths can’t penetrate clouds • Big problem in remotely sensed imagery—tropics especially • Temporal compositing can be used to get rid of clouds • Cloud shadows are a problem too

31. The NE corner of the Laramie image that you’ve used in lab Clouds and cloud shadows!

32. Summary – Radiometric Corrections • Change the DNs of pixels from the values that the satellite measured • Usually done to remove radiance not directly from the target (e.g. path radiance) • Should be considered carefully because you alter the original radiometry