Unmanned aircraft systems help to map aquatic vegetation

Abstract

Questions: Do high-resolution (sub-decimetre) aerial images taken with unmanned aircraft systems (UASs) allow a human interpreter to recognize aquatic plant species? Can UAS images be used to (1) produce vegetation maps at the species level; and (2) estimate species abundance? Location: One river and two lake test sites in northern Sweden, middle boreal sub-zone. Methods: At one lake and at the river site we evaluated accuracy with which aquatic plant species can be identified on printouts of UAS images (scale 1:800, resolution 5.6 cm). As assessment units we used homogeneous vegetation patches, referred to as vegetation stands of one or more species. The accuracy assessment included calibration and validation based on field controls. At the river site, we produced a digital vegetation map based on an UAS orthoimage (geometrically corrected image mosaic) and the results of the species identification evaluation. We applied visual image interpretation and manual mapping. At one of the lake sites, we assessed the abundance (four-grade scale) of the dominating Phragmites australis and produced a cover map. Results: We identified the species composition of vegetation stands at the lake and the river site with an overall accuracy of 95.1% and 80.4%, respectively. It was feasible to produce a digital vegetation map, albeit with a slight reduction in detail compared to the species identification step. At the site for abundance assessment, P. australis covered 20% of the total lake surface area, and 70% of the covered area had cover ≤25%. Conclusions: The tested UAS facilitates lake and river vegetation identification and mapping at the species level, as well as abundance estimates. Unmanned aircraft systems are convenient providers of sub-decimetre-resolution aerial images. We found that such images allow for the identification and mapping of non-submerged aquatic and riparian vegetation at the species level as well as for abundance estimates. © 2013.