SRTM_Topo     (last update 11/21/02)

SRTM Documentation (best viewed with mono-spaced font, such as courier)

1.0  Introduction

The SRTM data sets result from a collaborative effort by the National
Aeronautics and Space Administration (NASA) and the National Imagery and
Mapping Agency (NIMA), as well as the participation of the German and
Italian space agencies, to generate a near-global digital elevation model
(DEM) of the Earth using radar interferometry. The SRTM instrument
consisted of the Spaceborne Imaging Radar-C (SIR-C) hardware set modified
with a Space Station-derived mast and additional antennae to form an
interferometer with a 60 meter long baseline. A description of the SRTM
mission, can be found in Farr and Kobrick (2000).

Synthetic aperture radars are side-looking instruments and acquire data
along continuous swaths. The SRTM swaths extended from about 30 degrees
off-nadir to about 58 degrees off-nadir from an altitude of 233 km, and
thus were about 225 km wide. During the data flight the instrument was
operated at all times the orbiter was over land and about 1000 individual
swaths were acquired over the ten days of mapping operations. Length of the
acquired swaths range from a few hundred to several thousand km. Each
individual data acquisition is referred to as a "data take."

SRTM was the primary (and pretty much only) payload on the STS-99 mission
of the Space Shuttle Endeavour, which launched February 11, 2000 and flew
for 11 days. Following several hours for instrument deployment, activation
and checkout, systematic interferometric data were collected for 222.4
consecutive hours. The instrument operated virtually flawlessly and imaged
99.96% of the targeted landmass at least one time, 94.59% at least twice
and about 50% at least three or more times. The goal was to image each
terrain segment at least twice from different angles (on ascending, or
north-going, and descending orbit passes) to fill in areas shadowed from
the radar beam by terrain.

This 'targeted landmass' consisted of all land between 56 degrees south and
60 degrees north latitude, which comprises almost exactly 80% of the total
landmass.

2.0 Data Set Characteristics

2.1 General

SRTM data are being processed in a systematic fashion using the SRTM Ground
Data Processing System (GDPS) supercomputer system at the Jet Propulsion
Laboratory. Data are being mosaicked into approximately 15,000 one degree by
one degree cells and formatted according to the Digital Terrain Elevation
Data (DTED) specification for delivery to NIMA, who will use it to update
and extend their DTED products. Data are being processed on a
continent-by-continent basis beginning with North America. NIMA will apply
several post-processing steps to these data including editing, spike and well
removal, water body leveling and coastline definition. Following these
"finishing" steps data will be returned to NASA for distribution to the
scientific and civil user communities, as well as the public.

However, in advance of that, some portion of the data processed by the GDPS
is being released to Principal Investigators selected by NASA under
the Solid Earth and Natural Hazards program, in response to requests for
specific research programs.

2.2 Organization

SRTM data will be delivered in individual rasterized cells, or tiles, each
covering one degree by one degree in latitude and longitude. Sample spacing
for individual data points is either 1 arc-second or 3 arc-seconds,
referred to as SRTM-1 and SRTM-3, respectively. Since one arc-second at the
equator corresponds to roughly 30 meters in horizontal extent, the sets are
sometimes referred to as "30 meter" or "90 meter" data.

2.3 Elevation mosaics

Each SRTM data tile contains a mosaic of elevations generated by averaging
all data takes that fall within that tile. Since the primary error source
in synthetic aperture radar data is speckle, which has the characteristics
of random noise, combining data through averaging reduces the error by the
square root of the number of data takes used. In the case of SRTM the
number of data takes could range from a minimum of one (in a very few
cases) up to as many as ten.

3.0 Data Formats

The names of individual data tiles refer to the longitude and latitude of
the lower-left (southwest) corner of the tile (this follows the DTED
convention as opposed to the GTOPO30 standard). For example, the
coordinates of the lower-left corner of tile N40W118 are 40 degrees north
latitude and 118 degrees west longitude. To be more exact, these
coordinates refer to the geometric center of the lower left pixel, which in
the case of SRTM-1 data will be about 30 meters in extent.

SRTM-1 data are sampled at one arc-second of latitude and longitude and
each file contains 3601 lines and 3601 samples. The rows at the north
and south ecges as well as the columns at the east and west edges of each
cell overlap and are identical to the edge rows and columns in the adjacent
cell.

