13 



Mapping techniques 



To map eelgrass beds, aerial prints were overlaid with a sheet of 

 acetate, eelgrass beds were outlined, and other notes were recorded. 

 The photographs and overlays were subsequently photographed with B&W 

 slide film, and this image was projected onto a map of 1:25,000 scale or 

 smaller. The eelgrass beds were then redrawn by hand and distortions in 

 the image were compensated for by eye or manipulating the image on a 

 film enlarger. These bed outlines were re-traced using a digitizing pad 

 connected to a microcomputer. Digitizing and mapping programs for a 

 microcomputer were used for data storage, area analysis, and plotting 

 at different scales. 



The maps produced here have -25 m resolution. The process of 

 projection, tracing, and digitizing, however, introduced random errors 

 in bed position. These errors were small, and the position of eelgrass 

 beds on the maps in this report were generally accurate within 40 m for 

 beds adjacent to the shore, 60 m for beds within 0.5 km of shore, and 

 within 80 m for eelgrass beds more than 0.5 km from any shoreline when 

 compared to bed positions measured directly from the source photographs. 



Each subarea is shown with political boundaries and site names and 

 again with eelgrass beds drawn. In the latter, eelgrass beds are drawn 

 with dashed lines and coastlines as solid lines. Bed areas were 

 computed from the stored coordinates and reported as hectares [1 ha = 

 2.47 acres] . 



