Study area: The Zwin is a nature reserve situated along the Belgian North Sea coast, northeast of Knokke, in the province of West-Flanders, Flanders, Belgium. The area is managed by the Flemish Agency for Nature and Forest and consists of a tidal marsh, coastal dunes with Ammophila arenaria, dune grasslands and/or shrub (Hippophae rhamnoides, Salix repens), and a transitional grassland zone that stretches from the inner edge of the coastal dunes into the polders.

Data processing: The raw data were processed to Digital Surface Models and orthophotos by the Flemish Institute for Technological Research (VITO) in 2017. Images with coarse GPS coordinates were imported and processed in Agisoft PhotoScan Pro 1.4.x, a structure-from-motion (SfM) based photogrammetry software program. After extraction and matching of tie points, a bundle adjustment leads to a sparse point cloud and a refined set of camera position and orientation values. Ground control points (either artificially installed markers on the terrain, or other photo-identifiable points, measured on the ground with RTK GNSS) were used to further refine the camera calibration and obtain a pixel-level georeferencing accuracy. From there, a point cloud densification and classification into ground and non-ground points was performed, leading to a rasterized digital surface model (DSM) and digital terrain model (DTM). Finally, a true orthomosaic was projected onto the DTM.

Coordinate reference system: All geospatial data have the coordinate reference system EPSG:31370 - Belgian Lambert 72.

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