pling 
ge. 
Ss area 
  
ruction of 
by sonar 
Ship's.course Ship 
  
Figure 4 Perspective images observed from the 
left side (A) and the right side (B) of the bow. 
opaqueness of the background, so the threshold level 
was set to the data value based on the histogram of 
the echo level to transparentize the volume data 
(Figure 3). This volume data could be enlarged, 
rotated, moved, and painted to facilitate processing 
the three-dimensional images. Figure 4 is perspective 
image observed from the left (A) and right sides (B) of 
the bow, respectively. 
3. RESULTS 
3.1 Spatial Distribution of Fish School by Two 
Perpendicular Plane Projections 
Figure 5 shows the spatial distribution of a fish school 
and the sea-bottom features from the top-view 
projection and the side-view projection. The upper 
chart (A) indicates the top-view projection, and the 
horizontal central broken line represents the course of 
the ship. As the ship proceeded from left to right in the 
figure, the vertical axis over the track line indicates the 
horizontal distance to the left broadside, and the lower 
vertical axis indicates the distance to the right 
broadside. The middle chart (B) indicates the side- 
view projection. The horizontal axis indicates the 
cruising distance, and the vertical axis indicates the 
depth. 
These displays help us easily understand the spatial 
distribution of fish schools. For example, an apparent 
single fish school in the vertical projection seems to 
731 
   
     
    
   
   
Horizontal pr 
   
jection 
  
  
Depth 
  
Figure 5 Spatial distribution of fish school by 
horizontal projection (A), and vertical projection 
(B), and echogram obtained by vertical echo- 
sounder (C). 
consist of several fish schools in the horizontal 
projection, and vice versa. On the other hand, the 
echogram of the vertical echo-sounder (C) resembles 
the vertical projection display, but the vertical 
projection provides more information about the depth 
of the image. The echogram of the vertical echo- 
sounder is only a vertical cross section of the ship's 
track. 
Thus, the display method proposed here helps us to 
understand the precise distribution of fish schools. 
3.2 Three-Dimensional Shape of Fish School 
As mentioned above, it is impossible to determine the 
spatial distribution and solid shape of fish schools 
using only echograms from a vertical echo-sounder. 
The three-dimensional analysis technique described 
above can be used to determine the shapes of 
underwater objects. As the vertical sectional images 
are sampled every 1.5 seconds, the distance between 
each section is 7.7 m when a ship cruises at 10 knots. 
Figure 6 shows the solid shape of the fish school cut 
down from Figure 4 by surface rendering techniques. 
The upper figure (A) shows the perspective image of a 
fish school from the right side of the bow, middle figure 
(B) shows the perspective image of side direction, and 
the lower figure (C) shows the image from the bow.