Sea cave length measurement.

Caver Bob maintains a list of long sea caves. I'm looking for instruments to measure the length of sea caves.

So far, I've used a laser to measure two small caves, 33 and 55 m long.

But I think the best solution might be sonar with an active transponder. The master sends a ping, the transponder answers pong - on another frequency, to avoid confusion with all the echoes in a cave.

The master then divides the delay by two, and multiplies with the speed of sound.

This solution should work, even around corners.

A simpler one-frequency set-up could be a conventional depth sounder turned horizontally, and a submerged radar reflector. But I think false echoes would be a problem.

Also see Faroese caves on Facebook and Google Maps.

Google update: JW Fishers make this stuff.

Automatic kayak photography

When the kayaking gets real interesting, I'm busy with my paddle, unable to handle the camera.

I'd like a camera that automatically takes photos when the going gets interesting. In a sea kayak, the triggers might be vertical acceleration, sudden listing, and being upside down. (While when skiing, I guess free fall is interesting...)