See the complete article: Publicly available global environmental layers

  • Edited by: T. Hengl and H.I. Reuter

Global SRTM Digital Elevation Model is probably the most well-known global environmental dataset (Rabus et al. 2003). The area covered is between 60° North and 58° South. It was recorded by X-Band Radar (NASA and MIL, covering 100% of the total global area) and C-Band Radar (DLR and ASI, covering 40%). The non-public DLR-ASI data is available with a resolution of approximately 30 m (1 arcsec). The most up-to-date version of the 1 km resolution SRTM DEM you can obtain via the USGS website. An updated 1 km resolution global topography map (SRTM30 PLUS; used by Google Earth) has been prepared by Becker et al. (2009).

A complete land surface model ETOPO1 Global Relief Model (includes bathymetry data) is available at resolution of 1 km and can be obtained from the NOAA's National Geophysical Data Center (Amante and Eakins, 2008). An updated version of the ETOPO is the Global Land One-km Base Elevation Project (GLOBE) DEM. Global bathymetry data (GEBCO data set) can also obtained from the British Oceanographic Data Centre.

Via the Worldclim website, you can download global DEMs at different resolutions from 1 km to 2.5, 5 and 10 arc-minutes. The 90 m SRTM DEMs can be also obtained from the CGIAR (Consortium for Spatial Information) website. Another DEM produced from topographic data provided by countries is the Global Land Survey Digital Elevation Model (GLSDEM), distributed by the University of Maryland.

Jonathan de Ferranti and Christoph Hormann have released in 2014 a global 3-arc seconds DEM with no serious voids or artifacts (a fusion between the SRTM DEM and GDEM). The 3-arc seconds or about 100 m resolution global land surface DEM (about 15GB of data compressed) can be downloaded from WiewFinderPanoramas.org website (see also the processing steps used to generate this global DEM). The National Geospatial-Intelligence Agency (NGA) announced in 2014 that the global SRTM-Level 2 (30 m resolution), previously only available for the United Stateshas, will be made available in 2015.

Global Relief Model based on SRTM DEM and ETOPO.
Slope map in percent based on the DEMSRE.

DEM-derivatives can be used to extract or classify landform units i.e. areas of relatively homogeneous morphology or geomorphology (if combined with geological data). Iwahashi and Pike (2007) produced a landform classification map of the world at 1 km using the SRTM data. The output map was based on unsupervised nested-means algorithm and uses three DEM-derivatives as inputs: slope gradient, local convexity, and surface texture. Dragut and Eisank (2012) produced a global map of physiographic units at three levels using the object based classification algorithms that consequently shows more homogeneous units: flat plains, high mountains, high hills, low mountains, tablelands, rough low hills, smooth low hills, irregular plains. Jasiewicz and Stepinski (2012) have generated an automated landform map of the world at 100 m resolution using the geomorphon classifier (flat, peak, ridge, shoulder, hollow, slope, spur, footslope, valley and pit). Geomorphons can be derived locally using the algorithm in GRASS GIS.

Physiographic landform units (product of the SCALA project).

USGS jointly with ESRI has released in 2014 a Global Ecological Land Units map at 250 m resolution. GELU also includes map of general landforms (the data can be downloaded from the USGS site).

From June 2009, ASTER-based Global Digital Elevation Model (GDEM) at resolution of 30 m has been made available to the worldwide public. The GDEM was created by stereo-correlating the 1.3 million-scene ASTER archive of optical images, covering almost 98% of Earth's land surface. The one-by-one- degree tiles can be downloaded from NASA's EOS data archive and/or Japan's Ground Data System.

Robinson et al. (2014) have produced a merged SRTM DEM / ASTER DEM product (called 'EarthEnv-DEM90') that can be obtained from EarthEnv.org website. EarthEnv-DEM90 contains significantly less artifacts and noise than the original SRTM DEM, but the vertical accuracy is still at the order of SRTM DEM (RMSE of 10-15 m).

SPOT 5 has produced collected a large archive of stereo-mosaics (from 2002 until today) using the HRS camera images (Kornus et al. 2006). DEM images at 30, 10 m and even few meters resolution can be obtained via the GEO Elevation service. No global DEM based on SPOT has ever been released publicly.

USGS has released in 2011 a global multiscale DEM product called GMTED2010 (Danielson and Gesch, 2011). This is basically a merged product that is based on: global Digital Terrain Elevation Data (DTED) from the Shuttle Radar Topography Mission (SRTM), Canadian elevation data, and data from the Ice, Cloud, and land Elevation Satellite (ICESat). GMTED is available at three resolutions (approximately): 1,000, 500, and 250 meters. Geotiffs can be downloaded in tiles from the USGS GMTED viewer.

Japan Aerospace Exploration Agency has released in 2015 ALOS World 3D (AW3D30) a Global Digital Surface Model at 30 m resolution. AW3D30, similar to GDEM, is based on heights derived from stereoscopic images acquired by the Advanced Land Observing Satellite “DAICHI” (ALOS) satellite. Data can be ordered and downloaded (without restrictions) via a registration from JAXA ALOS pages.

TanDEM-X is the new generation satellite (owned by the German Aerospace Center) that aims at producing new generation radar-based DEMs with a horizontal detail of 12 meters) and a height accuracy of better than 2 m (Krieger et al. 2007). The data will likely not be publicly available (for more info refer to the WorldDEM product), but the global DEM will likely cause dramatic changes in terms of accuracy and usability of elevation data for mapping and monitoring.

Some large areas are also being surveyed by airborn LiDAR surveys. Via OpenTopography.org you can access some publicly available LiDAR data from a range of countries. LiDAR data is at the order of magnitude higher accuracy than the space-born elevation data, and can be used for spatio-temporal monitoring of changes in topography.

Google provides an elevation service i.e. returns elevations for any given location on the globe (including oceans). This is not the best place to obtain elevations for large areas, but it works perfectly for smaller size vectors (points, lines, polygons). Likewise, ESRI serves not only elevation data but also results of hydrological analysis, erosion risk assessment and similar. Microsoft also provides REST Elevations API, which is based on the 90 m resolution SRTM data (for US 10 m resolution NED is used).

A range of satellites (Topex/Poseidon-ERS, Jason-Envisat) are used to track the sea altimetry, tides, significant wave height and wind speed, and alike oceanographic features. Most of these images are available only at course resolution of 0.125-1 arcdegree. Merged data products are available for download via the AVISO website. One needs to register to gain the FTP access to auxiliary products such as the long term Mean Sea Surface, Global tide (heights of tidal constituents) and similar.

If you are curious to see how the world (continents) looked like 50, 100 and more millions ago, look at the global paleogeography maps contributed by Ron Blakey (2008).