Geometric and Radiometric errors
- When image data is recorded by sensors on satellites and aircraft it can contain errors in geometry, and in the measured brightness values of the pixels.
- The second are referred to as radiometric errors and can result from
- the instrumentation used to record the data the wavelength dependence of solar radiation and
- the effect of the atmosphere.
- Geometric errors can also arise in several ways.
- the relative motions of the platform.
- non-idealities in the sensors themselves,
- the curvature of the earth and uncontrolled variations in the position,
- velocity and attitude of the remote sensing platform can all lead to geometric errors of varying degrees of severity.
Geometric and Radiometric errors (2)
- It is usually important to correct errors in image brightness and geometry.
- That is certainly the case if the image is to be as representative as possible of the scene being recorded.
- It is also important if the image is to be interpreted manually.
- If an image is to be analysed automaticali (by machine), using the specific algorithms, it is not always necessary to correct the data beforehand;
- that depends on the analytical technique being used.
- Some institutions (experts) of thought recommend against correction when analysis is based on pattern recognition methods.
Instrumentation Radiometric Errors
- Mechanisms that affect the measured brightness values of the pixels in an image can lead to two broad types of radiometric distortion.
- First, the distribution of brightness over an image in a given band can be different from that in the ground scene.
- Secondly, the relative brightness of a single pixel from band to band can be distorted compared with the spectral reflectance character of the corresponding region on the ground.
- Both types can result from the presence of the atmosphere as a transmission medium through which radiation must travel from its source to the sensors, and can also be the result of instrumentation effects.
Sources of Radiometric Distortion (1)
- An ideal detected radiation should be linear.
- Real detectors will have some degree of non-linearity.
- There will also be a small signal out, even when there is no radiation in.
- Historically that is known as dark current and is the result of residual electronic noise present in the detector at any temperature other than absolute zero.
- In remote sensing it is usually called a detector offset.
- The slope of the detector curve is called its gain, or sometimes transfer gain.
Sources of Radiometric Distortion (2)
- An ideal radiation detector has a transfer characteristic such as that shown in Figure a.
a) Linear radiation detector transfer characteristic, and
b) hypothetical mismatches in detector characteristics
How dark current can be recorded?
Sources of Radiometric Distortion (3)
Atmospheric and particulate scattering.
Sources of Radiometric Distortion (4)
- Most imaging devices used in remote sensing are constructed from sets of detectors.
- In the case of the Landsat ETM+ there are 16 per band.
- Each will have slightly different transfer characteristics, such as those depicted in previous Figure b.
- Those imbalances will lead to striping in the across swath direction.
- For push broom scanners there are as many as 12,000 detectors across the swath in the panchromatic mode of operation.
- For monolithic sensor arrays that is rarely a problem, compared with the line striping that can occur with mechanical across track scanners that employ discrete detectors.
Sources of Radiometric Distortion (5)
Landsat-7 detector projection at the prime focal plane.
Bands: 1) 0.45 - 0.52 VNIR; 2) 0.52 - 0.60 VNIR; 3) 0.63 - 0.69 VNIR; 4) 0.76 - 0.90 VNIR; 5) 1.55 - 1.75 SWIR; 7) 2.08 - 2.35 SWIR; 6) 10.4 - 12.5 TIR
Sources of Radiometric Distortion (6)
Original Landsat MSS image with striping noise.
Sources of Radiometric Distortion (7)
- Another common instrumentation error is the loss of a complete line of data resulting from a momentary sensor or communication link failure, or the loss of signal on individual pixels in a given band owing to instantaneous drop out of a sensor or signal link.
- Those mechanisms lead to black lines across or along the image, depending on the sensor technology used to acquire the data, or to individual black pixels.