After the aerial photo analysis, the sample plots, excluding inaccessible forests, will be visited next. Since the NFI3 the shrub forest has also been recorded.
The field teams measure 13 trees on the average on every sample plot. Additionally, they describe the stand and assess the condition of the site. Enquiries at the local forest service and the road network of the national maps 1:25000 provide further information about ownership as well as timber harvest and forest transportation system.
In recent years, the increasing demand for information has led to an expansion of the data catalog with every new survey. So far, ecological characteristics and information about forest functions have been requested.
Data for the first NFI was noted down on forms while on the terrain, whereas since the second NFI, data has been entered into field computers and checked for completeness on location.
Since the NFI1 some of the sample plots have been assessed a second time by an independent team to guarantee high data quality. In addition, the field teams are instructed and trained annually.
The NFI's staff comprises several people from the research unit Forest Resources and Management. Additionally, 8 to 15 teams of two were employed for the field work. Since the introduction of the continuous survey with the NFI4, only three groups have been doing field work. These foresters and forest wardens receive additional training and further education for their job periodically.
The expenditure for data collection and verification amounted to 100 man years in the first NFI. The total cost per sample plot were some CHF 550. After the first inventory the permanent sample plots did not have to be marked anymore, but this cost saving was offset by price increases in NFI2. Because of a lower budget allocation for NFI2, the number of sample plots had to be reduced.
Field teams are equipped with a minibus. Each NFI group is also supplied with a GPS, a vertex hypsometer, and a Finnish parabolic caliper. A Finnish parabolic caliper is used to measure stem diameters at 7 meters above the tree base. This diameter, the diameter measured at 1.3 m and the height of the tree, are used to calculate the volume of the tree.