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. 2015 Sep 22;10(9):e0137765. doi: 10.1371/journal.pone.0137765

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This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration, which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

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Fig 2 — Demonstration of a. original, vs. b. OpenCV-calibrated lateral tree image obtained with a UAV-based GoPro camera at an above-ground altitude of 18 meters. Horizontal red line drawn to illustrate form of horizon in each version of the image.