A photogrammetric technique is described that can be applied to the problem of measuring the linear dimensions of free-swimming salmon contained in aquaculture pens. These dimensions are then used to statistically estimate the animal's weight. The system is non-invasive and is small and logistically easy to deploy from small boats. The system employs a very simple stereoscopic calculation that is ratio-metric. This means that image measurements are made in arbitrary units and converted to physical units by a multiplicative constant. This constant is simply the inter-pupil distance between the lenses of the stereoscopic pair of video cameras. Additionally, by choosing images in which the target is swimming perpendicular to the optical axis of the apparatus, the calculation is independent of the distance from the cameras to the fish, although full-frame images produce the greatest accuracy. The video cameras are frame synchronized for greater noise immunity and to ensure image-pair simultaneity. That is, the signals representing the 30 frames per second produced by each video camera are coincident in time. When combined they are perfectly matched. The usual practice would be to record each video signal using separate VCRs with synchronized time-coding for image identification and matching. This would be cumbersome and subject to operator error. The system under discussion avoids these difficulties by using an electronic device to "split" each of the two signals and to record the useful parts of each as a combined image using a single VCR.
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