Radiometric Calibration of "Commercial off the Shelf' Cameras for UAV-Based High-Resolution Temporal Crop Phenotyping of Reflectance and NDVI
Radiometric Calibration of "Commercial off the Shelf' Cameras for UAV-Based High-Resolution Temporal Crop Phenotyping of Reflectance and NDVI
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DOI：
10.3390/rs11141657
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发表时间：
2019-07-02
期刊：
影响因子：
5
通讯作者：
Hawkesford, Malcolm J.
中科院分区：
文献类型：
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作者：
Holman, Fenner H.;Riche, Andrew B.;Hawkesford, Malcolm J.
植被指数，如归一化差异植被指数(NDVI)，是在作物表型鉴定中用来衡量感兴趣的性状的常见指标。然而，这些指标的传统测量往往受到多种混杂因素的影响，如冠层盖度和林隙中可见的下垫面土壤的反射率。安装在无人机上的数码相机提供了调查这些混杂因素的空间分辨率，然而，不完全的反射单位辐射校准方法限制了数据如何应用于表型分析。在这项研究中，我们评估了用商用现成(COTS)数码相机拍摄的极高空间分辨率(1 Cm)无人机图像在获取校准后的反射率图像和将植被冠层反射率与下层土壤的反射率分离方面的适用性。我们提出了新的方法，成功地归一化曝光和太阳辐射效应的COTS相机图像，生成我们的目标田间小麦作物试验的多光谱(RGB-NIR)正射影像。对地面光谱仪测量的验证表明，反射率(R-2>；=0.6)和归一化植被指数(R-2>；=0.88)都得到了很好的结果。应用整个生长季收集的图像，并使用超额绿红指数进行掩蔽，以评估树冠复盖对NDVI测量的影响。结果表明，在冠层发育较低的早期，冠层覆盖人为降低了小区的NDVI值。
Vegetation indices, such as the Normalised Difference Vegetation Index (NDVI), are common metrics used for measuring traits of interest in crop phenotyping. However, traditional measurements of these indices are often influenced by multiple confounding factors such as canopy cover and reflectance of underlying soil, visible in canopy gaps. Digital cameras mounted to Unmanned Aerial Vehicles offer the spatial resolution to investigate these confounding factors, however incomplete methods for radiometric calibration into reflectance units limits how the data can be applied to phenotyping. In this study, we assess the applicability of very high spatial resolution (1 cm) UAV-based imagery taken with commercial off the shelf (COTS) digital cameras for both deriving calibrated reflectance imagery, and isolating vegetation canopy reflectance from that of the underlying soil. We present new methods for successfully normalising COTS camera imagery for exposure and solar irradiance effects, generating multispectral (RGB-NIR) orthomosaics of our target field-based wheat crop trial. Validation against measurements from a ground spectrometer showed good results for reflectance (R-2 >= 0.6) and NDVI (R-2 >= 0.88). Application of imagery collected through the growing season and masked using the Excess Green Red index was used to assess the impact of canopy cover on NDVI measurements. Results showed the impact of canopy cover artificially reducing plot NDVI values in the early season, where canopy development is low.