Data collection in the forest and by the forest service
After the aerial photo interpretation, with the exception of inaccessible forest areas, the sample plots are visited in the field. Since NFI3 (2004–2006), the shrub forest has also been surveyed.
The field teams measure an average of 13 trees on the sample plots. Additionally, they describe the forest stand and assess site conditions. Any information about the sample plots that the field teams cannot gather directly – such as ownership conditions or information on timber harvesting and the forest road network – is requested from the local forest service.
The increasing information demands on the part of politics, science and society have resulted in the expansion of the NFI data catalogue available for each inventory. For example, in the current NFI5 (2009–2017), tree microhabitats – tiny habitats for many forest-dwelling (animal) species – are being recorded for tally trees for the first time. The NFI has thus changed from a more forestry-oriented inventory to a comprehensive forest inventory that can provide information on the various tasks and functions of the forest.
Quality control in the NFI begins during field surveys, when the acquisition program checks whether the data are being entered consistently. Further, independent repeat surveys are completed to blind-check whether the data have been recorded reproducibly in the field, and additional control surveys are carried out by the field management team to check whether the survey manual has been interpreted correctly. Finally, the competence of the field teams is evaluated through periodic training courses, with the aim of achieving a uniform survey standard.
For the field surveys, 8 to 15 teams of two persons were hired in each of the inventories NFI1 to NFI3. Since the introduction of the continuous survey method in NFI4 (2009–2017), only three teams have been carrying out the surveys. These forest engineers and foresters receive individual instruction, and they attend an annual weekly training course before the start of the field surveys and one to two training days during the field season.
Each field team is equipped with a wide assortment of material. In addition to the classical calliper for measuring the diameter at breast height (dbh, measured at 1.3 m) and a tablet, this material includes also a GPS, a distance and tree height meter, and a Finnish calliper. The last of these tools is used to measure tree diameter at a height of 7 m. This diameter is used, together with dbh and tree height, to calculate tree volume.