Forest inventories have always been important for a rational forest management. Norway has always been one of the leading countries in this particular field. The techniques applied have of course changed through the years, and now it is LiDAR that dominates in Norway and other countries.
National forest inventory
Forest inventories are important as a basis for a rational forest management. Such inventories have long traditions in Norway. Already in 1919 a systematic inventory for the entire country was established in Norway. This was the world’s first national forest inventory (NFI) based on statistical sampling techniques. The inventory has been through changes since then, and is now based on permanent sample plots which are re-visited every five years.
Forest inventories on property level
In addition to the NFI, property level inventories have been important in Norway, and they became custom in the 60s. As opposed to the NFI, these inventories are meant to be the basis for rational decisions on forest management on property level. Since management almost always are carried out on the productive area, property level inventories have been concentrated on this area. The data sources that have been used in these inventories are aerial photographs and field measurements. Since field measurements have been a large part of these inventories, they have been quite expensive to perform. The accuracy depends on the measurements, the sample size together with the variation in the population.
Need for new inventory methods
Due to a need for reducing inventory costs, more frequent updates of inventories together with a need for also including unproductive forests near the alpine and arctic tree line, there is a need for more effective and cheap inventory methods. Demands for accurate and timely information on forest quantity, quality and location, support this need for new methods. This need has driven the development of inventory methods based on LiDAR (Light Detection And Ranging).
Distance measurements by means of laser
Airborne LiDAR has been used as a standard technique for topographic mapping and for seabed mapping in shallow waters for some time. Today the same technique is used for forest inventories. LiDAR is based on pulses of laser light that are emitted, reflected and received. The pulses are emitted form a sensor mounted in a fixed-wing plane or in a helicopter. The emitted pulses are reflected back from whatever object they hit on the ground and the distance from the sensor to the hit is calculated from the difference in time between emission and reception. Since also the positions and orientations of the sensor are known when the pulses are emitted, and three directional coordinate can be calculated for each laser hit. Multiple hits per square meter yields a dens point cloud (see picture) that can be utilized to estimate forest properties.
The new method was embraced by private and public companies and organizations that deals with forest inventory and mapping. After several full scale tests it was concluded that inventories based on LiDAR have 50% less random error than the traditional methods. Furthermore, high resolution LiDAR data with five to ten pulses per square meter enables single tree identification with fully automated procedures. Airborne LiDAR is also the remote sensing technique that seems superior in order to survey forest resources and carbon stock on regional and national level.
The world follows
By now, many operational inventories have been carried out on commercial terms in Norway Sweden and Finland. In Norway LiDAR has become the standard method for forest inventor. The method is also getting more and more common outside the Nordic countries.
LiDAR has a great potential related to forest inventories. It is expected that in the future, applications where the goal is to predict basal biological processes will be carried out based on a detailed canopy description resulting from LiDAR data. LiDAR will also be important in order to understand the large scale carbon dynamics in forests. It will also be an important technique for mapping biomass over large areas. For example, in a few years a satellite borne LiDAR designed for biomass measurements will be available.