Supervised classification (Tutorial)

Classification with Orfeo Toolbox

Image statistics

1. Add the Sentinel-2 imagery Subset_S2A_MSIL2A_20170619T_MUL_BOA.tif into a QGIS project.
2. Calculate mean and standard error for each band of the Sentinel-2 imagery using the OTB Graphical User Interface.

• In the Search box on the Windows Start menu type OSGeo4W Shell. You should be able to open the shell by clicking on it.
• Type into the shell: otbgui_ComputeImagesStatistics.

Select a multiband input file and an output XML file as seen in the screenshot.

Train image classifier

1. Add the training areas as vector polygon file lab05_training_input.shp into QGIS.
2. Open Orfeo Toolbox --> TrainImageClassifier (libsvm) to use the Support Vector Machine SVM algorithm (see figure B).
3. Set Subset_S2A_MSIL2A_20170619T_MUL_BOA.tif as Input image list.
4. Set lab05_training_input.shp as Input vector list.
5. Set Subset_S2A_MSIL2A_20170619T_MUL_BOA.tif.xml as Input XML image statistics file.
6. Set Name of discrimination field to C_ID (C_ID refers to the column that contains the LUC code).
7. Save the Output confusion matrix as ConfusionMatrixSVM.csv.
8. Save the Output model as SVM.model.
9. Calculation of accuracies :
   Open ConfusionMatrixSVM.csv in LibreOffice or MS Excel and calculate overall, producer and consumer accuracies.

Classification

1. Open Orfeo Toolbox --> Image Classification (see figure C).
2. Set Subset_S2A_MSIL2A_20170619T_MUL_BOA.tif as Input image.
3. Set SVM.model as Model file.
4. Set Subset_S2A_MSIL2A_20170619T_MUL_BOA.xml as Statistical file.
5. Save the Output image as su_svm.tif.
6. Evaluate classification results.
   1. Add the classification result su_svm.tif to QGIS.
   2. Right click su_svm.tif in the TOC and select Properties --> Style --> Style --> Load Style.
   3. Load lab05_MinDist.qml.

Compute a confusion matrix with independent reference data

1. Open Orfeo Toolbox --> ComputeConfusionMatrix (Vector).
2. Set su_svm.tif as Input image.
3. Set lab05_validation.shp as Input reference vector data.
4. Set Field name to C_ID.