The landscape team at our partner federation, Groundwork North East, asked us to provide them with elevation data for use in modelling flood risk along the catchment of the River Team. In the past, the only data available at a suitable resolution was too costly to purchase. However, since 2015 the Environment Agency have released their LiDAR (Light Detection And Ranging) data under an Open Government License. This covers over 70% of England and Wales at a resolution of 2m or greater.
The LiDAR is supplied as either a DSM (Digital Surface Model) or a DTM (Digital Terrain Model). For flood modelling, a DTM is most suitable as this covers the bare earth i.e excludes buildings and vegetation. To download the data you need to use the web portal located here. The LiDAR is supplied in 5km2 chunks in the ASCII file format. Use the composite DTM rather than the time stamped tiles. To work with the data in ArcGIS we need to transform it into a raster using the ASCII to Raster tool which can be found in the Arc Toolbox under Conversion -> To Raster -> ASCII to Raster. Be sure to change the ‘output data type’ from integer to float.
Once you have converted all your ASCII files to Raster you will need to merge the results into one final raster. This is achieved through the Mosaic to New Raster tool found under Data Management -> Raster -> Raster Dataset -> Mosaic to New Raster. Use the following parameters:
Input Location: Select the rasters to merge
Output Location: The folder where you want to save the output raster
Spatial Reference: British_National_Grid
Number of bands: Always one
Other inputs: leave as default
You can now visualise your DTM in ArcScene which gives a 3D overview of the data. This can be accessed from the 3D Analyst Toolbar in ArcMap. Add the layer we have just created into the Table of Contents, by default no height values are shown. To add in the 3D or Z component go to Layer Properties -> Base Heights -> Floating on a custom surface. Use the Z factor 'Factor to convert layer elevation values to scene units’ to show a greater delineation of the elevation - the higher the value the greater the height exaggeration.