The SRTM X-SAR Digital Elevation Model

The SRTM X-SAR digital elevation models (DEMs) have been generated from X-band synthetic aperture radar (SAR) data acquired during the Shuttle Radar Topography Mission (SRTM) in the year 2000. SRTM mission was conducted jointly between the German Aerospace Center (DLR), the Italian Space Agency (ASI), the US National Aeronautics and Space Administration (NASA) and the US Jet Propulsion Laboratory (JPL). From February 11 to 22, 2000, two interferometric radar systems onboard the space shuttle Endeavor acquired data with the objective to generate a global high resolution digital elevation model. The American radar system, operating in C-band (SIR-C), was complemented by the German-Italian X-SAR, a higher resolution X-band synthetic aperture radar (SAR) system. At this time, the SRTM DEMs were the first elevation dataset with worldwide uniform quality. The SRTM X-SAR DEM (1 arcsec) offered here for download were processed from data of the German-Italian X-SAR instrument.

• The SRTM X-SAR DEM represents a Digital Surface Model (DSM): That means it does not represent the bare earth surface, but the top layer of earth's surface as seen from space. Canopies or man-made objects can therefore also contribute to height values of individual cells.
• The 'SRTM X-SAR 10° Regional Mosaics' release is the most recent reprocessed and repacked version of the original product: The original 'SRTM X-SAR 15’ Tiles', first released in 2001, have been merged to 10° tiles and converted from DTED to GeoTIFF-format in order to provide this interesting dataset in an easy-to-use and state-of the-art format. For more information on the release history of the SRTM X-SAR DEM see chapter Download the SRTM X-SAR 10° Regional Mosaics.

DEM Accuracy

One pixel of the SRTM X-SAR DEM has a resolution of 1 x 1 arcsecond on the ground. The metric correspondence of this value is latitude dependent and roughly equals 30m x 30m. Elevation values are provided as integer numbers at 1 m intervals.

No data voids are a result of the original processing logic applied and a NoData value of -32767 is attributed to them. As normally those are single or few neighboring pixels, which can easily be interpolated employing standard geographic image processing software.

The DEMs have not been edited with respect to coastlines and water bodies. Therefore, coastlines may not be well defined and water bodies may not be perfectly flat.

The product specification concerning accuracy of the SRTM X-SAR 25m DEM products is defined as:

• Absolute horizontal precision: CE90 +/- 20m (circular error at 90% confidence level)
• Relative horizontal precision: CE90 +/- 15m (circular error at 90% confidence level)
• Absolute vertical precision: VE90 +/- 16m (vertical error at 90% confidence level)
• Relative vertical precision: VE90 +/- 6m (vertical error at 90% confidence level)

DEM Coverage

The orbit of the SRTM mission was chosen in a way that the US SIR-C instrument could cover all land mass between 57° S und 60° N at least once. Unlike the C-band SAR, the X-band SAR had no steering capacity and hence couldn't operate in ScanSAR mode. Also, at the benefit of a higher resolution, the swath of the X-SAR was with 50 km considerably narrower than the 225 km wide ScanSAR swath of the SIR-C instrument. This is why there are inevitable gaps between the individual ground-tracks of the X-SAR. Those gaps narrow down with latitude increase. Other gaps are a result of shuttle motion caused by orbit maneuvers, during which the radar instruments were not able to record. In total the X-SAR covered during the mission an area of about 112 million square kilometers of which 64 million square kilometers were land surface. That corresponds with roughly 43 percent of the global land surface.

Height Error Map

Product file structure of the 'SRTM X-SAR 10° Regional Mosaics' Release

The SRTM X-SAR 10° Regional Mosaics are delivered in a flat .zip format without internal subfolders. Each .zip file contains the following layers:

1. Digital elevation model (DEM)
2. Height error map (HEM)
3. Quicklook (png image)
4. Quicklook (kmz overlay)
5. Readme document; containing a.o. license information

Data type of the 'SRTM X-SAR 10° Regional Mosaics' Release

DEM and HEM files are of file format GeoTIFF. Compression is applied at individual file level, not at zip level.

• Type: signed integer 16 bit (Int16)
• Compression: Deflate with predictor 2
• Tiled
• 2/4/8/16/32 overviews with blocksize 256 x 256
• Projection: Geographic (Lat/Long), WGS84 (EPSG: 4326)

Download Instructions for the 'SRTM X-SAR 10° Regional Mosaics'

EOC download service offers three different download options:

1. HTTP map-based interface: Select and download using your web-browser by marking individual tiles on a map
2. Bulk download a list of tiles: Download using a download manager
3. HTTP browse: Use your web-browser to directly browse through the directory structure and download tiles

HTTP map-based Interface

This download option is recommended for users who wish to download some selected regional products and prefer the usage of a map driven interface for the selection of the products. Visit the EOC Download service SRTM X-SAR page. The page will appear with basic information about the SRTM X-SAR mission on the left and the map interface (showing a color coded version of the SRTM X-SAR DEM) with the polygon outline for each individual DEM tile on the right. You can alter the map layer(s) using the icon in the top right corner above the map canvas. Zoom to your region of interest and select the desired products by a left-click onto the respective tile. Name and volume of the product now appears in a list on the left side. Alternatively you can also draw a search rectangle (CTRL C and open search area with the mouse) in order to download the products for a larger region of interest. Selected products can be downloaded one by one by clicking on the download icon (arrow down) next to each item. Depending on the browser you use, products may eventually be saved to your default download folder without further notification.

