Data Sources

Mapzen Terrain Tiles are powered by several major open data sets and we owe a tremendous debt of gratitude to the individuals and communities which produced them.

Attribution is required for some of our data providers. See the Attribution document for more information.

List of sources

The underlying data sources are a mix of:

  • 3DEP (formerly NED) in the United States, 10 meters outside of Alaska, 3 meter in select land and territorial water areas
  • SRTM globally except high latitudes, 30 meters in land areas
  • GMTED globally, coarser resolutions at 7.5”, 15”, and 30” in land areas
  • ETOPO1 to fill in ocean bathymetry at 1’

What is the ground resolution?

Ground resolution per tile pixel varies per zoom level, the given pixel cell’s latitude, and input data source.

This formula generates the following table:

ground_resolution = (cos(latitude * pi/180) * 2 * pi * 6378137 meters) / (256 * 2^zoom_level pixels)

Ground resolution per zoom in meters at a given latitude:

zoom 45° 60°
0 156543.0 110692.6 78271.5
1 78271.5 55346.3 39135.8
2 39135.8 27673.2 19567.9
3 19567.9 13836.6 9783.9
4 9783.9 6918.3 4892.0
5 4892.0 3459.1 2446.0
6 2446.0 1729.6 1223.0
7 1223.0 864.8 611.5
8 611.5 432.4 305.7
9 305.7 216.2 152.9
10 152.9 108.1 76.4
11 76.4 54.0 38.2
12 38.2 27.0 19.1
13 19.1 13.5 9.6
14 9.6 6.8 4.8
15 4.8 3.4 2.4

Note: Esri has documentation about converting web map zoom integers to conventional map scales.

What is sourced at what zooms?

Generally speaking, GMTED is used at low-zooms, and ETOPO1 is used to show ocean bathymetry at all zooms (even bathymetry oversampled at zoom 15), and SRTM is relied on in mid- and high-zooms on land. In the United States, data from USGS supplements the SRTM land data to provide greater detail at 10 meters and in some areas at up to 3 meters.

It should be noted that both SRTM and GMTED fill oceans and other bodies of water with a value of zero to indicate mean sea level; in these areas, ETOPO1 provides bathymetry (as well as in regions which are not covered by SRTM and GMTED).

Data sources per zoom:

zoom ocean land
0 ETOPO1 GMTED
1 ETOPO1 GMTED
2 ETOPO1 GMTED
3 ETOPO1 GMTED
4 ETOPO1 GMTED
5 ETOPO1 GMTED
6 ETOPO1 GMTED
7 ETOPO1 GMTED
8 ETOPO1 GMTED
9 ETOPO1 SRTM with GMTED in high latitudes above 60°
10 ETOPO1 SRTM with GMTED in high latitudes above 60°
11 ETOPO1 SRTM with GMTED in high latitudes above 60°
12 ETOPO1 SRTM with GMTED and NED/3DEP in USA (10 meter)
13 ETOPO1 SRTM with GMTED and NED/3DEP in USA (10 meter)
14 ETOPO1 SRTM with GMTED and NED/3DEP in USA (mostly 10 meter, some 3 meter)
15 ETOPO1 SRTM with GMTED and NED/3DEP in USA (mostly 10 meter, some 3 meter)

Sources native resolution

You might be wondering why we source from different data sets at different zooms. Besides bandwidth reasons, it’s helpful to know the native resolution of each data set which is expressed as a nominal arc resolution which maps roughly to a web map zoom for “best displayed at”.

In more practical terms, this results in some datasets being “oversampled” for a given zoom and map location.

  • In the water, most bathymetry values at zoom 15 for a pixel that has a ground resolution of 5 meter will actually be showing an oversampled zoom 6 ETOPO1 value (nominally 2.5 km).

  • On the land, most elevation values at zoom 15 for a pixel that has a ground resolution of 5 meter will actually be showing an oversampled zoom 12 SRTM value (nominally 30 meters).

This formula generates the following table:

ground_resolution = (cos(latitude * pi/180) * 2 * pi * 6378137 meters) / (256 * 2^zoom_level pixels) / 30.87 meters per arc second

Arc resolution per zoom and data sources, per pixel:

zoom meters at equator arc seconds nominal arc degrees minutes seconds source nominal ground units
0 156543.0 5071.0 1.5 arc degrees
1 78271.5 2535.5 40 arc minutes
2 39135.8 1267.8 20 arc minutes
3 19567.9 633.9 10 arc minutes
4 9783.9 316.9 5 arc minutes
5 4892.0 158.5 2.5 arc minutes
6 2446.0 79.2 1 arc minutes ETOPO1 2.5 km
7 1223.0 39.6 30 arc seconds GMTED2010 1km (not used)
8 611.5 19.8 15 arc seconds GMTED2010 500m (not used)
9 305.7 9.9 7.5 arc seconds GMTED2010 250m
10 152.9 5.0 5 arc seconds
11 76.4 2.5 3 arc seconds SRTM 90m (not used)
12 38.2 1.2 1 arc seconds SRTM 30m
13 19.1 0.6 2/3 arc seconds
14 9.6 0.3 1/3 arc seconds 3DEP 10m
15 4.8 0.2 1/5 arc seconds
16 2.4 0.1 1/9 arc seconds 3DEP 3m

Data updates

Terrain tiles were built during 2016Q2 and 2016Q3 based on available sources at that time. Regular updates are not planned.

Future updates will be on an as-needed basis for smaller regions to incorporate additional 3DEP coverage in the United States and additional country specific data sources globally (such as NRCAN in Canada).

We are always looking for better datasets. If you find a data issue or can suggest an open terrain datasets, please let us know by filing an issue in tilezen/joerd.

Known issues

Many classical DEM and LIDAR related issues occur in terrain tiles. It is not uncommon to see large variations in elevation in areas with large buildings and other such structures. Resampling and merging artifacts are also observed along coastlines (where different datasets are seamed together).