Juha Hyyppä 
  
data collection merely along the flight track, such as in Nelson et al. (1984, 1988) or the pulse rate of the laser scanner 
hindered the capability to detect individual trees (Nässet, 1997). However, the situation changes when the number of 
pulses transmitted by the laser scanner increases. In the boreal forest zone and in many forest areas, there exist gaps 
between the forest crowns. For example in Finland, roughly speaking more than 30 % of the first pulse data reflects 
directly from the ground without any interaction with the canopy. By increasing the number of pulses, it is possible to 
have samples from each individual tree and also from the gaps between the trees. Basically this means that several laser 
pulses can be recorder per m“. This allows a detailed investigation of forest areas and the creation of 3-dimensional tree 
height map. The tree height map can be calculated from the digital terrain and crown models, both obtained with the laser 
scanner data. By analysing the 3-dimensional tree height model by using image vision methods, it is possible to locate 
individual trees, estimate individual tree heights, crown area and derive using that data stem diameter, number of stems, 
basal area and stem volume. 
This paper discusses automatic assessment of forest attributes by means of laser scanning. 
2 MATERIAL 
2.1 Test Site 
The boreal forest test site, Kalkkinen, is located in southern Finland, 130 km north of Helsinki. This rather typical boreal 
forest area was selected for the study in order to maximize the availability and adequacy of good field inventory data 
and remote sensing data. An intensive area of 100 hectares (2-km-by-0.5-km) was selected for the detailed study. The 
test site is dominated by minor hills, otherwise it is flat and situated about 110 m above sea level. The main tree species 
are Norway spruce and Scots pine whereas the mean stand size is 1.2 hectares. 
2.2 Field Inventory 
Standwise field data. Conventional stand-wise forest inventory was carried out on August-October 1996 using sample 
plots and personal experience. From these data, mean tree height [m], basal area [m?/ha], and stem volume per hectare 
[m3/ha] were obtained for each stand basically as means of the sample plot values. In order to monitor the cutting 
activity and other changes occurring between autumns 1996 and 1998 (laser acquisition), aerial photograph was taking 
in parallel with the field inventory and laser campaign. Changes were monitored visually and changed stands were 
rejected from further analysis. The descriptive statistics of the stand attributes information used for analysis are shown in 
Table 1 (41 stands). 
Table 1. Descriptive statistics of the field inventory data. 
  
  
  
  
  
Character Mean height Basal area Volume 
Mean value 16.9 m 19.3 m?/ha 174.7 m°/ha 
Standard deviation 7.0m 10.5 m?/ha 115.4 m°/ha 
Minimum value 3.0m 0.3 m"/ha 1.0 m'/ha 
Maximum value 24.2m 34.3 m’/ha 361.4 m’/ha 
  
Treewise measurements. A systematic sample plot network with 100-m spacing was designed for the test site. The 
location of center of each sample plot was determined with an accuracy of better than 1 m using advanced 
GPS/GLONASS system by Finnish Road Administration. From each plot (Figure 1), basal area with stratification by 
species, diameter and tree species of each tree, height and age (of at least 3 trees of every species and stratum) was 
measured. From every 5" plot, the location, diameter at breast height (1.3 m) and height of every tree were recorded. 
The location of every tree was measured as a reference to the center of the sample plot. Distance and angle deviating 
from compass north were recorded. From this data, 25 trees were used to derive the ratio between the crown area and 
stem diameter. 
2.3 Laser Scanning Measurements 
Laser scanning is based on distance measurements and precise orientation of these measurements between a sensor (the 
position of which is well known) and a reflecting object (the position to be defined). By knowing the sensor position, the 
distance and the incidence angle of each measurement, one can easily calculate the co-ordinates of the reflecting object. 
The scanning mechanism sweeps the laser beam across the flight line providing coverage across the flight track. Along 
track coverage is provided by the aircraft's motion. Concerning forest inventory, measurement density, incidence angle, 
  
422 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.