Country Specific – France

Our elevation model of France is based primarily on the Digital Terrain Model (fr. Modèle Numérique de Terrain) named RGE ALTI®, made available by the IGN (Institut national de l'information géographique et forestière). 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 France including upstream areas of all rivers, we have extended the DTM from the government of France with the 30-meter EU-DEM data set, which in turn is based on SRTM and ASTER GDEM data.

A full overview of when the elevation data in France was acquired is available by going to the Library and enabling the "Acquisition date" layer, found in the Elevation category.

Bridges, underpasses and hydrological corrections

Major bridges have generally been removed from the model, but for many smaller bridges and underpasses, additional hydrological corrections that allow water to flow through such structures may be necessary. SCALGO Live France includes a nationwide hydrological correction set based on layers from the BD TOPO® dataset, made available by the IGN. Corrections have been generated at locations where rivers intersect roads or railroads, as well as at river sections that overlap with sluices (écluses). Secondly, corrections have been generated at road overpasses and tunnels. Each correction thus follows a line in the BD TOPO® river or road network, with end points adjusted to match the elevation model as well as possible. In places where the elevation model is already hydrologically corrected (e.g. at large bridges), corrections are not generated.

This data set is machine-generated, so some errors should be expected. However, since we only include corrections along known river and road lines, we believe it to be conservative in terms of water flow.

The set of corrections is available under the Hydrological Corrections category in the Library.

The national analyses use these corrections, and workspaces created using the predefined "Flash Flood Map" or "Sea-Level Rise" buttons also include them by default. If you create a workspace through any other means than the predefined buttons (e.g. if you upload your own model), you can include corrections in that workspace through the workspace Actions tab by clicking Import corrections, they will not be included automatically.

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 BD TOPO® Batîment dataset.

Base map and aerial photography

The default map view when you go to SCALGO Live shows an overview map from IGN. The street names and place labels are sourced from OpenStreetMap.

You also have the option of viewing an aerial photography map from IGN.