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Radiometric and Geometric Errors. Mirza Muhammad Waqar Contact: [email protected] +92-21-34650765-79 EXT:2257. RG610. Course: Introduction to RS & DIP. Outlines. Digital Image Advantages of Digital Image Constraints of Remote Sensing System Image Preprocessing - PowerPoint PPT Presentation
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RADIOMETRIC AND GEOMETRIC ERRORS
Course: Introduction to RS & DIP
Mirza Muhammad WaqarContact:
[email protected]+92-21-34650765-79 EXT:2257
RG610
Outlines
Digital Image Advantages of Digital Image Constraints of Remote Sensing System Image Preprocessing Geometric Distortions Radiometric Distortions
Digital Image
A metric Cell Spatial information Spectral information
Satellite data mostly available in grid file format
Used for Spatial Analysis (Quantitative Analysis) Spectral Analysis (Qualitative Analysis)
For information extraction form satellite imagery, we normally perform both, qualitative as well as quantitative analysis.
Advantages of Digital Image
1. Flexible structure 2. All mathematical and statistical operations can
be applied3. Advance image processing packages are
available to process digital imagery.
Constraints of Remote Sensing Systems
Remote sensing systems are not yet perfect and contains four types of resolution constraints:
Spatial Spectral Temporal Radiometric
These constraints (plus complexity of land and water surfaces) cause errors in data/image acquisition process.
This leads to degradation of quality of remote sensing data/image. Before remote sensing data is analyzed, data/image needs to be
preprocessed to restore image quality. Image restoration involves correction of distortion, degradation, and
noise introduced during the imaging process.
Image Preprocessing
During image processing, anomalies are removed which can create problem during information extraction.
1. Spatial Anomalies (Geometric Distortions)2. Spectral Anomalies (Radiometric Distortions)
Geometric Distortions
There are two types of geometric distortions exists in satellite data
1. Systematic Errors2. Non-Systematic Errors
Systematic Errors
These errors are system dependent also called platform based errors.
If the quantity of error is know These errors can be removed
Mostly found in mechanical sensors
For example, velocity of Landsat scanners’ motor varies and its variation is known. A mathematical model can be develop to remove such
distortions.
Systematic Errors
1. Scan Skew Distortion2. Earth Rotation Effect3. Platform Velocity4. Mirror Scan Velocity5. Panoramic Distortions6. Perspective Distortions
Scan Skew Distortion
During the time the scan mirror completes one active scan, the satellite moves along the ground track.
Therefore, scanning is not at right angles to the satellite velocity vector (ground track) but is slightly skewed, which produce along track geometric distortion, if not corrected
Earth Rotation Effect
26-28 seconds required to capture a Landsat image.
In Landsat TM/ETM+, up till 16 scan line, distortion is gradual, however after 16 lines, distortion is greater
Satellites having small swath width have less earth rotation effect.
Earth Rotation
Earth Rotation Effect
Platform Velocity
Variation of pixel size in terms of information content. 1:100 => Less information => Large pixel size 1:10 => More information => Small pixel size
When information is increasing, pixel size is decreasing.
Platform Velocity
Image Scale Distortions
Size of the pixel is changing
For satellite, we have equal sampling rate
Due to Scale distortions, Dwell Time is changing
Information content varies
Information content is the indicator of scale
Mirror Scan Velocity
Mirror scan velocity of landsat scanner is not constant It is slower first then it increases
Perspective Distortions
As all remote sensing satellites exit at high altitude. Earth curvature effect become very prominent
which cause perspective distortions
This effect can be removed by rectification (we will
study in next lecture).
Distortion in Scale due to Scanning System
Distortion in Scale due to Scanning System
Non-Systematic Error
All the terminologies make for non-systematic distortions was developed for aerial platforms.
Two types of non-systematic distortions:1. Developed due to Altitude2. Developed due to Attitude
Altitude Distortions
Due to altitude variation, FOV and IFOV changes. Causing scale distortions.
Attitude Distortions
Geometric Distortions
Radiometric Error
Internal cause: When individual detectors do not function properly or are improperly calibrated.
External cause: Atmosphere (between the terrain and the sensor) can contribute to noise (i.e., atmospheric attenuation) such that energy recorded does not resemble that reflected/emitted by the terrain.
Radiometric Error
Internal Error Correction (Correction for Sensor System Detector Error) Element(s) ij:
may go bad at the beginning of the scan line (line-start problem) may go out of calibration or adjustment (line stripping or banding), or may drop out (line drop out) completely.
Detect: Take average of brightness value (BV) of surrounding pixels and compare to BVij.
Correct: Assign average BV if BVij is beyond a given threshold. Or, correct from overlapped images. Improves fidelity of brightness value magnitude. Improves visual
interpretability.
Radiometric Correction
External Error Correction (Correction for Environmental Attenuation Error) Two sources of environmental attenuation:
Atmospheric attenuation Topographic attenuation
Atmospheric attenuation (caused by scattering and atmosphere) Not a problem for most land-cover-related studies because signals from soil,
water, vegetation, and urban area may be strong and distinguishable. Problematic for biophysical information from water bodies (e.g., chlorophyll a,
suspended sediment, or temperature) or vegetated surfaces (e.g., biomass, NPP, % canopy closure) because there is only subtle difference in reflectance.
Error correction: data is calibrated with in situ measurements, and/or, data is corrected with a model atmosphere. Error minimized using multiple "looks" at the same object from different vantage
points or using multiple bands.
Radiometric Correction
Topographic attenuation Slope and aspect effects include shadowing of areas of interest. Goal of slope-aspect correction: To remove topographically induced illumination
variation (so that two objects having same reflectance show same BV even though they may have different slope and aspect).
Forest stand classification is improved when slope-aspect errors are corrected.
Correction is based on illumination (proportion of direct solar radiation hitting a pixel). Digital Elevation Model (DEM) required. DEM and remote sensing data must be geometrically registered and resampled to same spatial resolution.
Amount of illumination depends on relative orientation of pixels toward the sun.
Questions & Discussion