1 Dr. Dr. Dr. Tsehaie Tsehaie Tsehaie Woldai Woldai Woldai EREG Module 2,3,4 Radiometric Correction EREG Module 2,3,4 Radiometric Correction EREG Module 2,3,4 Radiometric Correction Radiometric Distortions in RS Images and Their Correction Radiometric Distortions in RS Radiometric Distortions in RS Images and Their Correction Images and Their Correction EREG RS Core Module Dr. Tsehaie Woldai (With data from C. Pohl and E. Schetselaar) EREG RS Core Module EREG RS Core Module Dr. Tsehaie Woldai (With data from C. Pohl and E. Schetselaar)
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1Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Radiometric Distortions in RS Images and Their CorrectionRadiometric Distortions in RS Radiometric Distortions in RS Images and Their CorrectionImages and Their Correction
4Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Image DistortionsImage Distortions
Distortions (errors) in RS images (optical systems)
RadiometricSystem errors minimized by cosmetic
corrections
Atmospheric distortions minimized by atmosphericcorrections
Geometric minimized by geometriccorrections
5Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Radiometric DistortionRadiometric Distortion
What is a radiometric distortion?It’s an error that influences the radiance or radiometric value of a scene element (pixel).
Why?Signal travelling through atmosphere; atmosphere affects the signalSun illumination influences radiometric valuesSeasonal changes affect radiometric valuesSensor failures or system noise affects valuesTerrain influences radiance
Multi-temporal or multi-sensor analysis requires corrected images
6Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Radiometric Distortion Radiometric Distortion ~ ~ Optical CaseOptical Case ~~
Sensor noise or failures
Seasonal variations
Effect often neglectable due to larger influence of atmospheric effects
7Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Sensor noise and failures (1)Sensor noise and failures (1)((Eg., a whiskbroom LandsatLandsat--TM Scanning PatternTM Scanning Pattern))
16 detectors scan the scene alternating from left to right and back by use of an oscillating mirror
At any time 100 detectors are operating simultaneously (6x16 VNIR plus 1x4 TIR).
8Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Sensor noise and failures (2) Sensor noise and failures (2)
Types of sensor noise and failuresTypes of sensor noise and failures
1. Line Striping
2. Periodic line drop outs
3. Random or spike noise
9Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
1. Line Striping1. Line Striping
Cause:non-identical response of one or more detectors resulting from drift in response after calibration of the detectors
Correction method (one of them):
1. Compute the histogram of one detector as standard
2. Match the histograms of the other detectors to the histogram of the standard detector
10Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Line StripingLine Striping
DeDe--stripedstripedStripingStriping
11Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Line StripingLine StripingLandsat TM ExampleLandsat TM Example
DeDe--stripedstripedStripingStriping
12Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Line StripingLine StripingSPOT XS ExampleSPOT XS Example
DeDe--stripedstripedStripingStriping
13Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Line Striping The amplitude of every sixth line is multiplied by a factor of 0.5 to produce the correct digital value from which the restored image is plotted.
Line Striping
14Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
2. (Periodic) line drop outs2. (Periodic) line drop outs
Cause:erroneous radiance values for pixels, lines or areasdefective scanner, transmission, receiving or media system
Correction method:Correction by repetition of neighbouring values or averaging or,The amplitude of every sixth line is multiplied by a factor of 0.5 to produce the correct digital value from which the restored image is plotted.
15Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Line drop outsLine drop outs
Original
Line drop-out
Corrected
16Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Line DropLine Drop--outs (Landsat ETM data)outs (Landsat ETM data)
17Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Line DropLine Drop--outsouts
18Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Line drop outsLine drop outsDropped Signal Dropped Signal -- ExampleExample
(CCRS Remote Sensing Tutorial)
19Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
3. Random noise or spikes3. Random noise or spikes
Cause: Transmission errors or temporary disturbances
Correction method:
1. Detect spike by comparing DN with DN of its surrounding pixels (neighbours)
2. Replace DN with DN value interpolated from the surrounding pixels
20Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Random noise or spikesRandom noise or spikes
Example of Spikes in Landsat MSS
21Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Random noise or spikesRandom noise or spikes
Example of correcting for ‘spikes’
56
Image with spike
De-spiked image
22Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Microwave BackscatterMicrowave Backscatter
23Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Radiometric Effects Radiometric Effects ––SAR CaseSAR Case
Speckle‘Salt and pepper effect’Effect of surface roughness and system factors in image productionAccounts for interference contribution of individual scatterersIt is a random and multiplicative effect
24Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Correction Methods Correction Methods ––
Multi-look processingMovement of sensor - receive backscattered signal from one target several timesCreate several images (=looks)Average images to produce ‘multi-look’ image
Spatial averagingUse full resolution single look imageApply low-pass filter
Rayleigh principle (the shorter the wavelength, the greater the scattering)
(T. Woldai, ITC)
35Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Atmospheric EffectsAtmospheric EffectsRayleigh Scattering causes blue skies during daytime and red skies at sunset
Blue sky
Day timeEarth
Atmosphere
BG
R
Earth
Sunset
Blue
Red
Sun
Sun
RayleighRayleigh ScatteringScattering
(T. Woldai, ITC)
36Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
MieMie--scatteringscattering
Mie scattering occurs when the wavelength of the incoming radiation is similar in size to the atmospheric particles (e.g., aerosols: a mixture of gases, water vapor and dust).
