Juha Hyyppä
Depending on the terrain type and slope, the difference dz, value has to be specified. In Pyysalo (2000), it was found that
the hits between the minimum and 60 cm above it formed an ideal cloud of point to be used for final DTM calculation.
Mean and median values of classified ground hits were used for DTM calculation. The missing terrain heights was
interpolated by using the near-by heights and Delaunay algorithm.
In Pyysalo (2000) and Hyyppi et al. (2000c), the accuracy of the DTM algorithm was verified to be 22 cm in boreal
forest area. The terrain type and slope affected strongly the accuracy.
3.2 DTHM Generation
The maximum value of all the points within the resolution cell was calculated. It was found to represent rather well the
tree tops. When there were holes (no data), the value for these points were obtained by interpolation and using the
knowledge of nearby pixels. Diverging points in the digital crown model were detected by gradient method and
thresholding. The final DTHM was calculated as the difference between the DCM and the DTM. The demonstration was
carried out using first pulse data since it appeared that the first pulse mode was enough to provide the needed
information. The use of the both modes would be likely to further improve the results obtained in this paper.
Figure 1.Obtained digital tree height model for a sample area. Individual tree crowns can be easily seen.
3.3 Segmentation
The segmentation of an image can be defined as its partition into different regions, each having certain properties
(Soille, 1999). In the segmented image, the elementary picture elements are no longer the pixels but connected set of
pixels.
During the segmentation process, tree crown shape and location of individual trees were determined. Trees were found
by looking at the local maxima in the laser-derived tree height model. Before the maxima finding, tree height model was
low-pass-filtered. Without any filtering, the amount of tree crowns is typically too high. Too much filtering causes
oversize for the tree crowns (undersegmentation). The tree crown segments were determined by the modified watershed
424 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.
