Data from: Rapid assessment of forest canopy and light regime using smartphone hemispherical photography

Hemispherical photography (HP), implemented with cameras equipped with “fisheye” lenses, is a widely used method for describing forest canopies and light regimes. A promising technological advance is the availability of low-cost fisheye lenses for smartphone cameras. However, smartphone camera sensors cannot record a full hemisphere. We investigate whether smartphone HP is a cheaper and faster but still adequate operational alternative to traditional cameras for describing forest canopies and light regimes. We collected hemispherical pictures with both smartphone and traditional cameras in 223 forest sample points, across different overstory species and canopy densities. The smartphone image acquisition followed a faster and simpler protocol than that for the traditional camera. We automatically thresholded all images. We processed the traditional camera images for Canopy Openness (CO) and Site Factor estimation. For smartphone images, we took two pictures with different orientations per point and used two processing protocols: (i) we estimated and averaged total canopy gap from the two single pictures, and (ii) merging the two pictures together, we formed images closer to full hemispheres and estimated from them CO and Site Factors. We compared the same parameters obtained from different cameras and estimated generalized linear mixed models (GLMMs) between them. Total canopy gap estimated from the first processing protocol for smartphone pictures was on average significantly higher than CO estimated from traditional camera images, although with a consistent bias. Canopy Openness and Site Factors estimated from merged smartphone pictures of the second processing protocol were on average significantly higher than those from traditional cameras images, although with relatively little absolute differences and scatter. Smartphone HP is an acceptable alternative to HP using traditional cameras, providing similar results with a faster and cheaper methodology. Smartphone outputs can be directly used as they are for ecological studies, or converted with specific models for a better comparison to traditional cameras.半球摄影法（Hemispherical Photography, HP）是采用配备鱼眼镜头（fisheye lenses）的相机开展的技术手段，目前已成为描述森林冠层与光照状况的常用方法。近年来一项颇具前景的技术进展，便是面向智能手机相机的低成本鱼眼镜头问世。然而智能手机相机的传感器无法记录完整的半球视野。本研究旨在探究智能手机半球摄影法是否可作为传统相机的替代方案：既具备更低成本与更高效率，同时仍能满足森林冠层与光照状况描述的操作需求。 我们在223个森林样点中，分别使用智能手机相机与传统相机采集半球影像，样点涵盖不同上层林木物种与冠层密度。相较于传统相机的图像采集流程，智能手机的图像采集流程更为简便快捷。我们对所有影像均进行了自动阈值分割处理。针对传统相机影像，我们开展了冠层开度（Canopy Openness, CO）与立地因子的估算。针对智能手机影像，我们在每个样点拍摄两张不同朝向的照片，并采用两种处理流程：其一，分别估算两张单张照片的总冠层间隙后取平均值；其二，将两张照片拼接为更接近完整半球视野的影像，再据此估算冠层开度与立地因子。我们对比了不同相机获取的同类参数，并针对二者构建了广义线性混合模型（Generalized Linear Mixed Models, GLMMs）。 第一种处理流程下，智能手机影像估算得到的总冠层间隙，其平均值显著高于传统相机影像估算得到的冠层开度，尽管二者存在一致的系统偏差。第二种处理流程下，拼接智能手机影像估算得到的冠层开度与立地因子，其平均值同样显著高于传统相机影像的结果，但二者的绝对差值与离散程度均相对较小。 智能手机半球摄影法可作为传统相机半球摄影法的可行替代方案，其采集处理流程兼具高效性与经济性，且所得结果具有可比性。智能手机的输出结果可直接应用于生态学研究，或通过特定模型进行转换，以实现与传统相机结果的更优对比。