Bulk download a list of tiles

Mark the tiles you want to download in a way as described in the paragraph above. Select the 'List' icon to save the download list to an ASCII text file (with an absolute pathname for each file per line), or to an XML-like metalink file by selecting the metalink icon.

The ASCII file list can then be fed to a command line client (e.g. wget or cURL), or a download manager (e.g. aria2). The following code examples illustrate the usage of those command line tools.

Supposed you have saved the list to the name ‘SRTM_XSAR-url-list.txt’ to your current working directory and the download directory will be the current working directory as well: then the wget command would be the following one-liner:

Built a wget command according to the following:

wget -i SRTM_XSAR-url-list.txt

Built a cURL command according to the following:

curl $(printf ' -O %s' $(<SRTM_XSAR-url-list.txt))

or:

Built an aria2 command according to the following:

aria2c -i SRTM_XSAR-url-list.txt

The metalink XML file can be fed to a download manager tool (e.g. aria2). Suppose you have saved the metalink to the file name ‘SRTM_XSAR.meta4’ in your current working directory, then the command line for aria2 would be:

aria2c -M SRTM_XSAR.meta4

HTTP Browse

The files can be also accessed via HTTPS by entering the following URL path into your web browser:
https://download.geoservice.dlr.de/SRTM_XSAR/files/

When the Space Shuttle Endeavour lifted off into space for STS-99 Shuttle Radar Topography Mission on February 11, 2000, it carried as payload two radar antennas. While one was installed inside the Shuttle's cargo bay, the other one was mounted at the end of a 60 m long mast extending from the spacecraft. By this time, this mast was the longest manmade object in space.

While the shuttle orbited around the surface of the earth, a transmitter-receiver in the cargo bay sent radar beams to the Earth. When reflected back to the shuttle, they were received and recorded by both, by this equipment and by the antenna at the end of the mast. The radar images thus depicted the earth’s surface from two different viewing positions imaged practically at the same time. Such acquisition pairs or series allows scientists to calculate precise elevation model of the surface of the earth. using the so-called 'Single Pass Interferometry' technique.

Over the course of the mission, Endeavour flew 176 repeats on a circular orbit at an altitude of about 233 km and an inclination of 57°. Operations of the 13,600 kg payload could be started within less than 12 hours after launch. During the 11 days campaign from February 11 to 22, a total volume of about 12 TerraByte (TB) mission payload data were acquired (8.6 TB by the C-band radar, 3.7 TB by the X-band radar).

Further SRTM Data Products: SIR-C DEM Products from the US Radar Instrument

Several SRTM DEM products have as well been processed from the US SIR-C instrument.
Information about these datasets and terms of access can be found at the NASA/JPL website.

Further SRTM Data Products: X-SAR Image Products

Besides the SRTM X-SAR digital height models, also the original X-SAR images are preserved at DLR. These image products exist in three different representations and image geometries:

• Single Look Complex (SSC)
• Multi-Look Ground Range (MGD)
• Geocoded Terrain Corrected (GTC)

For the SSC product, real and imaginary parts are supplied for each pixel in the sensor-specific signal travel-time geometry. For the MGD product the brightness values are determined for each pixel and transformed into so-called ground range geometry. This produces homogenous rastering over the range (perpendicular to the flight direction). Since the three-dimensional orientation of every pixel is determined, the image product can also be orthorectified and transformed to a map projection. These steps are carried out for the GTC products. All the data sets cover an area of about 150 km in the flight direction and 50 km in range and are supplied in CEOS format The product specifications of the image products are as follows:

The use of the SRTM X-SAR DEMs is governed by the following conditions of use. When accessing and using the data the user accepts the conditions of use detailed below.

The SRTM X-SAR DEMs are intended primarily for scientific purposes. Redistribution of the original SRTM XSAR DEMs is not permitted, neither for commercial nor for non-commercial purposes. No further restrictions, except those contained herein, are being imposed on the use of the DEMs or derived products.

The SRTM X-SAR DEM products have been generated to the highest possible standards of accuracy using state-of-the-art technology. However, the products are provided as is. No warranty of any kind, whether explicit or implied is given. Furthermore, since the products are provided free-of-charge, DLR shall not be liable for any damage arising out of the SRTM X-SAR products’ use. The user is responsible for observing that no damage is caused to anyone or anything by his use of the SRTM X-SAR products. By obtaining the SRTM XSAR products, the user therefore agrees to hold the German Aerospace Center (DLR) harmless from and against any and all claims which might arise by himself or any third party from the use of the SRTM X-SAR products.

For all derived products based on the SRTM X-SAR DEM products and for all publications including these data or derived products, using the following copyright information is mandatory: ©DLR/ASI 2000.

The EOC Geoservice Team

In case of any problems or questions, please contact the EOC Geoservice: