Country Specific – Great Britain

Our elevation model of Great Britain is based primarily on open LIDAR composite DTM models made available by the Environment Agency, Natural Resources Wales, and the Scottish Environment Protection Agency. The Environment Agency National LIDAR Programme started in November 2016 and aims to have coverage of all of England by 2021. We strive to keep our model up to date with the latest sources.

In order to use an elevation model for hydrological analysis such as watershed and flow accumulation computations, two primary conditions need to be met:

• The upstream area of any river should be covered by the elevation model.
• Structures on top of the terrain should only be present in case they actually block water from flowing under or through them.

Below, we discuss how we process the model to fulfill these conditions as well as possible.

Extensions

In order to cover all of England, Wales and Scotland, we have extended the open LIDAR data from the UK government in the following areas:

• For the parts of England not yet covered by the National LIDAR Programme, we have used the older Environment Agency 1m DTM, and for parts not covered by the 1m DTM we have used the 2m DTM.
• For the parts of Wales not covered by the Wales 1m DTM, we have used the Wales 2m DTM.
• For the parts of Scotland not covered by the LiDAR for Scotland Phase 4 DTM, we have used the Phase 3, Phase 2 or Phase 1 DTM, as well as Environment Agency's "LIDAR Composite DTM for Scotland".
• For the parts not covered by open LIDAR datasets mentioned above, we use the 30-meter EU-DEM dataset, which in turn is based on SRTM and ASTER GDEM data.

A full overview of which data source is used for which part of the model is available clicking the gear icon next to an elevation layer, selecting the "Source" tab, and "Show source information". Use the point query tool to highlight a coverage area and see the name of the source.

Bridges and underpasses

Major bridges have generally been removed from the models delivered by the Environment Agency, making most bigger rivers able to flow freely to the sea. However, for many smaller bridges and underpasses, additional "hydrological corrections" may be necessary. These can e.g. be made through workspaces in SCALGO Live.

Apart from vegetation and major bridges, also buildings have been removed from the terrain model during construction. When computing water flow paths, more realistic results are generally obtained when the elevation model does include buildings so water can be simulated to flow around them. In SCALGO Live, we accomplish this by adding buildings back into the model using a dataset of building footprints, where we raise all grid cells covered by a building to a height 10 meters above the highest terrain point within the building footprint. This model is called "Terrain/Buildings" and is the basis for all nationwide hydrological computations.

The building footprints are taken from the Ordnance Survey OpenMapLocal Building dataset.

Rivers

Major rivers have been lowered in the elevation model to ensure that water can flow freely to the coast. The river lines are taken from the Ordnance Survey Open Rivers dataset.

Coastline

We have removed parts of the LIDAR models that are off the coast, as the data is generally unreliable. The coastline is taken from the Ordnance Survey Open ZoomStack vector dataset, layer named "Land".

Base map and aerial photography

The default map view when you go to SCALGO Live shows the Ordnance Survey Open ZoomStack map, rendered in the "Outdoor" style. Other map styles are available through the gear menu on the "Base Map" layer. The street names and place labels are sourced from OpenStreetMap. You also have the option of viewing Aerial photography provided by Mapbox.