Geometric correction is to correct the geometric distortions; internal and external distortions as shown in Figure 5.6.
internal distortions: caused by sensor, such as lens distortion, misarrangement of detectors, variation of sampling rate etc.
external distortions: caused by external parameters other than sensor, including variation of attitude and position of platform, earth curvature, topographic relief etc.
Geometric correction is made according to the following steps.
step 1: Data input of uncorrected image data
step 2: Selection of geometric correction method and transformation equation depending on the sensor geometry and the
estimated types of geometric distortions
step 3: Determination of transformation parameters using ground control points (GCP) (see Figure 5.7)
step 4: Resampling and interpolation ( see 2.6, Chapter 2)
step 5: Output georeference image, known as geo-coded image
There are three types of geometric correction.
System correction: geometric correction using the geometry of the sensor with sensor calibration data and attitude/positioning measurements. But this is not enough to produce geo-coded image with high accuracy.
Mapping transformation: determination of transformation function, usually the second of third order polynomials to transform image to map coordinate system based only on ground control points (see Figure 5.7).
Combined method: at the first stage, system correction is adopted to avoid major systematic distortions and then mapping transformation with simpler functions such as affine or pseudo-affine is applied with fewer number of GCPs.