Radiometric Correction

1. Radiometric Correction The radiance measured by any given system over a given object is influenced by: Changes in scene illumination Atmospheric conditions Viewing geometry variations: Greater in the case of airborne data collection than in satellite image acquisition Instrument response characteristics

2. Radiometric Correction Sun elevation correction and earth-sun distance correction Haze compensation

3. Radiometric Correction Noise correction electronic noise - both random and periodic Sun-angle correction for comparison and mosaic images acquired from different time of the year Correction for atmospheric scattering subtract the haze DN values from different bands DN

4. Radiometric Correction

6. Radiometric Correction

7. Noise Removal Image noise is any unwanted disturbance in image data that is due to limitations in the sensing, signal digitization, or data recording process. Potential source: electronic interference between sensor components Noise can either degrade or totally mask the true radiometric information content of a digital image Noise removal usually precedes any subsequent enhancement or classification of the image data

8. Noise Removal The objective is to restore an image to as close an approximation of the original scene as possible Line striping or banding ? destriping Line striping occurs due to non-identical detector response Although the detectors for all satellite sensors are carefully calibrated and matched before the launch of the satellite With time the response of some detectors may drift to higher or lower levels, resulting in relatively higher or lower values along every sixth line in the image data Line striping is corrected using histograms per detector

9. Noise Removal Line drop: Occurs due to recording problems when one of the detectors of the sensor in question either gives wrong data or stops functioning. The Landsat ETM, for example, has 16 detectors in all its bands, except the thermal band A loss of one of the detectors would result in every sixteenth scan line being a string of zeros that would plot as a black line on the image Dropped lines are normally 'corrected' by replacing the line with the pixel values in the line above or below, or with the average of the two. Detector: Component of a remote sensing system that converts electromagnetic radiation into a recorded signal Atmospheric Path Radiance Is a term that refers to that component of radiation received by a sensor that did not originate from the target but through scattering in the earth's atmosphere.

10. Noise Removal

11. Striping-Landsat

12. Partially missing lines-Example

15. Radiometric Correction

20. Rayleigh Scattering

22. Radiometric Correction

23. Absolute Radiometric Correction

25. Absolute Correction Convert DN to radiance, Lapp • Sensor dependent • Lapp=Ai*DN+Bi (Landsat) • Lapp=DN/Ai (Spot) • Ai calibration gain, Bi calibration offset • Which values Ai and Bi to use? Usually both analytical (derived from pre-launch measurements) and empirical (derived from post-launch measurements) exist MSS shows 8-12% difference ; sensor vs. ground processing (Markham and Barker, 1987)

26. Landsat ETM+ DN to Radiance L=gain*DN+offset L= (LMAX-LMIN/255) DN+LMIN L=Spectral radiance measured (over the spectral bandwidth of the channel) LMAX=The minimum radiance required to generate the maximum DN (here 255) LMIN=The spectral radiance corresponding to a DN response of 0 DN=Digital number value recorded G=Slope of response function (channel gain) B=Intercept of response function (channel offset)

27. Landsat ETM+ DN to Radiance

28. Landsat ETM+ DN to Radiance

29. Landsat 7 ETM+ DN to Radiance

30. Atmospheric Effects

31. Sun angle correction Position of the sun relative to the earth changes depending on time of the day and the day of the year Solar elevation angle: Time- and location dependent In the northern hemisphere the solar elevation angle is smaller in winter than in summer The solar zenith angle is equal to 90 degree minus the solar elevation angle Irradiance varies with the seasonal changes in solar elevation angle and the changing distance between the earth and sun

32. Sun angle correction An absolute correction involves dividing the DN-value in the image data by the sine of the solar elevation angle Size of the angle is given in the header of the image data

33. Sun angle correction

34. Spectral Irradiance & Earth-Sun Distance

35. Haze Reduction Aerial and satellite images often contain haze. Presence of haze reduces image contrast and makes visual examination of images difficult. Due to Rayleigh scattering Particle size responsible for effect smaller than the radiation’s wavelength (e.g. oxygen and nitrogen) Haze has an additive effect resulting in higher DN values Scattering is wavelength dependent Scattering is more pronounced in shorter wavelengths and negligible in the NIR

36. Haze Reduction One means of haze compensation in multispectral data is to observe the radiance recorded over target areas of zero reflectance For example, the reflectance of deep clear water is zero in NIR region of the spectrum Therefore, any signal observed over such an area represents the path radiance This value can be subtracted from all the pixels in that band

37. Haze Reduction

38. Haze Reduction

39. Haze-Example Indonesia

40. Haze removal