I am using a CIR orthophoto and tried to make a land use classification with the use of Semi-Automatic Classification Plugin in QGIS. I work in QGIS 3.2 but I get errors that my ROI are too small (error creating signature), but my these areas are quite large. Furthermore, the classification does not work at all. I want to know the built-up area on the orthophoto, so if there is another methodology in QGIS, I would love to know.
You can find the orthophoto on this website: http://www.geopunt.be/download?container=omz15vl&title=Orthofotomoza%C3%AFek,%20middenschalig,%20zomeropnamen,%202015,%20Vlaanderen#