Digital Elevation Model (DEM) data for the Conwy estuary, Wales

a) Seabed bathymetry for the Menai Strait, Conwy Bay and the mouth of the Conwy estuary extending upstream to the Conwy Bridge were obtained from marine Digital Elevation Models (DEMs) distributed by OceanWise and made available through Digimap. These marine DEMs consist of bed elevations measured with single- and multibeam sonar sensors and converted into raster tiles of 1 arc second horizontal resolution. They are referenced to the WGS 1984 coordinate system and the recorded bathymetry is referenced to the local Chart Datum at Llandudno.

b) The majority of land elevations for this catchment (73%) have been surveyed by Natural Resources Wales (NRW) using Light Detection and Ranging (Lidar) technology through a series of aerial surveys spanning multiple years (2016-2020). The composite-derived Digital Terrain and Surface Models (DTMs and DSMs, respectively) were obtained from Digimap. The raster tiles have 1 m resolution, are projected to the British National Grid and the elevations are referenced to the national Ordnance Survey vertical datum at Newlyn. The difference between DTMs and DSMs is that the former represents earth terrain and as such has all ‘above-surface’ features (e.g. vegetation, human-made structures) removed, which are included in the latter.

Elevation data for the remaining land areas not covered by the Lidar-based composites (27%) were extracted from the Ordnance Survey Terrain 5 m DTM also available through Digimap. This lower-resolution dataset has national coverage and consists of elevation data obtained from multiple sources including airborne Lidar, photogrammetry and field surveys. The data are provided as raster tiles of 5 m resolution projected to the British National Grid with elevations referenced to the national Ordnance Survey vertical datum at Newlyn.

c) Locations and heights of flood defences were obtained from the NRW data catalogue. These data consist of vector shapefiles projected to the British National Grid with polylines detailing the location of flood defences including information on the type of defence (e.g. wall, embankment) and the height of the crest above the terrain in metres.

All spatial data outlined above were imported into ArcGIS Pro 3.0 and processed to generate the DEM. Individual raster files pertaining to the various datasets outlined above were mosaiced into a single raster tile and resampled to 5 m resolution using a bilinear interpolation method.