Abstract
Although many more types of in situ data collection exist, notably in oceans and freshwater, this chapter leans towards the vegetation inventories in terrestrial biodiversity monitoring: how to collect data in the field or from aerial photos, orthophotos, and drones, which can then be used as auxiliary data in any area monitored or as a stand-alone result. The chapter takes up classifications and design of plots and practical issues of how to run a monitoring programme, exemplified by different running monitoring schemes. We also discuss the importance of calibrating observations between field staff and other tools to minimize observer variation and illustrate why these are necessary.