Revision as of 09:23, 29 June 2020

Per pixel classification with QGIS and OTB processing plugin

In OTB Version 7.0.0 the requirements for the input training vector data are very restrictive because of some bugs.

• • File format needs to be Shapefile (GPKG does not work).
  • The column containing the class label needs to integer (whole number). Data type Integer 64bit does not work!
  • Any other columns with other data types (real or text (string) with umlauts or blank are not correctly imported which leads to failure of the modul TrainImagesClassifier.

A work around is to make a copy of your training data and delete all other columns than the class label column

All these issues have been fixed in OTB Version 7.1.0!

Training phase

• In the search engine of Processing Toolbox, type TrainImages and open TrainImagesClassifer.
• In the Input Image List select a (or optional: several) multispectral images: mosaic_S2A_MSIL2A_20200521T102031_N0214_R065_T32UNC_UNB_UPC.tif.
• In the Input Vector Data List choose a vector polygon file with training areas: lucc_training_2020-05-21.shp.
• In the Validation Vector Data List and choose a vector polygon file with an independent sample of validation areas: leave empty.
• Training and validation sample ratio: 0.25. This is a 4-fold cross-validation with split 0.75 per cent for training and 0.25 per cent for testing.
• Type Code in the Field Name text field.
• Choose Support Vector Machine Classifer libsvm from the drop down list.
• SVM Model Type is csvc
• User defined input centroids: Type any character, no need for a filename. This field is required and but ignored (a bug in the OTB plugin!)
• The SVM Kernel Type is Linear.
• Switch checkbox Parameters optimization off. The optimization results in a higher accuracy but takes much time (> 1 hour computation).
• In the Output model specify an model file: e.g. svm.model
• Define an output file for Output confusion matrix or contingency table (e.g.svm_matrix.csv).
• Click Run.
• Click on the Log tab and inspect the model quality measures: Precision, Recall, F-score and Kappa index.

Classification phase

• In the search engine of Processing Toolbox, type ImageClassifier and double click ImageClassifier.
• Set mosaic_S2A_MSIL2A_20200521T102031_N0214_R065_T32UNC_UNB_UPC.tif as Input image.
• Set Input _mask to blank (top of drop-down list).
• Set lucc_svm.model as Model file.
• Set the number of classes in your model. This is the number of classes in your training file.
• Set Output pixel type to uint16
• Save the Output image as svm_classification.tif.
• Uncheck Confidence map: Open output file after running algorithm.
• Uncheck Probability map: Open output file after running algorithm.
• Add svm_classification.tif to QGIS canvas.
• Find the prepared style file rast_classifcation.qml in the Tutorial data of Workshop 05.
• Right click svm_classification.tif in the TOC and select Properties --> Symbology --> Style --> Load Style.
• Select the style file svm_classification.tif. OK.

Postprocessing

This application uses a majority filter with a circular structure element.

• In the search engine of Processing Toolbox, type classification and double click ImageClassifier.
• Define Input classification image as an classification output raster.
• Set Structuring element radius to 2 pixels.
• Set Output pixel type to uint16.
• Save the Output regularizd image as svm_classification_majority.tif.
• Click Run.

Per pixel classification with OTB standalone

Training phase

• Type into the search box of the Windows taskbar: mapla.bat. Click on mapla.bat to open Monteverdi Application Launcher.
• In the search engine of mapla, type TrainImages and double click TrainImagesClassifer.
• In the Input Image List click on + and select a (or optional: several) multispectral images: Subset_S2A_MSIL2A_20170619T_MUL.tif .
• In the Input Vector Data List choose a vector polygon file with training areas: lucc_training_input.shp.
• Activate the checkbox Validation Vector Data List and choose a vector polygon file with an independent sample of validation areas: lucc_validation.shp
• In the Output model specify an output model file: e.g. lucc_svm.model
• Activate the checkbox and save the Output confusion matrix or contingency table as lucc_svm_confusion.csv.
• In the Bound sample number by minimum field type 1.
• Set the training and validation sample ratio to 0. (0 = all training data).
• Mark C_ID in the Field containing the class integer label (C_ID refers to the column that contains the LUC code in the training and validation vector file).
• Choose LibSVM classifier from the drop down list as Classifier to use for the training.
• The SVM Kernel Type is Linear.
• The SVM Model Type is C support vector classification.
• Switch the Parameters optimization to on.
• Check user defined seed and enter an integer value.
• Click on Execute.
• Click on the Log tab and inspect the quality measures: Precision, Recall, F-score and Kappa index.

Classification phase

• In the search engine of mapla, type ImageClassifier and double click ClassifcationMapRegularization
• Set Subset_S2A_MSIL2A_20170619T_MUL.tif as Input image.
• Set SVM.model as Model file.
• Save the Output image as svm_classification.tif.

• Evaluate classification results:
• Add svm_classification.tif to QGIS canvas.
• Download the style file classifcation.qml from Stud.IP.
• Right click svm_classification.tif in the TOC and select Properties --> Style --> Style --> Load Style.
• Select the style file \lucc\classification.qml. OK.
• Open the text file lucc_svm_confusion.csv with LibreOffice Calc or MS Excel and calculate overall, user, producer accuracy and kappa index.

Postprocessing

This application uses a majority with a circular structure element.

• In the search engine of mapla, type, regular and double click ClassifcationMapRegularization.
• Define Input classification image as ans classification output raster.
• Save the Output regularizd image as svm_classification_majority.tif.
• Set Structuring element radius to 3 pixels.
• Set Output pixel type to uint8.
• Click Execute.