1. Department of Forest Sciences, University of Helsinki, Helsinki, FI-00014, Finland;
2. Department of Remote Sensing and Photogrammetry, Finnish Geospatial Research Institute, Masala, FI-02431, Finland;
3. Centre of Excellence in Laser Scanning Research, Finnish Geospatial Research Institute, Masala, FI-02431, Finland;
4. School of Forest Sciences, University of Eastern Finland, Joensuu, FI-80101, Finland