Nevertheless, beside this remarkable achievements, reports about failed remote sensing missions with UASs are frequent. On the very extreme, GSDs < 0.001 m paved the way for recent works that estimated seed emergence. The segmentation of images in pixels related to either plants or soil demands comparable GSDs ≤ 0.01 m and allowed to measure canopy cover, leaf area index and crop density. GSDs ≤ 0.02 m served as base for structure-from-motion (SfM) processing followed by plant height extraction a method that was used to subsequently estimate plant biomass, lodging, yield, and other plant height related parameters. GSDs ≤ 0.1 m were sufficient for empirical estimations of general crop characteristics such as leaf area index, crop nitrogen content, and ripening processes, or weed detection. Especially in crop monitoring, the trend for finer resolved photos has led to an increasing number of applications. This development enables identification of details on the plant or even plant organ level. Recent UAS and sensor technologies allow ground sampling distances (GSD) in the centimeter, millimeter or even sub-millimeter range. Unmanned areal systems (UAS) are increasingly used as platforms to monitor vegetation by means of optical remote sensing.
