Jagielo et aL: Demersal groundfish densities in trawlable and untrawlable habitats off Washiington 



563 



1) «', was measured to the nearest milli- 

 meter, 2 ) an object on the seafioor adjacent 

 to the laser spots was identified, 3) the 

 videotape was advanced until the object 

 appeared at the bottom of the video moni- 

 tor display, and 4) w^ was measured again 

 (Fig. 2). The distance that was swept during 

 this interval (T) is calculated trigonometri- 

 cally by using the angle of the camera and 

 constants estimated with the following pro- 

 cedures of Davis and Tusting (1991). The 

 process is illustrated in Figures 3 and 4. 



The variables of interest are 



T = the geodetic distance between the lo- 

 cation of the laser spots on the sea- 

 floor and the bottom edge of the cam- 

 era's field of view (distance swept); 

 the height of the video camera above 

 the sea floor; 



o = the angle of the camera lens; 



8 = the tilt angle of the camera; 



D = the distance between the focal point 

 of the camera and the reflection of 

 the laser spots on the seafioor; 

 D] = the horizontal distance from the 

 camera to a point on the sea floor 

 at the center of the camera's field of view; 

 Dg = the horizontal distance from the camera to a point 

 on the sea floor at the bottom edge of the field of 

 view; and 



D' = the distance from the camera lens to the refiection 

 of the laser spots on the seafioor; 



H = 



Figure 3 



Schematic representation of the submarine and camera orientation to the 

 bottom (the line labeled D' represents the center line of the camera). Defini- 

 tions for the variables are provided in the left column of text on this page. 



w = the distance measured between the laser spots as 

 they appear on the video monitor display; 



W = the known distance (20 cm) between the lasers 

 mounted in parallel on the camera housing. 



Note the following relationships: 



