The terrestrial survey carried out by the NFI involves great time and effort. In order to optimize input, the sampling areas are interpreted in stereo aerial photographs prior to the field season. Digital sensor data from swisstopo is used in this process. There are no terrestrial on-site visits to clearly identified non-forest plots.
This sampling allows for a reliable estimate over larger areas. To reduce estimation errors even for small areas, information from remote sensing data is used for multi-phase analysis.
In addition to interpreting aerial photographs, digital surface models are calculated from stereo aerial images through image correlation. From there, crown height information can be derived. Tree species are classified using spectral data. These products will be available nationwide and will be updated every six years.