Radiometric is a pre-processing technique to reconstruct physically calibrated values by correcting the spectral distortions caused by sensors, sun angle, topography and the atmosphere as shown in Figure 5.3.
Radiometric correction is classified into two types; absolute and relative correction.
Absolute correction: Correct radiance or reflectance should be measured or converted by using the sensor calibration data, the sun angle and view angle, atmospheric models and ground truth data. The incident energy input to sensors as shown in Figure 5.4 should be analyzed correctively by radiometric correction. However it can not be applied in most applications, therefore the relative correction is applied because the atmospheric model is so complicated and the exact measurement of atmospheric condition is difficult.
Relative Correction: Relative correction is to normalize multi-temporal data taken on different dates to a selected reference data at specific time.
The following techniques will be typical.
- Adjustment of average and standard deviation values.
- Conversion to normalized index: for example the normalized difference vegetation index (NDVI).
- Least square method: linear function of y = ax + b is determined, where y is reference data and x is data to be normalized.