Setting up normalization

You can reduce a topographic effect in digital imagery by applying a Lambertian or non-Lambertian reflectance model.

A Lambertian reflectance model assumes that the surface reflects incident solar energy uniformly in all directions, and that variations in reflectance are due to the amount of incident radiation.

A non-Lambertian reflectance model applies an empirical topographic normalization either linearly or exponentially. A correction factor is also applied, which can be based on the incidence angle or the incidence and exitance angles.

To avoid overcorrection, specify a threshold illumination angle. By doing so, you can reduce the correction on pixels that have an illumination angle higher than the threshold you set.

The effect of the threshold illumination is scene-dependent, and the effect varies according to local topography. If the topographic correction must be increased on the steepest slopes, defining the threshold angle as described in the following table is recommended.

If the solar angle is	Set the threshold angle to (solar zenith) plus
Less than 20 degrees	20 degrees
Between 20 and 45 degrees	15 degrees
Greater than 45 degrees	10 degrees

To prevent over-reducing the reflectance of pixels with extreme incidence angles, specify a factor by which to reduce the correction.

To use a Lambertian reflectance model
1. Make sure Lambertian correction is selected.
2. Click OK.
To use a non-Lambertian reflectance model
1. Click Empirical topographic normalization.
2. Under Function type, click one of the following according to how you want to set the correction factor:
  - Linear
  - Exponential
3. Under Correction factor, click one of the following according to what you want to base the correction factor on:
  - Incidence angle
  - Incidence and exitance angles
    For use when the sensor has a tilt angle.
4. In the Threshold angle box, type or select the threshold of the illumination angle, in degrees.
  By default, the value is the angle of the solar zenith.
5. In the Reduce correction factor by box, type or select the amount, in percentage, by which to constrain the correction factor.
6. Click OK.