Abstract
Plant species that negatively affect their environment by encroachment require constant management and monitoring through field surveys. Drones have been suggested to support field surveyors allowing more accurate mapping with just-in-time aerial imagery. Furthermore, object-based image analysis tools could increase the accuracy of species maps. However, only few studies compare species distribution maps resulting from traditional field surveys and object-based image analysis using drone imagery. We acquired drone imagery for a saltmarsh area (18 ha) on the Hallig Nordstrandischmoor (Germany) with patches of Elymus athericus, a tall grass which encroaches higher parts of saltmarshes. A field survey was conducted afterwards using the drone orthoimagery as a baseline. We used object-based image analysis (OBIA) to segment CIR imagery into polygons which were classified into eight land cover classes. Finally, we compared polygons of the field-based and OBIA-based maps visually and for location, area, and overlap before and after post-processing. OBIA-based classification yielded good results (kappa = 0.937) and agreed in general with the field-based maps (field = 6.29 ha, drone = 6.22 ha with E. athericus dominance). Post-processing revealed 0.31 ha of misclassified polygons, which were often related to water runnels or shadows, leaving 5.91 ha of E. athericus cover. Overlap of both polygon maps was only 70% resulting from many small patches identified where E. athericus was absent. In sum, drones can greatly support field surveys in monitoring of plant species by allowing for accurate species maps and just-in-time captured very-high-resolution imagery.
Highlights
Drones, or UAS, are becoming a valuable tool for mapping problematic or endangered plant species (Alvarez-Taboada et al 2017; Rominger and Meyer 2019)
We successfully conducted a UAS flight campaign in a saltmarsh ecosystem, classified the area covered by the tall grass Elymus athericus and compared our results to the distribution of E. athericus mapped by a field survey in the study area
We found that field-based maps of E. athericus patches were in good spatial agreement with the object-based image analysis (OBIA)-based maps, showing the same amount of area covered by Elymus
Summary
UAS (unmanned aerial systems), are becoming a valuable tool for mapping problematic or endangered plant species (Alvarez-Taboada et al 2017; Rominger and Meyer 2019). UAS are cost friendly and allow flexible application in space and time, and very-high-resolution (VHR) image data can be acquired within 1 h by a single person, e.g., before or during a field survey. Precise capture of relatively large areas, e.g., up to 400 ha, can be done in one single flight (Samiappan et al 2017a). Further limitations are often high geometric errors and high data volumes as the low flying UAS can provide VHR imagery with ground sampling distances of a few centimeters. Just-in-time VHR image data can support conservation and natural resource management, e.g., by mapping plant individuals at the scale of a field survey
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