Tree Top Detection in UAV Data: Evaluating Accuracy of Different Estimation Techniques

Vegetation greenness was monitored with digital cameras in three northern peatlands during five growing seasons. The greenness index derived from the images was highest at the most nutrient-rich site. Greenness indicated the main phases of phenology and correlated with CO₂ uptake, though this was mainly related to the common seasonal cycle. The cameras and Sentinel-2 satellite showed consistent results, but more frequent satellite data are needed for reliable detection of phenological phases.

The capability of time-lapse photography to retrieve snow depth time series was tested. We demonstrated that this method can be efficiently used in three different case studies: two in the Italian Alps and one in a forested region of Finland, with an accuracy comparable to the most common methods such as ultrasonic sensors or manual measurements. We hope that this simple method based only on a camera and a graduated stake can enable snow depth measurements in dangerous and inaccessible sites.

The surface albedo time series CLARA-A2 SAL was used to study trends in the timing of the melting season of snow and preceding albedo value in Finland during 1982–2016 to assess climate change. The results were in line with operational snow depth data, JSBACH land ecosystem model, SYKE fractional snow cover and greening-up data. In the north a clear trend to earlier snowmelt onset, increasing melting season length, and decrease in pre-melt albedo (related to increased stem volume) was observed.

Monitoring ecosystems using low-cost time lapse cameras has gained wide interest among researchers worldwide. Quantitative information stored in image pixels can be analysed automatically to track time-dependent phenomena, e.g. seasonal course of leaves in the canopies or snow on ground. As such, cameras can provide valuable ground references to earth observation. Here we document the ecosystem camera network we established to Finland and publish time series of images recorded between 2014–2016.

Digital photography has become a widely used tool for monitoring the vegetation phenology. The seasonal cycle of the greenness index obtained from photographs correlated well with the CO₂ exchange of the plants at our wetland and Scots pine forest sites. While the seasonal changes in the greenness were more obvious for the ecosystem dominated by annual plants, clear seasonal patterns were also observed for the evergreen forest.

The Intergovernmental Technical Panel on Soils has completed the first State of the World's Soil Resources Report. The gravest threats were identified for all the regions of the world. This assessment forms a basis for future soil monitoring. The quality of soil information available for policy formulation must be improved.