Effect of Target Moisture on Laser Scanner Intensity

We have studied the effect of moisture on the backscatter reflectance of mineral aggregate (such as sand and gravel) targets that are used as reflectance standards in airborne laser scanning intensity calibration. Target moisture has turned out to be a crucial factor when using external reference targets (either commercial or naturally available at the measurement site). The most common targets used thus far are different types of gravel and sand, which emphasizes the role of moisture because of the long time required for drying. We carried out laboratory and field experiments to find out how moisture affects the performance of these targets as reference standards. We found that even small amount of moisture has a crucial effect on target reflectance properties: The backscattered reflectance decreases strongly up to 10% gravimetric water content, after which the reflectance level mostly remains constant. The moisture effect has to be further studied and taken into account in the retrieval and calibration of laser scanner intensity data acquired in moist conditions. We also demonstrate a practical method of deriving the surface backscattered reflectance at different moisture contents with a digital camera.