Automatic thresholding for hemispherical canopy-photographs based on edge detection

The analysis of hemispherical photographs is nowadays an established method for assessing light indirectly and describing canopy structures. In this article, we present an automatic threshold algorithm for separating canopy and sky by edge detection. The algorithm was evaluated under different canopy conditions by comparing its results for canopy openness, fractal dimension and diffuse transmittance with those from multiple manual thresholding and direct measurements of the percent photosynthetic photon flux density (PPFD). We show that the automatic threshold algorithm is appropriate to replace the widely used manual interactive processing. It also improves the accuracy of results, especially in comparison with single manual thresholding. Whereas manual threshold setting has often been criticised as subjective and a major source of error the less time-consuming edge detection approach is objective, reproducible and can be applied to a large number of images.

[1]  C. Field,et al.  Photographic estimation of photosynthetically active radiation: evaluation of a computerized technique , 1987, Oecologia.

[2]  R. Hall,et al.  A comparison of digital and film fisheye photography for analysis of forest canopy structure and gap light transmission , 2001 .

[3]  Heinz-Otto Peitgen,et al.  The science of fractal images , 2011 .

[4]  G. A. Van Sickle,et al.  Tree-crown defoliation measurement from digitized photographs , 1983 .

[5]  Sven Wagner,et al.  Calibration of grey values of hemispherical photographs for image analysis , 1998 .

[6]  A. Zehm,et al.  Multiparameter analysis of vertical vegetation structure based on digital image processing , 2003 .

[7]  D. Clark,et al.  Long-term study of solar radiation regimes in a tropical wet forest using quantum sensors and hemispherical photography , 1993 .

[8]  P. Reich,et al.  Evaluation of several measures of canopy openness as predictors of photosynthetic photon flux density in deeply shaded conifer-dominated forest understory , 1999 .

[9]  D. Clark,et al.  Evaluation of digital and film hemispherical photography and spherical densiometry for measuring forest light environments. , 2000 .

[10]  François de Coligny,et al.  Simulating radiation distribution in a heterogeneous Norway spruce forest on a slope , 2003 .

[11]  L. Olsson,et al.  Evaluation of forest-canopy photographs with diode-array scanner OSIRIS , 1982 .

[12]  D. Coombe,et al.  Hemisperical and Woodland Canopy Photography and the Light Climate , 1959 .

[13]  P. Rich.  Characterizing plant canopies with hemispherical photographs. , 1990 .

[14]  P. Becker,et al.  Analysis of forest light environments part I. Computerized estimation of solar radiation from hemispherical canopy photographs , 1989 .

[15]  Jason R. Roxburgh,et al.  USES AND LIMITATIONS OF HEMISPHERICAL PHOTOGRAPHY FOR ESTIMATING FOREST LIGHT , 1995 .

[16]  Kazukiyo Yamamoto,et al.  Estimation of Relative Iilluminance using Digital Hemispherical Photography , 2002 .

[17]  S. Hale,et al.  Comparison of film and digital hemispherical photography across a wide range of canopy densities , 2002 .

[18]  Frédéric Baret,et al.  Review of methods for in situ leaf area index determination Part I. Theories, sensors and hemispherical photography , 2004 .

[19]  M. A. Halverson,et al.  Forest mediated light regime linked to amphibian distribution and performance , 2003, Oecologia.

[20]  P. Comeau,et al.  A comparison of several methods for estimating light under a paper birch mixedwood stand , 1998 .

[21]  Richard F. Voss,et al.  Fractals in nature: from characterization to simulation , 1988 .

[22]  Sven Wagner,et al.  Relative radiance measurements and zenith angle dependent segmentation in hemispherical photography , 2001 .

[23]  J. Bellow,et al.  Comparing common methods for assessing understory light availability in shaded-perennial agroforestry systems , 2003 .

[24]  Christian Messier,et al.  A simple and efficient method to estimate microsite light availability under a forest canopy , 1996 .

[25]  T. Spies,et al.  Evaluating Forest Vegetative Cover with Computerized Analysis of Fisheye Photographs , 1986, Forest Science.

[26]  Margaret C. Anderson.  Studies of the Woodland Light Climate: I. The Photographic Computation of Light Conditions , 1964 .

[27]  Leon Bren,et al.  A study of the effect of logging on the understorey light environment in riparian buffer strips in a south-east Australian forest , 2003 .

[28]  JASON R. ROXBURGH,et al.  USES AND LIMITATIONS OF HEMISPHERICAL PHOTOGRAPHY FOR ESTIMATING FOREST LIGHT ENVIRONMENTS , 2004 .

[29]  Christian Messier,et al.  Comparison of various methods for estimating the mean growing season percent photosynthetic photon flux density in forests , 1998 .

[30]  Jurij Diaci,et al.  Ein Vergleich verschiedener Lichtmessmethoden in Buchennaturwäldern Sloweniens aus verjüngungsökologischer Sicht | A comparison of solar radiation estimation methods in old-aged beech forests in Slovenia from the point of view of ecological regeneration , 2002 .