Vegetation of the Gwydir Wetlands 2022

This is a vegetation map of the Gwydir wetlands. It was produced using air photo interpretation from high resolution aerial imagery collected in August 2022 and January 2023.

Map development began with the collection of high-resolution aerial colour (Red-Green-Blue) imagery. The imagery was provided as an orthographic mosaic (ie a straight down view) with a 40 cm ground sampling distance covering the whole study area at each wetland. This formed the primary input of information for vegetation extent mapping. This aerial imagery was acquired in August 2022 for the Gwydir Wetlands. In addition, 15 cm high-resolution colour imagery, collected in January 2023, was also sourced from another project and provided as an orthomosaic. This additional imagery helped inform the aerial interpretation of vegetation community extents for an eastern portion of the Gwydir Wetlands study area. Several interpreters were then trained in Aerial Photographic Interpretation (API) to visually analyse the imagery to identify and delineate different vegetation types. This was done based on their spectral characteristics, colour, texture, shape, spatial patterns and associations with predictive environmental layers (such as flood frequency categories, elevation and geomorphology type). Existing survey data was also used to help identify vegetation types from imagery. This included BioNet species data, floristic data and other grey literature. Oblique aerial handheld photos captured from a helicopter were also sourced from another project to inform the aerial imagery interpretation. A subset of the available oblique handheld photos was selected to correspond to the timing (within two years) of the 40cm aerial imagery acquired for vegetation map development. The subset of oblique handheld photos adopted to inform the air photo interpretation included photos collected between January-December 2022. A polygon layer divided into small regions was sourced to overlay on the 40cm aerial imagery. This spatial layer was produced using the Definiens eCognition software package. A computer-based image analysis tool known as segmentation was applied to a set of raster datasets with a 5m grid cell size. This produced a spatial layer of ‘segments’ or very small polygons based on the combined spectral and textural features of the input rasters (Roff et al., 2022). The segmented layer was overlayed on the 40cm aerial imagery. Interpreters then manually selected groups of segments and assigned classes (‘attributes’) to the polygons to delineate vegetation patterns. The use of the segmented spatial layer enabled more efficient mapping, as interpreters did not have to manually draw polygon linework with a mouse.
Vegetation patterns were interpreted from the high-resolution 40cm aerial imagery at a scale of 1:25 000 for non-flood dependent vegetation and at a scale of 1:10 000 for wetland communities. The minimum map unit (smallest polygon) was 2 ha. Selected polygons from the segmentation process were initially assigned to an artificial class referred to as a Vegetation Photo Pattern (VPP), analogous to NSW Vegetation Classes (for more information on NSW Vegetation Classes see https://www.environment.nsw.gov.au/topics/animals-and-plants/biodiversity/nsw-bionet/the-nsw-vegetation-classification-framework ). The VVPs were aligned with plant community types (PCTs) as described in the NSW BioNet Vegetation Classification Database (see https://vegetation.bionet.nsw.gov.au/). Each PCT was also aligned to a vegetation functional group corresponding to the vegetation objectives in the Gwydir Wetlands and Macquarie Marshes LTWPs. The accuracy of the map vegetation functional groups was assessed using 780 independently collected field validation points. The overall accuracy was 0.77 and the Kappa statistic was 0.7. Accuracies and 95% confidence intervals for map individual map classes were: Non woody wetland: 0.78 (0.73-0.87) Flood dependent woodland 0.81 (0.76-0.86) River red gum forest: 0.77 (0.68-0.86) River red gum woodland: no field data, not assessed. Terrestrial vegetation: 0.68 (0.62-0.75) Non-native or other (includes pasture, cropping, infrastructure, dams): 0.89 (0.81-0.98)

This mapping project was funded by the NSW Water for the Environment Program and the Gwydir Reconnecting Watercourse Country Program.

This mapping project was funded by the NSW Water for the Environment Program, and the Gwydir Reconnecting Watercourse Country Program.