There are more than 500 million trees in Switzerland - far too many to investigate individually. Random sampling, however, yields adequate information. For that purpose a 1km-grid was mapped over Switzerland in the first NFI. The intersections defined the location of the sample plots in the forest.
Since the second NFI, only half of these plots, roughly 6500, have been located in the field. The grid, which originally had a mesh size of 1 km, was extendend to 1.4 km. To compensate for this reduction, the aerial photos were interpreted in a grid of 500 m.
The same methods have been carried out since switching from a periodic to a continuous survey in the fourth NFI, but the sample plots are now located over a period of nine years. Thereby another ninth of the sample plots, which are evenly distributed all over Switzerland, are surveyed every year.
The center of the sample plot is marked by a metal pole in the ground. Roughly 130,000 sample trees were measured in the NFI1 and marked so as they can be found again in later inventories. Thanks to the exact sketches, about 98% of the sample plots could be found directly during the NFI2 without having to search for them. In the NFI4, the position of the centers of the sample plots are located exactly with a GPS.
Within a 200 m² circle, every tree which has a diameter larger than 12 cm is recorded, and within a 500 m² circle, every tree which has a diameter larger than 36 cm is recorded. These diameters are measured at a height of 1.3 m (diameter at breast height DBH). The radii are 7.98 m (r₁) and 12.62 m (r₂) on level terrain.
Trees and bushes having a diameter of less than 12 cm and a height of at least 10 cm are registered on a separate circle (satellite) 10 m west of the center. Additionally, the dead wood on the ground is recorded on three transects if the diameter is larger than 7 cm.