The Secchi disk is a white disk that is lowered into the water and the depth at which it disappears and reappears from sight is logged. This depth is proportional to the clarity or transparency of the water. The Forel Ule colour scale is used to classify the colour of natural waters.It consists of 21 colours ranging from blue to green to yellow to brown, and is used alongside the Secchi disk with the observer typically recording the colour of a submerged Secchi disk with this app. The app will guide you through each measurement, store location data, photos and additional observations.