Automatic thresholding for hemispherical canopy-photographs based on edge detection
Introduction
Light availability is an important site factor in ecology and the management of forests or agricultural land. It can be assessed indirectly in hemispherical photographs, a technique first used for forests by Evans and Coombe (1959) and Anderson (1964), which is nowadays widely applied (recently Bellow and Nair, 2003, Courbaud et al., 2003, Dignan and Bren, 2003, Halverson et al., 2003). With technological progress, several approaches for automation, especially computerised methods, have been proposed (Bonhomme and Chartier, 1972, Olsson et al., 1982, Chan et al., 1986, Chazdon and Field, 1987, Becker et al., 1989). In addition, several software packages for the image analysis are now available (Frazer et al., 2000), and the change from black and white film to digital camera systems has been evaluated (Englund et al., 2000, Frazer et al., 2001, Hale and Edwards, 2002).
Although using hemispherical photographs has several advantages over direct light assessment and technological and methodological progress have led to improvements in the method, it also has limitations. One of the most critical points is separating the canopy from the sky during image analysis. The usual initial step is a binary transformation generally done by setting a threshold manually, although there are some approaches that work directly with grey or colour values (Olsson et al., 1982, Wagner, 1998, Wagner, 2001). Several authors have pointed out that manual thresholding can be a relevant source of error because it is somewhat arbitrary and subjective (Chan et al., 1986, Rich, 1990, Machado and Reich, 1999, Frazer et al., 2001, Diaci and Thormann, 2002, Jonckheere et al., 2004).
In this article, we present an automatic threshold algorithm for hemispherical canopy-photographs based on edge detection. The algorithm is evaluated under different canopy conditions by comparing it with single and multiple manual thresholding and direct radiation measurements. The results are discussed in relation to manual thresholding and the known limitations of using hemispherical canopy-photographs to assess radiation indirectly.
Section snippets
Optimal threshold algorithm
The principle of the method is the search of a threshold value that gives highest local contrast at the edges between classified canopy and sky. Accordingly, the threshold value t with the maximum mean brightness difference at the edges is defined as optimal threshold topt (Eq. (1); Fig. 1).The image domain S* of calculating the mean is defined as:with
Results
Fig. 2 shows the comparison of single manual and automatic thresholds with the mean manual threshold values. There are striking scatters for both methods showing similar magnitude of variation. However, the automatic algorithm tends to over- or underestimate the thresholds for the majority of images, this is particularly noticeable for the conifer forest and the uncleared windthrow subsets.
Results for the canopy openness and fractal dimension are given according to the different thresholding
Discussion
As shown in Fig. 2, the single manual thresholds vary greatly, which means that they are a potential source of error. Even more parameters calculated after single manual thresholding may vary remarkably too (Fig. 3). The influence of the variation in single manual thresholds on subsequent analyses depends on both the parameter and the image properties. After transformation, photographs with a high contrast between the vegetation and sky will show only slight differences in a wide range of
Conclusions
The results show that the suggested automatic threshold algorithm by edge detection has advantages over manual interactive processing. It is objective, comprehensible and reproducible, whereas manual thresholding has often been criticised as subjective and a major source of error. The edge detection approach may also improve the accuracy of the results. But its main advantage is that it is much less time-consuming than manual thresholding and can be applied to a large number of images. For
Program source
The automatic threshold algorithm is implemented in the software tool SideLook. It can be downloaded as shareware at http://www.appleco.ch.
Acknowledgements
We are especially grateful to J. B. Stewart and two anonymous reviewers for their valuable comments on the manuscript. The authors would like to thank S. Martin for providing the conifer forest images. We are grateful to C. Dähler, L. von Fellenberg, A. Ghiringhelli, R. Häner, M. de Montmollin and M. Zubler for their help with manual thresholding. We also thank C.W. Hoffmann for his help with mathematical equations and wording. This work was supported by the Swiss Agency for the Environment,
References (31)
- et al.
Analysis of forest light environments. 1. Computerized estimation of solar-radiation from hemispherical canopy photographs
Agric. For. Meteorol.
(1989) - et al.
Comparing common methods for assessing understory light availability in shaded-perennial agroforestry systems
Agric. For. Meteorol.
(2003) - et al.
Simulating radiation distribution in a heterogeneous Norway spruce forest on a slope
Agric. Forest Meteorol.
(2003) - et al.
A study of the effect of logging on the understorey light environment in riparian buffer strips in a south-east Australian forest
Forest Ecol. Manage.
(2003) - et al.
A comparison of digital and film fisheye photography for analysis of forest canopy structure and gap light transmission
Agric. For. Meteorol.
(2001) - et al.
Comparison of various methods for estimating the mean growing season percent photosynthetic photon flux density in forests
Agric. For. Meteorol.
(1998) - et al.
Comparison of film and digital hemispherical photography across a wide range of canopy densities
Agric. For. Meteorol.
(2002) - et al.
Review of methods for in situ leaf area index determination. Part I. Theories, sensors and hemispherical photography
Agric. For. Meteorol.
(2004) - et al.
Long-term study of solar-radiation regimes in a tropical wet forest using quantum sensors and hemispherical photography
Agric. For. Meteorol.
(1993) Calibration of grey values of hemispherical photographs for image analysis
Agric. For. Meteorol.
(1998)
Relative radiance measurements and zenith angle dependent segmentation in hemispherical photography
Agric. For. Meteorol.
Multiparameter analysis of vertical vegetation structure based on digital image processing
Flora
Studies of the woodland light climate. 1. The photographic computation of light conditions
J. Ecol.
Interpretation and automatic measurement of hemispherical photographs to obtain sunlit foliage area and gap frequency
Isr. J. Agric. Res.
Evaluating forest vegetative cover with computerized analysis of fisheye photographs
Forest Sci.
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