Quote: (Originally Posted by
neilh)
If you used one with a handle then couldn't you rig a leash/boltsnap and job's a good 'un?
We brought a couple of boards along just in case last autumn. Funnily enough, even with the boltsnaps, they stayed on the boat (and not even the boat we dived off, but the boat left in the harbour).
Quote:
but can I ask why the NAS approach is impractical in this case? I know very little about this stuff and find it interesting, so I'm curious to hear why you're having to adapt the techniques.
The NAS method is very accurate, but also very time consuming. It also relies on being able to add permaent markers/tags as control points (which remain in place for the duration of the project (ie years) and temporary markers to each data point being surveyed. The method also works best (ie pretty much requires) the two divers to be at opposite ends of the tape measure. In the sort of vis we have, this would impose a maxium limit of about 5m for a measurement. Each point should, under the NAS method be measured from 4 other points (3 are required for 3D data, the 4th being for redundancy/error checking). The area we laid line accross last year was approx 65m long by 40m wide, we'll be extending that this year. We just couldn't cover the site with the NAS method in the available time, even it was possible to hammer permanent markers into the solid seabed for control points.
With the distance/bearing method, we'll should get reasonable data, which combined with video/stills is more useful than a very accurate survey of a small portion of the site.
David