Measuring a shark. You could do it by catching one, and holding up a tape measure from its fin to its head, without harming it or yourself. Or you could use a camera. That's what Dr Mark Meekan and Gabriel Vianna are doing in the shark sanctuary at Palau: Using two cameras mounted in a rig, a diode used to sync video frames, and the geometry between them to measure the sharks from afar.
The technique, which is highly accurate, has been described before in a 2009 paper entitled A Review of Underwater Stereo-Image Measurement For marine Biology and Ecology Applications. [save our seas via The Scuttlefish. Donate to Save Our Seas. Image Credit Peter Verhoog]