Using two cameras mounted in a frame, a diode used to sync video frames, and the geometry between them to measure the sharks from afar. The technique has been described before in a 2009 paper entitled A Review of Underwater Stereo-Image Measurement For Marine Biology and Applications. Donate to The Scuttlefish.com to see how you can measure sharks using the technique. For more information on the technique, visit www.scuttlefoss.com/sharknet.”]
Source: https://gizmodo.com/how-to-measure-a-shark-safely-5797155