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i'm currently trying to use the scp image classification plugin to find swimming pools in an aerial image. It is just an RGB image as if it had infra red this'd be a lot easier.

Does anyone know how i could improve my results in terms of image classification without having access to imagery with IR or NIR bands using this or another classification plug-in for QGIS?

The three algorithms used by the scp plugin were unable to provide raster files demarcating the swimming pools using RGB imagery.

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in the absence of NIR band, you can try the following index because the blue component is relatively high with respect to the red for most swimming pools.

(blue-red)/(blue+red) (Borja Rodríguez-Cuenca and Maria C. Alonso, Remote Sensing, 2014)

however, there might be confusions with shadows, therefore you need to predict shadow position (if you have a DSM) or use an intensity threshold. Also note that the quality of the results will depend on the pixel depth.

Finally, there is one possible confusion that you should be aware of: trampoline. I don't know where your study area is located, but in some countries it is quite popular and a real issue for automatic mapping.

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  • I second the note on trampolines. A mapping authority which shall remain anonymous contracted remote imagery tracing. The work was excellent, but to their surprise the area in question (high latitude) had just too many swimming pools. Some investigation led to the conclusion that swimming pools and trampolines are easy to mix up - even when the tracing is done manually.
    – ragnvald
    Apr 15, 2016 at 7:23
  • The area is in rural Australia. Shadows due to shade cloths could be an issue. Ill let you know how i get on.
    – QGISUser
    Apr 17, 2016 at 22:58
  • How did you get on? Im thinking of doing something similar in Sydney
    – jakc
    Jul 15, 2017 at 9:01
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Probably not easy as the reflectance of water is pretty special and is easy detected because of the drop in reflectance in the NIR-Band. The only chance is to detect low reflectance in general. But swimming pools often reflect in the color of the material of which they are made of because they are shallow and have clean water.

You can try supervised classification where you set training areas right on the swimming pools to see if they have a somewhat special statistical distribution.

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  • Thankfully i've managed to track down some NIR imagery from the state government body for it and will be having a go with that later today. I thought it was a long shot just using rgb but had to ask.
    – QGISUser
    Apr 17, 2016 at 22:59

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