SRTM-3 data are sampled at three arc-seconds and contain 1201 lines and
1201 samples with similar overlapping rows and columns. This organization
also follows the DTED convention. Unlike DTED, however, 3 arc-second data
are generated in each case by 3x3 averaging of the 1 arc-second data - thus
9 samples are combined in each 3 arc-second data point. Since the primary
error source in the elevation data has the characteristics of random noise
this reduces that error by roughly a factor of three.

This sampling scheme is sometimes called a "geographic projection", but of
course it is not actually a projection in the mapping sense. It does not
possess any of the characteristics usually present in true map projections,
for example it is not conformal, so that if it is displayed as an image
geographic features will be distorted. However it is quite easy to handle
mathematically, can be easily imported into most image processing and GIS
software packages, and multiple cells can be assembled easily into a larger
mosaic (unlike the pesky UTM projection, for example.)

3.1 DEM File (.HGT)

The DEM is provided as 16-bit signed integer data in a simple binary
raster. There are no header or trailer bytes embedded in the file. The data
are stored in row major order (all the data for row 1, followed by all the
data for row 2, etc.).

All elevations are in meters referenced to the WGS84 geoid. INote that this
from data processed by the "PI Processor", which uses the WGS84 ellipsoid.

Byte order is Motorola ("big-endian") standard with the most significant
byte first. Since they are signed integers elevations can range from -32767
to 32767 meters, encompassing the range of elevation to be found on the
Earth.

In these preliminary data there commonly will be data voids from a number of
causes such as shadowing, phase unwrapping anomalies, or other
radar-specific causes. Voids are flagged with the value -32768.


4.0  Notes and Hints for SRTM Data Users

4.1 Data Encoding

Because the DEM data are stored in a 16-bit binary format, users must be
aware of how the bytes are addressed on their computers. The DEM data are
provided in Motorola or IEEE byte order, which stores the most significant
byte first ("big endian"). Systems such as Sun SPARC and Silicon Graphics
workstations use the Motorola byte order. The Intel byte order, which
stores the least significant byte first ("little endian"), is used on DEC
Alpha systems and most PCs. Users with systems that address bytes in the
Intel byte order may have to "swap bytes" of the DEM data unless their
application software performs the conversion during ingest. 

4.3 SRTM Caveats

As with all digital geospatial data sets, users of SRTM must be aware of
certain characteristics of the data set (resolution, accuracy, method of
production and any resulting artifacts, etc.) in order to better judge its
suitability for a specific application. A characteristic of SRTM that
renders it unsuitable for one application may have no relevance as a
limiting factor for its use in a different application.

In particular, data produced by the PI processor should be considered as
"research grade" data suitable for scientific investigations and
development and testing of various civil applications.

No editing has been performed on the data, and the elevation data in
particular contain numerous voids and other spurious points such as
anomalously high (spike) or low (well) values. Water bodies will generally
not be well-defined - in fact since water surfaces generally produce very
low radar backscatter they will appear quite "noisy" or rough, in the
elevations data. Similarly, coastlines will not be well-defined.

5.0 References

Farr, T.G., M. Kobrick, 2000, Shuttle Radar Topography Mission produces a
wealth of data, Amer. Geophys. Union Eos, v. 81, p. 583-585.

Rosen, P.A., S. Hensley, I.R. Joughin, F.K. Li, S.N. Madsen, E. Rodriguez,
R.M. Goldstein, 2000, Synthetic aperture radar interferometry, Proc. IEEE,
v. 88, p. 333-382.

NIMA, 1994, Military Standard WGS84,
http://164.214.2.59/publications/specs/printed/WGS84/wgs84.html


Other Web sites of interest:

NASA/JPL SRTM: http://www.jpl.nasa.gov/srtm/

NIMA: http://164.214.2.59/nimahome.html

STS-99 Press Kit: http://www.shuttlepresskit.com/STS-99/index.htm

Johnson Space Center STS-99:
http://spaceflight.nasa.gov/shuttle/archives/sts-99/index.html

German Space Agency: http://www.dlr.de/srtm

Italian Space Agency: http://srtm.det.unifi.it/index.htm

U.S. Geological Survey, EROS Data Center: http://edc.usgs.gov/