1 DEM from Active Sensors – Shuttle Radar Topographic Mission (SRTM) Ben Maathuis, WRS-2004, Koert Sijmons IT/RSG/GTS Bali, Indonesia

2 SRTM (Shuttle Radar Topography Mission) 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

3 SRTM (Shuttle Radar Topographic Mission) The SRTM 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

4 SRTM (Shuttle Radar Topography Mission) DEMs with a 90 meter resolution can be down loaded free of charge from the Internet

5 SRTM (Shuttle Radar Topography Mission) 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 2004

6 l Knowledge of surface topography is of major importance to Earth Sciences, e.g. hydrology, geomorphology, but: 1: : : : Africa Asia Australia Europe N. America S. America Availability of Topographic Maps (%) Source: CNES, Paris/Toulouse 1997

7 World-wide status of Topographic maps (1997) * Former Sovjet Union Australia including Oceania

8 Actualization world wide 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 Africa and Latin America, average more than 50 years Actualization of Topographic maps 1:25,000 and 1:50,000 in Europe, average between 7 and 15 years

9 World-wide actualization status of Topographic maps, 1997

10 Although topographic contours supply fairly accurate information about elevation and slopes, being derived from an interpolation between precisely determined reference points (bench marks), the elevation is always estimated and stated to the nearest meter. The need for generalization in topographic maps results in a loss of detail and accuracy Present problems

11 Modern instruments like electronic theodolites and satellite navigation receivers (GPS) provide point measurements and generating terrain maps is a time consuming and costly process Aerial and space borne stereoscopic images produce wide area coverage using photogrammetric principles but are limited by the need for good visibility (and logistics of aircraft operations) Present problems

12 Integration of topographic data from different sources results in inhomogeneous data quality due to: different horizontal and vertical datums, map projections, formats, resolutions, etc Impossible to assess the accuracy of the resulting derived products Present problems

13 SRTM – advantages Homogeneity: The SRTM DEM is the first continuous large scale product that has not been mosaiced from data derived from different sensors, formats and dates (11 days mission) Resolution improvement: Compared to the only existing global DEM of 1 km. horizontal resolution (USGS) the present available SRTM DEM’s at 90 meter resolution, with a relative vertical accuracy of less than 10 meters, offers a great improvement

14 Availability and coverage: the SRTM data is not classified, data in C-band cover nearly 80 % of the Earth’s surface, home to 95 % of all humans (60 degree North to 56 degree South latitudes) Data (90 m. resolution) are basically for free, can be downloaded from the internet: l ftp://edcsgsg.cr.usgs.gov/pub/data/srtm/ – (1 degree by 1 degree cells, continent wise, for free) – (seamless, small areas for free, larger areas are charged) SRTM – advantages

15 SRTM – advantages

16 SRTM – advantages

17 The Mission - System Space Shuttle Endeavour, launch , 11 days mission, with modified radar instrument, called Spaceborne Radar Laboratory, Interferometric SAR, a 60 m. mast and X (3.1 cm) and C band (5.3 cm) antenna

18 Radar beam swath width of 225 kilometers across from an orbit altitude of 233 kilometers Day, night and all weather independent The Mission - System

19 Radar Interferometry The technique to generate three-dimensional images from the Earth’s surface. A transmit antenna illuminates the terrain with a radar beam which is scattered by the surface. Two receive antennas with a fixed separation between them (baseline) record the backscattered radar echo from slightly different positions

20 Radar Interferometry 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. 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: phase (in cycles) = distance from transmitter / wavelength (1)

21 Radar Interferometry 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)

22 Radar Interferometry 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.

23 Radar Interferometry 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.

24 Radar Interferometry 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

25 The two signals received at both ends of the baseline show a phase shift due to different signal paths. Through the calculation of the relationship between target-receiver distances and the phase difference one obtains elevation information Radar Interferometry

26 SRTM elevation products C-band Spatial resolution90 * 90 m Horizontal datumWGS 84 Vertical datumWGS 84 ellipsoid Physical unitsmeters Grid size (Lat-Long)3 * 3 arcsec Horizontal accuracy (90% circular error) Absolute< 60 meters Relative< 45 meters Vertical accuracy (90% linear error) Absolute< 16 meters Relative< 10 meters Geometric specifications Accuracy specifications Data format: 16-bit signed integer Reference origin: Southwest corner

27 SRTM data quality 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 areas without major vegetation influences No bathymetric info!!

28 Problems of SRTM use 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. Mosaic of 40 (1 by 1 degree) tiles 16S/59W 21S/53W

29 SRTM-DTM modification Modification through interpolation of the undefined values

30 SRTM - DEM modification ILWIS hydro functions A B C DEM optimization

31 SRTM - DTM modification Land cover correction factor: Satellite image classification of vegetated areas - 2 m - 5 m - 10 m

32 SRTM-Processing results Original DEM DEM processed using drainage and vegetation correction factors to produce “hydrological correct” DEM

33 SRTM-DTM additional parameters 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.