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DEM from Active Sensors – Shuttle Radar Topographic Mission (SRTM). Bali, Indonesia. Ben Maathuis, WRS-2004, Koert Sijmons IT/RSG/GTS. SRTM (Shuttle Radar Topography Mission). The Shuttle Radar Topography Mission obtained elevation data on the near-global scale to generate
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DEM from Active Sensors – Shuttle Radar Topographic Mission (SRTM)
The Shuttle Radar Topography Mission obtained
elevation data on the near-global scale to generate
the most complete high-resolution digital topographic
database of Earth.
SRTM consisted of a specially modified radar system
that flew onboard the Space Shuttle Endeavour
during an 11-day mission in February of 2000
TheSRTM radar contained two types of antenna
Panals, C-band and X-band. The near-global
topographic maps of Earth called Digital Elevation
Models (DEMs) are made from the C-band radar data
Data from the X-band radar are used to create
slightly higher resolution DEMs, but without the
global coverage of the C-band radar
DEMs with a 90 meter resolution can be down
loaded free of charge from the Internet
The released SRTM DEMs for the United States are
at 30-meter resolution. DEMs for the rest of the world
will be at 90 meters.
DEMs at 90 meters resolution are “seamless” available
for North America, Central en South America
For Eurasia the DEMs are available on 1 degree by
1 degree images
DEMs for Africa will be available in the middle of
Knowledge of surface topography is of major importance to Earth Sciences, e.g. hydrology, geomorphology, but:
Availability of Topographic Maps (%)
Source: CNES, Paris/Toulouse 1997
*Former Sovjet Union
Australia including Oceania
Actualization world wide of Topographic maps 1:25,000, average 20 years
Actualization world wide of Topographic maps
1:50,000, average 45 years
Actualization of Topographic maps 1:25,000 and
1:50,000 in Europe, average between 7 and 15 years
Actualization of Topographic maps 1:25,000 and
1:50,000 in Africa and Latin America, average
more than 50 years
Actualization world wide
If you were walking away from the transmitter, you would walk through many cycles of the repeating pattern. You would walk through a single cycle of the pattern when it repeated itself just once. A single cycle of the wave is indicated by the green line. The distance walked through a single cycle is called the wavelength, and is 2 cm in the example in the picture, represented by the blue line.
The phase of the wave is the total number of cycles of the wave at any given distance from the transmitter, including the fractional part. Therefore, the phase at any given distance from the transmitter is given by the distance divided by the wave length:
The electronic strength of the transmitted signal is shown on the y-axis, and the distance from the transmitter is shown on the x-axis. The signal is seen to oscillate, or exactly repeat itself over and over again along the x-axis.
phase (in cycles) = distance from transmitter / wavelength (1)
At the first peak of the wave (0.5 cm on the x-axis), the phase is 1/4 cycle. At the 1-cm mark, the phase is 1/2 cycle. At the 3-cm mark on the x-axis, the phase of the wave is 1.5 cycles. Therefore:
distance from transmitter = phase (nr. of cycles) * wavelength (2)
When a radar signal is transmitted from the Shuttle and hits a target on the Earth, part of the signal is reflected back toward the Shuttle. A receiver on the Shuttle measures the strength of the reflected wave, and that strength, when plotted versus distance from the target, would look much like the figure below.
The Shuttle has two receivers separated by a fairly big distance (60 m in the case of SRTM). The two receivers are said to be at the ends of the "interferometric baseline." An interferometer measures the difference in phase between two signals received at the ends of a baseline, as shown in the figure. The interferometer accomplishes the phase differencing by comparing the signals at the two ends of the baseline by a signal-processing technique called "complex cross correlation."
This phase difference is called the "interferometric phase."
Because each received phase depends on the distance between the receiver and the target, the interferometric phase is a measurement of the DIFFERENCE between the distances from each receiver to the target.
To see how radar interferometry is sensitive to topography (height of the target), the figure shows two different targets at two different heights. It can be seen that the differential distance of each of these targets between the ends of the baseline depends on the height of the target. For the higher target (target 2), the differential distance is greater than for the lower one (target 1). The interferometric phase for target 2 is therefore larger than that for target 1. The differential distance gets larger as the incidence angle (theta_1 < theta_2) to the target gets larger. The interferometric phase can be related to the incidence angle by :
interferometric phase = B sin(theta) / wave length (3)
B is the baseline
Data format: 16-bit signed integer
Reference origin: Southwest corner
Bathymetric info of reservoir
(by sounding) integrated in DTM
(oblique view with ASTER FCC)
Good correlation between
GPS field measurements
and SRTM-elevation values
when compared for areaswithout major vegetation influences
No bathymetric info!!
Mosaic of 40 (1 by 1 degree) tiles
Black areas are data voids, due to shadowing, phase unwrapping anomalies, other radar specific and environmental causes, such as the low backscatter especially over open water.
Modification through interpolation of the undefined values
Land cover correction factor:
- 2 m
- 5 m
- 10 m
Satellite image classification of vegetated areas
DEM processed using drainage
and vegetation correction factors to produce “hydrological correct” DEM
A drainage network can be generated after DEM pre-processing and flow accumulation are performed. Using different accumulation thresholds, different drainage “scales” can be derived.
The Wetness Index sets catchment area in relation to the slope gradient. This is basically the famous w = ln ( As / tan ( ß ) ). Gives an idea of the spatial distribution and zones of saturation or variable sources for runoff generation.
Stream power is the product of catchment area and slope and could be used to identify places where soil conservation measures that reduce the effect of concentrated surface runoff could be installed.
The Sediment transport (LS) factor accounts for the effect of topography on erosion. Here the two-dimensional catchment area is used instead of the one-dimensional slope length factor as in the USLE.