Frequently Asked Questions - DEM

Digital elevation data can now be obtained for any place on the Earth, at several resolutions. This document will discuss the major sources for reliable and freely available DEM data.  In order to choose the right source for DEM data, there are two main criteria to consider: geographical region of interest and spatial resolution. Below is a table that summarizes the DEM data sources that should be of interest to you.

The National Map is the primary source of the National Elevation Dataset (NED) data of the U.S. at:   http://nationalmap.gov/viewer.html..  When you first visit this site you should read the directions for downloading data from the National Map Viewer before searching for data. This will teach you to easily navigate the site and select exactly what you need.  These data have a Geographic “projection” (latitude/longitude) and use the NAD83 datum.  You will find that there are many other types of data available at this site.  You should explore this site at your convenience.

Within the U.S. you can access NED data at 1 arc second (~30 m), 1/3-arc-second (~10 m), and in some locations 1/9-arc-second (~3m) resolution.  For Alaska, the NED data are primarily 2-arc-second (~60 meters). All of the data are prepackaged in 1x1 degree tiles, except for 1/9 arc second data that are prepackaged in 15x15 minute tiles.

Zoom in to your area of interest and click on the Elevation Availability in the Table of Contents on the left.  If a certain checkbox is disabled or an error notification box appears, then zoom in for finer spatial resolution.  Reminder: the data is available for only North America (U.S., some Canada, and Mexico).

Once you have confirmed the dataset you are interested in is available, click on the Download Data button on the toolbar or in the upper right of the window. There are a few options for defining your region of interest; draw a rectangle; select a reference area, adjust current map extent, or enter coordinate input.  Select a method then specify the type of data you would like to download, in this case Elevation, and click Next.  

You are presented with a list of tiles at different resolutions, in different formats.  Currently the output formats available on the National Map Viewer are IMG, ArcGrid, and GridFloat.  You should choose the IMG format if working with ENVI.

Check off the product(s) you are most interested in and click Next.  The Table of Contents pane changes to a view of your Cart.  Check out by providing your email address. The link (or multiple links depending on the size of the region) to download the data will be sent immediately to your email address.

In February of 2000 the Space Shuttle mapped most of the land surfaces of the Earth, from 60° north to 56° south, to create the highest resolution global elevation dataset available to date.  Global data were released at a 3 arc-second (~90 m) resolution.  Data covering the United States were also released at a 1 arc-second (~30 m) resolution.  The Version 1 data had many data voids and irregular water surfaces and coastlines.

Version 2 SRTM Data

NASA released a “finished” version of these data called Version 2.  These data had most voids filled in by interpolation, lake surfaces were corrected, and coastlines properly defined and aligned.  These data are available at the USGS Earth Explorer site.  Data are available at 3 arc-second (~90 m) resolution globally, and also at 1 arc‑second (~30 m) in the U.S.  Care should be taken when using SRTM data in areas with extreme topographical change as there are still data voids in some of these areas.

Version 3 SRTM Data

In November 2013 NASA released Version 3 of the SRTM data.  These data had voids filled using other data sources such as the ASTER GDEM2, GMTED2010, and NED.  These are the highest quality SRTM data available to date.  Get these data at the NASA Reverb | Echo site.  As with the Version 2 data on the USGS site, these data are available at 3 arc-second (90 m) resolution globally, and also at 1 arc‑second (30 m) in the U.S. and most of the world.

To obtain these data on the Reverb site zoom in to your area of interest then enter the keyword SRTM in the Search Term box in the upper right.  Under Step 2: Select Datasets select the V003 dataset you wish, either NASA Shuttle Radar Topography Mission Global 3 arc second V003 or NASA Shuttle Radar Topography Mission Global 3 arc second V003, then click Search for Granules.  (Do not select the datasets that have the phrase “second number V003” as part of their tittle.)  Data are packaged in 1° x 1° tiles.  Use the Browse button to view the granule(s) and put those you want into your Cart.  From the Cart click on the Download button and a text file will be generated that can be used to download your data.  Simply paste this line of text into your browser to get the data.

These integer data use the Geographic “projection” (latitude/longitude) and the datum is WGS84.  The data are in “height” format with a file extension of .HGT.  To open these data in ENVI Standard, from the main menu select File | Open As | Digital Elevation | SRTM DEM.

Global 1 Arc-Second Data - best choice for 30 m resolution

Beginning in September of 2014 the USGS began distributing 1 arc-second Version 3 void-filled data globally.  Unlike the original Version 3 release, these data will be distributed through the USGS Earth Explorer site. Click on the Data Sets tab then Digital Elevation | SRTM.  Data are packaged in 1° x 1° tiles and distributed in three formats; DTED, BIL, and GeoTIFF.  GeoTIFF data may be the easiest to use in most geospatial software programs and is the recommended format.

The science teams at the NASA Jet Propulsion Lab (JPL) and Japan’s Ministry of Economy, Trade, and Industry (METI) used the ASTER sensor onboard the Terra satellite to produce 30 m resolution elevation data. These data cover 99% of the land surface from 83° north latitude to 83° south latitude. Version 2 of these data was released in October 2011 and is a significant improvement over the initial data release.  You can view the latest information about these data at the USGS LP DAAC

These ASTER data are now available at the USGS Earth Explorer site. You may zoom in, create polygon features, or enter path/row information to select your area of interest. Then select Digital Elevation, then ASTER GLOBAL DEM, and click Results.

These data may be packaged in many tiles.  You can turn on or off individual tile footprints and immediately download each tile.  If there are many tiles you can select them for Bulk Download, which will add the results to your “shopping cart.” You will receive an email when the data are available for retrieval and you then use the USGS Bulk Download Application (installed on the YCEO Lab computers) to easily retrieve your data.

The USGS has global DEM data at several resolution levels: 30-arc-second (1 kilometer), 15-arc-second (450 meters), and 7.5-arc-second (225 meters). The GMTED2010 dataset with multiple resolution levels replaces a former version called GTOPO30, which only had the resolution level of 30-arc-second (1 kilometer). These data have been collected from a variety of sources using aggregation methods.  These datasets are best used for working at the continental scale and with very large regions. More information is available in the USGS publication.

The data can be accessed via the Earth Explorer site.  First define your region of interest under the Search Criteria tab.  You can upload a Shapefile (in Lat/lon), a KML file, or use the view window extents.  Next select the Data Sets tab and under Digital Elevation select GMTED2010.  Click on the Results button at the bottom of the screen and data that meets your selection is placed in the Results tab.

Click on the Footprint icon to view the size of the tile.  These cover a very large area.  Click on the Download icon and you can select the DEM resolution; 7.5 Arc Sec (225m), 15 Arc Sec (450m), and 30 Arc Sec (1km).

These integer data use the Geographic “projection” (latitude/longitude), the datum is WGS84, and are provided in TIF format.  There are seven products in each order including min, max, mean, and standard deviation.  For most applications you will use the Mean identified with “gmted_mea” in the filename.  More information about these different products can be found in the USGS document referenced in the first paragraph.

A copy of the global 30-arc-second (1 kilometer) GMTED2010 data has been downloaded for internal use to a server at the Yale Center for Earth Observation (YCEO).  Please see a member of the YCEO staff for guidance in using these data.

ENVI Standard version 5.1 and above also has the 30-arc-second (1 kilometer) GMTED2010 dataset embedded into the application.  Access these data from ENVI; select File | Open World Data | Elevation (GMTED2010).  As of January 2016 ENVI software on the workstations in the YCEO Lab provide the 7.5 Arc Sec (~225m) GMTED2010 dataset in JPEG 2000 format.