## 3 Methods
### Test Field and Reference Data
The parameters were: (1) Maximum building size: 1 m \(\times\) 1 m. This is the maximum size of the local area where initial low points are searched for. (2) Terrain angle: 88\({}^{\circ}\). This is the local maximum steepness of the surface model. (3) Iteration angle: 12\({}^{\circ}\). This determines the maximum angle formed by the point, the closest triangle vertex, and the closest triangle plane. (4) Iteration distance: 1 m. This gives the maximum distance from a point to the nearest triangle plane. The breast height level was then determined as the point within given height limits from the corresponding ground-based points. The height limits used were 1.2 and 1.4 m. These points on the tree stems were then visually identified. Their location and diameter at breast height (DBH) were measured \begin{table}
\begin{tabular}{c c c} \hline
**Unit** & **Features** & **Specifications** \\ \hline NovAtel Flexpak6 & Frequencies & GPS L1 L2 L2C L5 \\ GNSS receiver and & & GLONASS L1 L2 \\ GPS-702-GG antenna & Horizontal positioning & \\  & accuracy (RMS) RTK & 1 cm + 1 ppm \\ NovAtel & Output rate & 200 Hz \\ UIMU-LCI Fiber & Acceleration accuracy (RMS) & 0.004 m/s\({}^{2}\) \\ optic gyro IMU & Gyro input range & \(\pm\)800 deg/s \\  & Gyro rate bias & \(<\)1.0 deg/h \\  & Gyro rate scale factor & 100 ppm \\  & Angular random walk & \(<\)0.05 deg/\(\backslash\)h \\  & Accelerometer range & \(\pm\)40 g \\  & Accelerometer bias & \(<\)1.0 mg \\ \hline \end{tabular} a: NovAtel Inc., Calgary, Canada, www.novatel.com.  
\end{table} 
Table 2: The technical specifications of the navigation system (NovAtel SPAN GNSS-IMU a). \begin{table}
\begin{tabular}{c c} \hline
**Features** & **Specifications** \\ \hline Maximum range & 153 m \\ Vertical field of view & 305\({}^{\circ}\) \\ Ranging error & \(\pm\)2 mm @ 25 m \\ Wavelength & 905 nm \\ Beam divergence & 0.19 mrad, 3 mm at exit \\ Scan frequency & 3–97 Hz \\ Point repetition frequency & 122–976 kHz \\ Angular resolution & 0.02–5.0 mrad \\ \hline \end{tabular} a: FARO Focus3D Features, Benefits & Technical Specifications, Lake Mary, FL, USA, www.faro.com.  
\end{table} 
Table 3: The technical specifications of the laser scanner (FARO Focus3D 120S a). by fitting a circle around the stem points (Figure 3a): the location of the tree was thus the center coordinates of the circle and DBH was the diameter of the circle. The circles were fitted manually using visual interpretation. Finally, the median GPS time of the points found within each circle was calculated and matched with the location and diameter data. The different driving directions were processed separately. In the case of \"multiplied trees\" near the turning points where trees had been scanned from several positions, each multiplication was measured individually (Figure 3b). The position error between the tree location in the point cloud and the corresponding reference tree location was then calculated for every tree and its multiplications were found in the point clouds. The median GPS time for each tree was used to study the variation in positioning error throughout the driving session. The accuracy of the reference trajectory was determined by comparing the tree locations derived from the total-station measurements with those measured from the point cloud. The RMSE values of the two reference trajectories are given in Table 4.