alls. 
  
315 
  
  
/ begin of snowfall 
- 580 
eo 
e 
N 
e 
   
y - Coordinate [cm] 
2? 8 
end of snowfall 
   
  
  
  
  
     
  
   
t 4 ta 
460 
(a) 
40594 595 596 57 598 599 60 601 602 603 
Julian Day 1994 
600 
ball 1 
580 ball 3 ball 2 
Trunk ball 4 . 
560 - > mistake 
   
— 
ball 4 
y - Coordinate [cm] 
8 
  
  
  
(b) 
260 280 300 320 340 360 380 400 420 440 460 
x - Coordinate [cm] 
  
  
  
  
Figure 2: Movement of balls of one branch during a snowfall event from February 28, 1100 to March 1, 615. 
Fig. (a) shows the movement of ball 1 at the edge of the branch during the observation period. The 
y-axis gives the coordinates in vertical direction. Intensive movement in t; indicates that the branch 
intercepted most of the snow according to the snowfall. In t snowfall intensity was smaller and less 
snow was intercepted. Fig. (b) shows the movement of the whole branch. The upper line shows the 
position of the branch before the snowfall, the lower line the position after the snowfall. The trunk 
of the tree corresponds to the left edge of the image. The x-axis gives coordinates in the horizontal 
direction of the image plane (horizontal movement) and the y-axis the vertical direction as it is seen 
on the video screen (vertical movement). The angle of the edge of the branch is changing from o, to 
da. In some cases errors occurred due to disturbing factors (e.g. wind). 
The search algorithm calculates the center of mass by finding the mean pixel coordinate by a subpixel calculation. 
That means that in a 7x7 matrix 3.5 pixel are on the left, right and above and below side of this point. The 
area boundary is assumed to be a simple polygon. The whole time series of the ball coordinates describes the 
line of motion of the ball during a snowfall. In total the tree is provided with 14 balls, so 14 ROT's had to be 
analysed. For this example only one branch in one snowfall event is described. 
The result of one analysis run for a snowfall event is given in Fig. 2(a,b). The figure shows the movement of a 
branch during a snowfall event which took place from February 28, 1994, 1100 to March 1, 1994, 615. In total 7 
cm of new snow with a water equivalent of 15 mm fell during this time period. From February 28, 1994, 1100 to 
February 28, 1994, 1825 the coordinates of the balls were digitized manually by creating points in the center of 
the balls each for a time interval of 30 min (see interrupted line in Fig 2(b)). This was necessary because of the 
heterogeneous light conditions in a forest which made automatic digitizing actually impossible. An example of 
an image at day is shown in Fig. 1(a). Coordinates of balls which could not be recognized were set to “missing 
value". From February 28, 1994, 1825 to March 1, 1994, 615 the balls were automatically digitized by the 
above mentioned algorithm in a 10 min interval. During the snowfall the branch intercepted snow which caused 
a bending of the branch (see section 2.1). The sum of movement of each ball is shown in Table 1. Greatest 
movement can be observed at ball 1, because it is mounted at the edge of the branch. The angle in relation to 
the horizontal is changing from a1= --11.8? to a5— -42.3? which yields a change of the angle of Ao — -54.1? . 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995