Shuttle Radar Topography Mission (SRTM) "Finished" FAQ

• What is SRTM "finished" 1 and 3 arc second data?
• What is the difference between SRTM and NED?
• Does the "finished" data still have void areas?
• What is the coverage area of SRTM 1 and 3 arc second?
• When will SRTM "finished" data be available on seamless?
• What are the specifications for SRTM "finished" 1 and 3 arc second data?
• What formats are available for SRTM "finished" 1 and 3 arc second data?
• What is the vertical accuracy of SRTM "finished" 1 and 3 arc second data?
• Which is better to use, NED or SRTM?
• Will there still be voids with the "finished" data?
• How are the void areas for SRTM filled in?
• Can I get SRTM "finished" 1 arc second data for areas outside of the United States?
• Are there more FAQ's directly from the NASA/JPL site?
• Is there a public forum to exchange information concerning SRTM data?
• Can I still obtain SRTM "research-grade" data?
• Can I obtain a copy of SRTM "finished" data in another way, such as pre-defined areas?

Q. What is SRTM "finished" 1 and 3 arc second data?

A. The Shuttle Radar Topography Mission (SRTM) is a joint project between NASA and NGA (National Geospatial-Intelligence Agency) to map the world in three dimensions. SRTM utilized dual Spaceborne Imaging Radar (SIR-C) and dual X-band Synthetic Aperture Radar (X-SAR) configured as a baseline interferometer, acquiring two images at the same time. These images, when combined, can produce a single 3-D image. Flown aboard the NASA Space Shuttle Endeavour February 11-22, 2000, SRTM successfully collected data over 80% of the Earth's land surface, for all area between 60 degrees N and 56 degrees S latitude.

Data Processing
SRTM data were processed from raw radar echoes into digital elevation models at the Jet Propulsion Laboratory (JPL) in Pasadena, CA. These original data files had samples spaced ("posted") at intervals of 1 arc-second of latitude and longitude (about 30 meters at the equator). These data were then edited by the National Geospatial-Intelligence Agency (NGA, formerly the National Imagery and Mapping Agency), and distributed as part of their DTED® product set. (http://edcsns17.cr.usgs.gov/srtmdted2)

The editing, also referred to as finishing, consisted of delineating and flattening water bodies, better defining coastlines, removing "spikes" and "wells", and filling small voids. This set is publicly available at two postings: 1 arc-second for the United States and its territories and possessions, and 3 arc-seconds for regions between 60 degrees N and 56 degrees S latitude.

The method NGA used to produce 3 arc-second data from the 1 arc-second set is "subsampling", namely selecting the center value from the set of nine centered on a particular posting location. This is the method that has been used to produce the 3 arc-second "finished" data that are available through The National Map Seamless Server. Note that in the course of creating the DTED®1 by "subsampling" the DTED®2 it is possible that residual voids smaller than 16 contiguous pixels may reside in the final DTED®1. It is also possible that some water bodies that are contained by higher elevations in the original DTED®2 may not be completely contained in the finished DTED® 1.

Research - Grade:
NASAs Jet Propulsion Laboratory is distributing a "research-grade" SRTM data set produced in a slightly different manner. The initial release consists of 1 arc-second data for the U.S. in its original unedited form, as well as 3 arc-second data produced by averaging the nine values centered on each posting. This averaging, as opposed to subsampling, reduces the high frequency "noise" that is characteristic of elevation data collected using radar interferometry and is the method preferred by the research community. To obtain the "research-grade" SRTM data go to http://www2.jpl.nasa.gov/srtm/.

The following products are available on The National Map Seamless Server:

United States "Finished" 1 Arc Second (30 meter):
1 arc second (30 meter) SRTM data currently available for the United States and its territories and possessions. The product consists of seamless raster data which is provided according to a user-specified area of coverage.

Global "Finished" 3 Arc Second (90 meter):
The SRTM global data is available 3 arc second (90 meter). The product consists of seamless raster data which is provided according to a user-specified area of coverage.

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Q. What is the difference between SRTM and NED?

A. The table below shows the differences. Basically, the advantage of using the SRTM over the NED is that SRTM is more current. This would especially be important over mining and quarry areas. The elevation from the NED is bare ground readings whereas SRTM is reflected surface based. To read more about the SRTM go to http://srtm.usgs.gov

	NED	SRTM
Resolution	1 Arc Second (~ 30m resolution)	1 Arc Second (~30m resolution)
Source Data	Maps / Aerial Photos	Radar Images
Source Resolution	10m & 30m DEMs	30m
Source Dates	1925-1999	February, 2000 Space Shuttle Endeavor
Surface Type	"Bare Earth"	"First Return"
Accuracy Specifications	7m RMSE	10m RMSE

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Q. Does the "finished" data still have void areas?

A. The "finished" data after editing still has void areas. The editing, also referred to as finishing, consisted of delineating and flattening water bodies, better defining coastlines, removing "spikes" and "wells", and filling small voids. Other elevation data, such as GTOPO30, would need to be used to fill the remaining void areas.

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Q. What is the coverage area of SRTM "finished" 1 and 3 arc second?

A. The coverage on the The National Map Seamless Server is 1 arc-second (30m) for the United States and its territories and possessions, and 3 arc-seconds (90m) for global coverage between 60 degrees N and 56 degrees S latitude.

