Canopy hemispherical photography (HP) is a technology for vegetation surveying that usually employs consumer-oriented devices. Variations on those devices are caused by technological advances and design. These variations, despite providing the potential for more reliable surveying, make the measurements challenging. The knowledge about how to standardize devices output is available, what is missing is a simple and inexpensive methodology to accomplish it. This is needed since the use of consumer-oriented devices creates an expectation of simplicity and affordability. To fill this methodology gap, we developed an easy-to-assemble calibration board of near-zero material cost (simple board) and outlined how to use it to develop device-specific functions: the projection function () and the vignetting function (). The former relates view zenith angle () with distance from image center, while models the radial decreasing of brightness known as vignetting effect. Explanations are accompanied by tailored R code. Altogether, we call it “the simple method”. Three devices were used to test it: a Nikon Coolpix 5700 with a FC-E9 converter (D1), a Nikon D610 with a Nikkor 8 mm fisheye lens (D2), and an iPhone 6 Plus with an Olloclip fisheye converter (D3). The reference data was taken with D1, corrected for vignetting effect with a photometric sphere and reprojected to the standard equidistant projection with a developed by Delta-T Devices (). The root mean square error (RMSE) of the simple method were 0.55° and 0.021 relative radiance () for D1; 1.8° and 0.1 for D2; and 2.1° and 0.066 for D3. This level of standardization will help to simplify the classification of canopy gaps, a key processing step in HP. And, if it is broadly adopted, will facilitate comparisons between independent studies and large time series.

中文翻译：

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用于森林冠层半球摄影的鱼眼镜头的简单校准


冠层半球摄影 (HP) 是一种植被测量技术，通常采用面向消费者的设备。这些设备的变化是由技术进步和设计引起的。尽管这些变化有可能提供更可靠的测量，但也给测量带来了挑战。关于如何标准化设备输出的知识是可用的，缺少的是一种简单且廉价的方法来实现它。这是必要的，因为面向消费者的设备的使用创造了简单性和经济性的期望。为了填补这一方法上的空白，我们开发了一种易于组装、材料成本接近于零的校准板（简单板），并概述了如何使用它来开发特定于设备的功能：投影函数 () 和渐晕函数 () 。前者将视图天顶角 () 与距图像中心的距离联系起来，同时对称为渐晕效应的亮度径向下降进行建模。解释附有定制的 R 代码。总而言之，我们称之为“简单方法”。使用三台设备进行测试：配备 FC-E9 转换器的尼康 Coolpix 5700 (D1)、配备尼克尔 8 毫米鱼眼镜头的尼康 D610 (D2) 和配备 Olloclip 鱼眼转换器的 iPhone 6 Plus (D3)。参考数据使用 D1 获取，使用光度球校正渐晕效应，并使用 Delta-T Devices 开发的标准等距投影重新投影到标准等距投影 ()。简单方法的均方根误差(RMSE)为0.55°，D1的相对辐射亮度()为0.021； D2 为 1.8° 和 0.1； D3 为 2.1° 和 0.066。这种标准化水平将有助于简化顶篷间隙的分类，这是惠普的一个关键处理步骤。 而且，如果它被广泛采用，将有助于独立研究和大型时间序列之间的比较。