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4.3 Area coverage 
In this section we analyze the percentage of seabed area which was actually covered by ALB points in several Secchi 
depth levels. The reference area is derived from the corresponding normalized seabed depth grid. Binary masks from the 
ALB points classified as seabed were generated for each campaign. At each grid cell (5x5 m 2 ) the mask was set to true if 
the cell contained at least one ALB seabed point, otherwise the grid cell was set to false. An approximation of the area 
covered by the ALB data was then determined by counting the grid cells corresponding to a particular depth level, that 
are set to true (see Figure 5). 
<0.5 0.5<0.7 0.7<1.0 1.0<1.3 1.3<1.5 >1.5 
Secchi depth 
1. Campaign ■ 2. Campaign 3. Campaign 
Figure 5. Coverage area by seabed points in comparison to the reference area. 
For the first campaign the coverage of the laser points decreases steadily with increasing depth. By far most of the area 
was detected in the very shallow water. After 0.7x Secchi depth the number of points and, thus, also the covered area 
rapidly decreases. In contrast, over 70% of the seabed area is covered by laser points for the second and third campaign 
up to 1.0 x Secchi depth. At a level of 1.0 * to 1.3 * Secchi depth the coverage decreases significantly, and regions deeper 
than 1.3 x Secchi depth are only rarely observed. 
4.4 Vertical accuracy 
Of particular interest for the evaluation of the data is the vertical accuracy because it is relevant for many applications. 
We analyze the vertical accuracy of the 3D points classified as seabed by computing the difference in depth for each 
echo sounding point to the closest cell of an interpolated laser grid. Only points with a horizontal distance of less than 
5 m to the grid cell are considered to cope with errors of wrong assignments. The differences between the laser grid and 
the echo sounding data are shown in Figure 6 and Table 3. 
Figure 6. The differences between laser grid and echo sounding points for each campaign in 0.1 m intervals. 
Proc. of SPIE Voi. 9638 96380Z-6