3.4.3.5. Signature threshold

Signature threshold

This tab allows for the definition of a classification threshold (only for Minimum Distance, Maximum Likelihood, and Spectral Angle Mapping algorithms) for each spectral signature. All the signatures contained in the Training input are listed. Thresholds defined in this tool are applied to classification only if Threshold value in Algorithm is 0.

This is useful for improving the classification results, especially when spectral signatures are similar. Thresholds of signatures are saved in the Training input.

If threshold is 0 then no threshold is applied and all the image pixels are classified. Depending on the selected Algorithm the threshold value is evaluated differently:

• for Minimum Distance, pixels are unclassified if distance is greater than threshold value;

• for Maximum Likelihood, pixels are unclassified if probability is less than threshold value (max 100);

• for Spectral Angle Mapping, pixels are unclassified if spectral angle distance is greater than threshold value (max 90).

3.4.3.5.1. Signature threshold

The table Signature threshold contains the following fields.

MC ID	MC Name	C ID	C Name	MD Threshold	ML Threshold	SAM Threshold
signature Macroclass ID	signature Macroclass Name	signature Class ID	signature Class Name	Minimum Distance threshold; this value can be edited	Maximum Likelihood threshold; this value can be edited	Spectral Angle Mapping threshold; this value can be edited

• : reset all signatures thresholds to 0 (i.e. no threshold used);

3.4.3.5.2. Automatic thresholds

Tool symbol and name	Description
Set threshold	set the defined value as threshold for all the highlighted signatures in the table
Set threshold = σ *	for all the highlighted signatures, set an automatic threshold calculated as the distance (or angle) between mean signature and (mean signature + (σ * v)), where σ is the standard deviation and v is the defined value; currently works for Minimum Distance and Spectral Angle Mapping