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On the exposure of hemispherical photographs in forests

P Beckschäfer (1)   , D Seidel (1), C Kleinn (1), J Xu (2)

iForest - Biogeosciences and Forestry, Volume 6, Issue 4, Pages 228-237 (2013)
doi: https://doi.org/10.3832/ifor0957-006
Published: Jun 13, 2013 - Copyright © 2013 SISEF

Research Articles


At least 10 different methods to determine exposure for hemispherical photographs were used by scientists in the last two decades, severely hampering comparability among studies. Here, an overview of the applied methods is reported. For the standardization of photographic exposure, a time-consuming reference measurement in the open land towards the unobstructed sky was required so far. The two Histogram Methods proposed here make use of the technical advances of digital cameras which enable users to assess a photograph’s histogram directly at the location of measurement. This avoids errors occurring due to variations in sky lighting happening in the time span between taking the reference measurement and reaching the sample location within the forest. The Histogram Methods speed up and simplify taking hemispherical photographs, and introduce an objectively applicable, standardized approach. We highlight the importance of correct exposure by quantifying the overestimation of gap fraction resulting from auto-exposed photographs under a wide range of canopy openness situations. In our study, gap fraction derived from auto-exposed photographs reached values up to 900% higher than those derived from non-overexposed photographs. By investigating the size of the largest gap per photograph and the number of small gaps (gaps contributing less than 0.1% to gap fraction), we concluded that the overestimation of gap fraction resulted mainly from the overexposure of vegetation surrounding large gaps.

  Keywords


Gap Fraction, Canopy Openness, LAI, Canopy Photography

Authors’ address

(1)
P Beckschäfer
D Seidel
C Kleinn
Chair of Forest Inventory and Remote Sensing, Georg-August-Universität Göttingen, Büsgenweg 5, D-37077 Göttingen (Germany)
(2)
J Xu
World Agroforestry Center, ICRAF-Kunming Office, Kunming, Yunnan 650201 (China)

Corresponding author

 

Citation

Beckschäfer P, Seidel D, Kleinn C, Xu J (2013). On the exposure of hemispherical photographs in forests. iForest 6: 228-237. - doi: 10.3832/ifor0957-006

Academic Editor

Francesco Ripullone

Paper history

Received: Jan 28, 2013
Accepted: Mar 26, 2013

First online: Jun 13, 2013
Publication Date: Aug 01, 2013
Publication Time: 2.63 months

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