SCALGO Live Documentation

Notes on the elevation model for Sweden

Our elevation model of Sweden is based primarily on Lantmäteriet's GSD-Höjddata grid 2+ with a grid resolution of 2x2 meter. The data was acquired from Lantmäteriet in early 2017 and is based on airborne LIDAR scans in the years from 2009 to 2016. It covers almost all of Sweden, except for some remote areas in the mountains of north-west Sweden. 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 fulfil these conditions as well as possible.

Extensions

In order to cover the upstream areas of all rivers in Sweden, we have extended the GSD-Höjddata model in the following areas:

  • In the uncovered mountainous areas of north-west Sweden we use data from the 30-meter EU-DEM data set, which in turn is based on SRTM and ASTER GDEM data. Even though this data is not of very high quality, it's the best available and ensures that downstream watershed sizes are as accurate as possible. In the figure below, areas with data from the EU-DEM are yellow.
  • Even though the GSD-Höjddata covers small parts of Norway and Finland, it does not extend far enough to include the full watersheds of rivers in Sweden. Where necessary, we have therefore included data from the national Norwegian and Finnish elevation models, which are of much higher quality than the EU-DEM data.
    Where available, outside of the coverage of the GSD-Höjddata, we use the Finnish 2-meter model from Lantmäteriverket (light blue in the figure below). For the rest of Finland and Norway we use the respective national 10-meter models: Lantmäteriverket's Höjdmodell 10 m (dark blue) and Kartverket's DTM 10 (green), both are based on varying sources such as contour lines and peak elevations.
  • In the big lakes (Vänern, Vättern, Siljan, Storsjön and Kallsjön), areas not covered by the elevation model have been filled to the elevation of the surrounding data (which is already flattened). This operation simply ensures all upstream water flows through these lakes and into the sea.

A full overview of which data source is used for which part of the model is available by enabling the "Sweden/Sources" layer under the "Vector" section in the layer list in SCALGO Live. Use the point query tool to highlight a coverage area and see the name of the source.

Overview of sources used for the elevation model.

Bridges and underpasses

Major bridges have generally been removed from Lantmäteriet's model, 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.

Major bridges have been removed from the model.

Buildings

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 data set 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 "Sweden/Buildings" and is the basis for all nationwide hydrological computations.

The building footprint data set used is GSD-Fastighetskartan from early 2017. This layer can be shown and downloaded individually and is called "Sweden/Buildings" under the "Vector" section.

Flow paths routed around buildings.

Coastline

Since the elevation of grid cells in the sea around Sweden is not consistent in the source data, we have masked out these cells using the Nationell strandlinje data set included in GSD-Fastighetskartan. This data can be shown individually and is called "Sweden/Coastline" under the "Vector" section.

Göteborgs skärgård in SCALGO Live.