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Radiometric Correction. Sun elevation correction and earth-sun distance correctionHaze compensation. 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 f
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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