Abstract
Abstract. Recently, with increasing use of unmanned aerial vehicle (UAV), radiometric calibration of UAV images has become an important pre-processing step for application such as vegetation mapping, crop field monitoring, etc. In order to obtain accurate spectral reflectance, some UAVs measure irradiance at the time of image acquisition. However, most of UAV systems do not have such irradiance sensors. In these cases, vicarious radiometric correction method has to be used. Digital numbers (DNs) of imaged ground reflectance targets are measured and spectral reflectance is acquired from with known reflectance values of the targets. For automated vicarious calibration, a technique for automatically detecting image location of ground reflectance targets has been developed. In this study, we report an improved version of automated reflectance target detection and a new semi-automatic reflectance target detection developed. Test results showed that among the 14 reflectance targets, 13 targets were detected with the automatic target detection method. The undetected target was extracted by the proposed semi-automatic target detect method. Additional test was conducted on the remaining targets to confirm the applicability of our semi-automatic target detection method. As a result, other targets were also detected. The proposed automated and semi-automated target detection method can be used for automated vicarious calibration of UAV images.
Highlights
unmanned aerial vehicle (UAV) images are being used with improved aircraft and sensors in various fields
Compact-sized multispectral image sensor with an irradiance sensor is mounted in a UAV to obtain spectral information (Mathews, 2015)
If more targets are used for reflectance conversion, more time is spent for setting target areas
Summary
UAV images are being used with improved aircraft and sensors in various fields. Users can acquire images at any location and time. In order to obtain accurate spectral reflectance, some UAVs measure irradiance at the time of image acquisition. Most of UAV systems do not have such irradiance sensors and do not provide radiometric correction coefficients. In these cases, vicarious radiometric correction method has to be used. If more targets are used for reflectance conversion, more time is spent for setting target areas To remove this manual process, a technique for automatically detecting image location of ground reflectance targets has been developed (Shin et al 2019). UAV images were acquired by a RedEdge-MicaSense camera with an irradiance sensor (Figure 2a).
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