SPECTRAL REFLECTANCE CHARACTERISTICS OF MANGROVE FORESTS IN MULTISPECTRAL UNMANNED AERIAL VEHICLE IMAGERY IN CA MAU PROVINCE, VIETNAM
DOI:
https://doi.org/10.70169/VJFS.1326Keywords:
UAV multispectral imagery, vegetation indices, flight altitude, radiometric calibration, mangrove forestAbstract
Flight altitude and radiometric calibration strongly influence the usability of multispectral imagery from unmanned aerial vehicles (UAVs) for mangrove forest monitoring. This study quantifies both effects for two different forest types, planted and natural mangrove forests, through three components: spectral reflectance characteristics, the role of radiometric calibration, and the effect of flight altitude. Images were collected with a DJI Mavic 3 Multispectral over a 66-ha plantation and a 71-ha natural mangrove stand in Ca Mau province during two time periods in May and November 2025, at flight altitudes of 100 m and 150 m, and processed in Pix4D Mapper with and without a reflectance panel. Reflectance of the green, red, red-edge and near-infrared (NIR) bands was highly stable within each flight at both altitudes, and the NIR and red-edge bands separated mangrove canopy from water most clearly. Classification accuracy of four surface classes reached 90.0% at 100 m and 96.0% at 150 m. Without the calibration panel, reflectance over forest canopy decreased by a factor from 11.2 to 18.5, making absolute values unusable. However, nine of the sixteen vegetation indices, all purely ratio-based (containing no additive constant), maintained correlation coefficients of 0.8 or higher with the calibrated dataset at 150 m, and six did so at 100 m. In terms of acquisition efficiency, flying at 100 m required 1.75 - 1.97 times the resources of 150 m (image count, flight time, processing time, and storage), with no corresponding gain in data quality under the illumination conditions of this study. Within the scope of the flights surveyed, the study suggests flying at 150 m and using a calibration panel whenever absolute reflectance is required; where panel deployment is impossible, the screened indices remain usable for relative monitoring.
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