Ghana Watershed Prototype Project


Introduction/Background

A number of satellite data sets are available through the U.S. Geological Survey (USGS) for monitoring land surface features. Representative data sets include Landsat, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and Shuttle Radar Topography Mission (SRTM). The Ghana Watershed Prototype Products cover an area within southern Ghana, Africa, and include examples of the aforementioned data sets along with sample SRTM derivative data sets.

      Landsat 7 arrow

    Description:

    This is a collection of Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery. ETM+ is a multispectral scanning radiometer on board the Landsat 7 satellite. The ETM+ instrument provides image data from eight spectral bands (band designations). The spatial resolution is 30 meters for the visible and near-infrared bands (1–5 and 7). Resolution for the panchromatic band (8) is 15 meters, and the thermal infrared band (6) is 60 meters. Scene size is about 170 x 183 kilometers (106 x 115 miles).

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    The sensor has provided nearly continuous acquisitions since July 1999, with a 16-day repeat cycle. An instrument malfunctioned on May 31, 2003, with the result that all Landsat 7 scenes acquired since July 14, 2003, have been collected in "Scan Line Corrector (SLC)-off" mode.

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    Additional information:

    Landsat 7 data

      MSS, TM, ETM+, and ETM+ Pan-sharpened Tri-Decadal Scenes arrow

    Description:

    The Tri-Decadal Global Landsat Orthorectified data collection consists of a global set of high-quality, relatively cloud-free orthorectified MSS, TM, and ETM+ imagery from Landsats 1–5 and Landsat 7. This data set was selected and generated through NASA’s Commercial Remote Sensing Program as part of a cooperative effort between NASA and the commercial remote sensing community to provide users with access to quality-screened, high-resolution satellite images with global coverage over the Earth’s landmasses. The data collection was compiled in accordance with the NASA contract with Earth Satellite Corporation (Rockville, MD) in association with NASA’s Scientific Data Purchase program.

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    The Tri-Decadal Global Landsat Orthorectified data collection consists of about 7,500 MSS (Landsat 1–5), 7,500 TM (Landsat 4–5), and 8,500 ETM+ (Landsat 7) images, which were selected to provide global coverage over an approximate 25-year interval (circa 1975, circa 1990, and circa 2000). All selected images were either cloud-free or contained minimal cloud cover. Only images with a high-quality ranking (based on the possible presence of errors such as missing scans or saturated bands) were selected. The following data products are available from the Tri-Decadal Global Landsat Orthorectified data collection:
    • Tri-Decadal Global Landsat Orthorectified MSS
      The average acquisition date for the Tri-Decadal Global Landsat Orthorectified MSS data is the mid-1970s relative to a 1990 acquisition baseline for the Tri-Decadal Global Landsat Orthorectified TM images (+/– three years of the 1990 baseline). However, individual scenes range from 1972 to 1987. These Landsat data were orthorectified using geodetic and elevation control data to correct for positional accuracy and relief displacement. Large blocks of Landsat data were adjusted through a patented procedure that uses pixel correlation to acquire tie-points within the overlap area between adjacent Landsat images.
    • Tri-Decadal Global Landsat Orthorectified TM
      The average acquisition date for the Tri-Decadal Global Landsat Orthorectified TM data is 1990 (+/– three years). However, individual scenes range from 1985 to 1996. These Landsat data have been orthorectified using geodetic and elevation control data to correct for positional accuracy and relief displacement. Large blocks of Landsat data were adjusted through a patented procedure using pixel correlation to acquire tie-points within the overlap area between adjacent Landsat images.
    • Tri-Decadal Global Landsat Orthorectified ETM+
      The average acquisition date for the Tri-Decadal Global Landsat Orthorectified ETM+ data is 2000 (+/– one year). All scenes were acquired between 1999 and 2001. These data have been orthorectified for co-registration with the earlier (circa 1990) Landsat TM coverage.
    • Tri-Decadal Global Landsat Orthorectified ETM+ Pan-sharpened
      These data products consist of pan-sharpened versions of the ETM+ scenes above. The pan-sharpening process involved a resolution merging process, which allows the fusion of the higher resolution panchromatic band (8) with selected lower resolution multispectral bands (7, 4, and 2) to create a higher resolution (pan-sharpened) color image.
    All Tri-Decadal Global Landsat Orthorectified data products have been pre-processed according to a standardized set of processing parameters.