Mie scattering is restricted to the lower atmosphere where larger particles are more abundant.
It influences the entire spectral region from the near ultra-violet up to and including the NIR.
37Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Selective Scattering in WaterSelective Scattering in Water
2
hi e igh
Blue light
HO
= colloidal particles less than 1 micron
Wt l
tBG
R
(T. Woldai, ITC)
38Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Atmospheric EffectsAtmospheric Effects
(CCRS Remote Sensing Tutorial)
NonNon--Selective ScatteringSelective Scattering NonNon--selective scattering is selective scattering is independent of wavelength, with independent of wavelength, with all wavelengths scattered about all wavelengths scattered about
equally. The most prominent equally. The most prominent example includes the effect of example includes the effect of
clouds (consisting of water clouds (consisting of water droplets). Since all wavelengths droplets). Since all wavelengths are scattered equally, a cloud are scattered equally, a cloud
appears white.appears white.
39Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Deleterious Factors of Cloud Deleterious Factors of Cloud CoverCover
CloudCloud
Cloud radiance contribution
Zone of no penetration
Shad
ow z
one
n radiance
Earth Shadow
(T. Woldai, ITC)
Susensor
40Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Atmospheric effectsAtmospheric effects
SkylightSun radiance is scattered in the direction of the Earth surface by small particles and molecules in the atmosphereCan cause increase in radiance received at the sensorMultiplicative effect
CorrectionMultiplication with factor of stable feature in timeEqualization of images for multiple dates
41Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Atmospheric effectsAtmospheric effects
HazeSun radiance is scattered in the direction of the sensor by small particles and molecules in the atmosphereCan cause increase in radiance received at the sensor
Correction (approximation)Select object that absorbs all radiance (e.g. water body); ideal DN = 0Due to haze DN ≠ 0; subtract radiance value from all pixels
42Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Haze Haze –– Example (Indonesia)Example (Indonesia)
Haze Effect Corrected Image
43Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Haze CorrectionHaze Correction
Dark object subtraction method
Assumption: infrared bands are not affected by Haze
• Identify black bodies: clear water and shadow zones with zero reflectance in the infrared bands
• Identify DN values at shorter wavelength bands of the same pixel positions. These DN are entirely due to haze
• Subtract the minimum of the DN values related to black bodies of a particular band from all the pixel values of that band
44Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Effects of Sun IlluminationEffects of Sun Illumination
Position of sunSun elevation (sun-angle)Sun - earth distance
Correction elevationDivision of each pixel value by the sine of solar elevation angle for particular time and location per spectral band
Correction distanceSun irradiance decreases with square of distance
45Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Effects of Sun Illumination Effects of Sun Illumination
Corrections are needed to compensate for sun illumination variations resulting from different scene acquisition dates
Applications
Change detection studies
Mosaics
46Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Correction for seasonal Correction for seasonal variations in Sun illuminationvariations in Sun illumination
Sun angle correction
Earth-Sun distance correction
)('
αSINDNDN =
2
365)5.93(2sin00167.01'
−
+=dDNDN π
Where d is the day number in a year
47Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Skylight CorrectionSkylight Correction
Dateref
Datea
FrefFa
BandNIR
Ban
d VIS
a
ref
FFi =θ
iii DD ×=θ'
48Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
We need to correct for atmospheric effects, variations in sun illumination and system defects
Atmospheric correction is the most complex correction
In many cases only haze correction is applied (dark object subtraction)
If we want to relate RS data with field spectrometric measurements we need to apply full atmospheric corrections, taking into account wavelength dependent factors, such as diffuse sky irradiance, transmission coefficients, path radiance and sensor calibration coefficients
49Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
ReferencesReferences
Bakx, W. (1995): “An introduction to digital image processing”, ITC Lecture Note RSD 65.Schreier, G. (1993): “SAR Geocoding: Data and Systems”, Karlsruhe: Wichmann, ISBN 3-87907-247-7, 435 pages.Wageningen UR, 1999,http://www.gis.wau.nl/cgirs/projects/rsbasics/overview.htm
50Dr. Dr. Dr. TsehaieTsehaieTsehaie WoldaiWoldaiWoldai EREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric CorrectionEREG Module 2,3,4 Radiometric Correction
Further ReadingFurther Reading
Richards, J.A., Remote sensing digital image analysis: an introduction, 2nd revised and enlarged edition, Berlin etc. - Springer Verlag, 1993, ISBN 3-540-54840-8.Lillesand, T.M. and Kiefer, R.W., Remote Sensing and Image Interpretation, 3rd Ed., 1993, J. Wiley & Sons, 720 pp.Jensen, J.R. & Schill, S.R. (1999): The Remote Sensing Core Curiculum, Vol. 3, Digital Image Processing, http://www.cla.sc.edu/geog/rslab/rsccnew/rscc-frames.htmlMather, P.M., Computer Processing of Remotely-Sensed Images: an Introduction, Wiley: New York 1999, ISBN 0-471-98550-3