Country Specific – United States

The elevation model for the United States of America is based on 3 products from the United States Geological Survey (USGS) 3D Elevation Progam (3DEP): The 1 meter, the ⅓ arcsecond, and the 1 arcsecond DEMs. These are combined such that the 1 meter DEM is preferred, with the ⅓ arcsecond DEM as a fallback and the 1 arcsecond DEM as a further fallback. The 1 meter product itself consists of multiple overlapping projects. We choose elevation data from the most recent tile for the cells that exist in multiple 1 meter projects.

The horizontal datum used is the North American Datum of 1983 (NAD83, EPSG: 4269) and the elevation model is split into UTM zones 1N to 22N and uses the corresponding UTM coordinate system in each zone (EPSG: 26901 to 26922). This matches the datum and projections of the 3DEP 1 meter product. We reproject the ⅓ arcsecond and 1 arcsecond DEMs to these zones and use a cell size of 8 meter and 16 meter, respectively.

The vertical datum varies, with the North American Vertical Datum of 1988 (NAVD88, EPSG: 5103) being used in the conterminous United States. For Hawaii, Puerto Rico, and the U.S. Virgin Islands the vertical datum is referenced to local mean sea level. For USGS-derived DEMs in other areas, e.g. parts of Canada and Mexico, the National Geodetic Vertical Datum of 1929 (NGVD29, EPSG: 5102) and local reference datums are used. This follows how the USGS uses vertical datums.

Workspaces in the US use a resolution of 1 meter, upsampling the national elevation model if needed. If you upload your own elevations, you may end up with workspaces with a different resolution.

Extensions

In order to cover the upstream areas of all rivers and lakes, especially the Great Lakes, we have extended the model in the following area(s):

• Canada:  We use the 1 meter and 2 meter elevation models from the High Resolution Digital Elevation Model (HRDEM) managed by the Natural Resources Canada (NRCan). These Canadian elevation models use the Canadian Geodetic Vertical Datum of 2013 (CGVD2013, EPSG: 6647).

In Canadian and Mexican areas not covered by these extension elevation models, we fall back to the 3DEP ⅓ arcsecond and 1 arcsecond DEMs.

Both the 3DEP and the HRDEM elevation models are updated regularly, and we strive to keep our models up to date with the latest sources.

In summary, the following table list all elevation model used, ordered by which model is chosen for the combined elevation model in the cases where they overlap.

A full overview of which data source is used for which part of the model is available by clicking the gear icon next to an elevation layer, selecting the "Source" tab, and "Show source information". Use point query to see more details for individual areas. Multiple styles are available for this layer to colour sources by e.g. date and resolution.

Buildings

Buildings are not included in the terrain models, since they represent the "bare earth" elevations. 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 "Terrain/Buildings" and is the basis for all nationwide hydrological computations.

The building footprint data set used is a combination of the following local sources as well as the Microsoft Building Footprints, which is used as a fallback.