To see availabilty of the data on The National Map Seamless Server:

Go to The National Map Seamless Server on the right side of the international map interface, under the Display tab, click the arrow next to the category Layer Extent. A list of layers will appear, find SRTM Finished 1 arc second Index, or SRTM Finished 3 arc second Index, and click the box next to the desired layer. This will automatically update the map with the index showing areas of coverage.

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Q. When will SRTM "finished" data be available on seamless?

A. The SRTM data currently being served on The National Map Seamless Server is considered the "finished" or edited data.

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Q. What are the specifications for SRTM "finished" 1 and 3 arc second data?

A.

Product	Resolution	Projection	Horizontal Datum	Vertical Datum	Vertical Units	Cell Size
Shuttle Radar Topography Mission (SRTM) 1 Arc Second (~30m resolution)	1 arc second (approx. 30m) conterminous US	Geographic	WGS84	WGS84/EGM96 geoid	Meters	0.00028
Shuttle Radar Topography Mission (SRTM) 3 Arc Second (~90m resolution)	3 arc second (approx. 90m)	Geographic	WGS84	WGS84/EGM96 geoid	Meters	0.00083

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Q. What formats are available for SRTM "finished" 1 and 3 arc second data?

A. ArcGrid is an Arc/Info proprietary format. Some other software packages can read the ArcGrid format. Customers who specify ArcGrid will be delivered a workspace. The workspace is not tarred or gzipped.

BIL is a non-proprietary format made by running the GRIDIMAGE command in Arc. The data is a simple binary raster format (signed integer data). There is an accompanying ASCII header file that provides file size information (number of rows and columns). The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.).

BYTEORDER	byte order in which image pixel values are stored I = Intel byte order (most significant byte first)
LAYOUT	organization of the bands in the file BIL = band interleaved by line (note: the DEM is a single band image)
NROWS	number of rows in the image
NCOLS	number of columns in the image
NBANDS	number of spectral bands in the image (1 for a DEM)
NBITS	number of bits per pixel (16 for a DEM)
BANDROWBYTES	number of bytes per band per row (twice the number of columns for a 16-bit DEM
TOTALROWBYTES	total number of bytes of data per row (twice the number of columns for a single band 16-bit DEM)
BANDGAPBYTES	the number of bytes between bands in a BSQ format image(0 for a DEM)

Example header file:

GridFloat is a non-proprietary format made by running the GRIDFLOAT command in Arc. The 32-bit (4 byte) is a simple binary raster format (floating point data). There is an accompanying ASCII header file that provides file size information (number of rows and columns). The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.). More information about binary floating point is at:

http://grouper.ieee.org/groups/754/ http://www.math.grin.edu/~stone/courses/fundamentals/IEEE-reals.html http://www.psc.edu/general/software/packages/ieee/ieee.html

Geotiff (32 bit floating point)is a 32 bit floating point grid format. More information on the 32 bit floating point TIFF is at:
http://partners.adobe.com/asn/developer/graphics/graphics.html

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Q. What is the vertical accuracy of SRTM "finished" 1 and 3 arc second data?

A. The SRTM "finished" data meet the absolute horizontal and vertical accuracies of 20 meters (circular error at 90% confidence) and 16 meters (linear error at 90% confidence), respectively, as specified for the mission. The vertical accuracy is actually significantly better than the 16 meters. It is closer to +/- 10 meters.

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Q. Which is better to use, NED or SRTM?

A. The decision to use NED or SRTM is basically up to the user and applications being applied. The NED gives bare earth elevations and the SRTM is first return radar and could bounce off the tops of heavy tree canopy. So, if the application is to locate cell phone towers, the NED would give a better representation. But, if the applications needs more current data, then SRTM would be the better representation.

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Q. Will there still be voids with the "finished" data?

A. Yes, in the processing of the "finished" data some voids will still be present, mainly, but not limited to, steep slopes. On The National Map Seamless Server map display, the void areas will be displayed as the color of the background layer. For example, if the GTOPO60 layer is turned on the voids will be green.

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Q. How are the void areas for SRTM filled in?

A. USGS does not supply a program to fill in the void areas. Other elevation data would need to be used, such as NED for the US or GTOPO30 for global coverage. There is a forum for SRTM information that might provide some information on filling voids at: http://pub7.bravenet.com/forum/537683448/

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Q. Can I get SRTM "finished" 1 arc second data for areas outside of the United States?

A. The SRTM data is a joint project between NGA and NASA/JPL. The USGS is just the distribution outlet and the other agencies make the policy on what to distribute. For further information contact: http://www2.jpl.nasa.gov/srtm/ or http://www.nga.mil

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Q. Are there more FAQ's directly from the NASA/JPL site?

A. Yes, there are more FAQ's concerning the actual mission and other information from NASA/JPL.

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Q. Is there a public forum to exchange information concerning SRTM data?

A. Yes, the public forum is located at: http://pub7.bravenet.com/forum/537683448/

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Q. Can I still obtain SRTM "research-grade" data?

A. Yes, to obtain the "research-grade" SRTM data, access this ftp site http://www2.jpl.nasa.gov/srtm/.

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Can I obtain a copy of SRTM "finished" data in another way, such as pre-defined areas?

A. Yes, to obtain pre-defined SRTM "finished" areas go to http://edcsns17.cr.usgs.gov/srtmbil2/ (1 arc second) or http://edcsns17.cr.usgs.gov/srtmbil/ (3 arc second)

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