Taking a bird's eye view
Some information can be obtained from aerial photos with less effort than by measuring it in the forest on site. Therefore, since the second NFI the data from the aerial photos has been recorded every 500m and, as a consequence, the number of terrestrial sample plots was able to be reduced by half.
Due to this densification and the development of particular statistical methods, data about the forest stand, the number of trees and the area covered with forest will be acquired with the same accuracy as in the NFI1. Additionally, the number of single trees and the length of narrow strips of groves will be measured on the aerial photos in areas outside of the forest. In the NFI3 digital color photos were used instead of grayscale photos for the very first time.
What counts as forest in the NFI?
Forest or non-forest? Specialists clarify this question by using aerial photos. The height and width of the tree stand determines whether an area is considered forest. In addition, the tree crowns have to cover at least 20% of the area (degree of cover).
Afforestations, regenerating areas, shrub forests and temporarily non-stocked areas (e.g. cutting areas) require no minimal height.