Abstract
Satellite data and aerial photos have proved to be useful in efficient conservation and management of mangrove ecosystems. However, there have been only very few attempts to demonstrate the ability of drone images, and none so far to observe vegetation (species-level) mapping. The present study compares the utility of drone images (DJI-Phantom-2 with SJ4000 RGB and IR cameras, spatial resolution: 5cm) and satellite images (Pleiades-1B, spatial resolution: 50cm) for mangrove mapping—specifically in terms of image quality, efficiency and classification accuracy, at the Setiu Wetland in Malaysia. Both object- and pixel-based classification approaches were tested (QGIS v.2.12.3 with Orfeo Toolbox). The object-based classification (using a manual rule-set algorithm) of drone imagery with dominant land-cover features (i.e. water, land, Avicennia alba, Nypa fruticans, Rhizophora apiculata and Casuarina equisetifolia) provided the highest accuracy (overall accuracy (OA): 94.0±0.5% and specific producer accuracy (SPA): 97.0±9.3%) as compared to the Pleiades imagery (OA: 72.2±2.7% and SPA: 51.9±22.7%). In addition, the pixel-based classification (using a maximum likelihood algorithm) of drone imagery provided better accuracy (OA: 90.0±1.9% and SPA: 87.2±5.1%) compared to the Pleiades (OA: 82.8±3.5% and SPA: 80.4±14.3%). Nevertheless, the drone provided higher temporal resolution images, even on cloudy days, an exceptional benefit when working in a humid tropical climate. In terms of the user-costs, drone costs are much higher, but this becomes advantageous over satellite data for long-term monitoring of a small area. Due to the large data size of the drone imagery, its processing time was about ten times greater than that of the satellite image, and varied according to the various image processing techniques employed (in pixel-based classification, drone >50 hours, Pleiades <5 hours), constituting the main disadvantage of UAV remote sensing. However, the mangrove mapping based on the drone aerial photos provided unprecedented results for Setiu, and was proven to be a viable alternative to satellite-based monitoring/management of these ecosystems. The improvements of drone technology will help to make drone use even more competitive in the future.
Highlights
With their luxuriant growth in tropical and subtropical latitudes along the land-sea interface, bays, estuaries, lagoons and backwaters [1], mangroves provide important ecosystem services, ranging from coastal protection to fisheries production, ecotourism, phytoremediation, carbon sequestration, and other services [2,3,4]
Regular monitoring/mapping of the mangroves based on remote sensing can provide authentic information that is needed for better management [19,20]
The capability of Pleiades-1B and DJI-Phantom-2 was evaluated through their image quality in terms of spatial, spectral, radiometric and temporal resolution; efficiency in terms of area coverage, data acquisition/processing time and user-cost, and accuracy in terms of object- and pixel-based classification approaches
Summary
With their luxuriant growth in tropical and subtropical latitudes along the land-sea interface, bays, estuaries, lagoons and backwaters [1], mangroves provide important ecosystem services, ranging from coastal protection to fisheries production, ecotourism, phytoremediation, carbon sequestration, and other services [2,3,4]. Regular monitoring/mapping of the mangroves based on remote sensing can provide authentic information (along with spatial-temporal dynamics) that is needed for better management [19,20]. Past and present vegetation maps are useful to reconstruct past events and predict future development scenarios [21,22,23]. To date, both satellite images and airplane-based aerial photographs have been used for mangrove mapping [17,19,21,23,24,25,26]. Availability of cloud-free satellite images for mangrove mapping is a known difficulty [15,33]
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