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 :ref:`training_input` are listed. Thresholds defined in this tool are applied to classification only if :guilabel:`Threshold` value in :ref:`classification_alg` is 0.
This is useful for improving the classification results, especially when spectral signatures are similar. Thresholds of signatures are saved in the :ref:`training_input`.
If threshold is 0 then no threshold is applied and all the image pixels are classified. Depending on the selected :ref:`classification_alg` 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).
The table :guilabel:`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);
| Tool symbol and name | Description |
|---|---|
:guilabel:`Set threshold`   |
set the defined value as threshold for all the highlighted signatures in the table |
| :guilabel:`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 |