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    Additional information:

    ETM+, ETM+ Pan-sharpened, TM, and MSS Scenes

      ETM+ Pan-sharpened and TM Mosaics arrow

    Description:

    The Tri-Decadal Global Landsat TM Orthorectified and ETM+ Pan-sharpened Mosaics data collections are derived from a global set of high-quality, relatively cloud-free orthorectified TM and ETM+ imagery from Landsats 4, 5, and 7. This data set was selected and generated through NASA’s Commercial Remote Sensing Program, as part of a cooperative effort between NASA and the commercial remote sensing community to make available quality-screened, high-resolution satellite images with global coverage over the Earth’s landmasses. The data collection was compiled in accordance with the NASA contract with Earth Satellite Corporation, now MDA Federal Inc. (MDA), (Rockville, MD) in association with NASA’s Scientific Data Purchase program.

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    The collections are derived from about 7,450 TM (Landsat 4–5) images and about 8,500 ETM+ (Landsat 7) images, which were selected to provide a full set of global coverage (circa 1990 and circa 2000). All selected images are either cloud-free or contain minimal cloud cover. Only images with a high-quality ranking (based on the possible presence of errors such as missing scans or saturated bands) were selected.
    • Tri-Decadal Global Landsat Orthorectified TM Mosaic
      The average acquisition date for the Tri-Decadal Global Landsat Orthorectified TM data is 1990 (+/– three years). However, individual scenes range from 1985 to 1996. These Landsat data have been orthorectified using geodetic and elevation control data to correct for positional accuracy and relief displacement. Large blocks of Landsat data were adjusted through a patented procedure using pixel correlation to acquire tie-points within the overlap area between adjacent Landsat images. The Landsat TM scenes were mosaicked by matching individual histograms and feathering overlapping areas together to create a TM mosaic product. The mosaics are contrast-enhanced color image data products that use Landsat TM bands 7, 4, and 2 composited in red, green, and blue, respectively. The resulting mosaics are made up of several Landsat scenes covering a 5-degree (north-south) segment of a standard 6-degree (east-west) Universal Transverse Mercator (UTM) partition.
    • Tri-Decadal Global Landsat Orthorectified ETM+ Pan-sharpened Mosaic
      These data products consist of pan-sharpened versions of the ETM+ scenes. The pan-sharpening process involved a resolution merging process, which allows the fusion of the higher resolution panchromatic band (8) with selected lower resolution multispectral bands (7, 4, and 2) to create a higher resolution (pan-sharpened) color image.

    Data Access:


    Additional information:

    Orthorectified TM Mosaics data

      ASTER arrow

    Description:

    This is a collection of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery. ASTER provides 14 spectral bands with 15- to 90-meter resolution, depending on the bands.

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    ASTER is a cooperative effort between NASA, Japan’s Ministry of Economy, Trade and Industry (METI) and Japan’s Earth Remote Sensing Data Analysis Center (ERSDAC).

    ASTER is one of many instruments on board the Terra platform, which was launched in December 1999. ASTER does not acquire data continuously, and its sensors are activated only to collect specific scenes upon request.

    The instrument consists of three separate telescopes, each providing different spectral range and resolution. The Visible and Near Infrared (VNIR) sensor provides four bands at 15-meter resolution. The Shortwave Infrared (SWIR) sensor provides six bands at 30-meter resolution. The Thermal Infrared (TIR) sensor provides five bands at 90-meter resolution. The swath width for all sensors is 60 kilometers.

    ASTER data distributed from the Land Processes Distributed Active Archive Center is available in the Universal Transverse Mercator (UTM) projection, with a resampling method of cubic convolution for daytime scenes and nearest neighbor for nighttime scenes. Files are in HDF-EOS format.

    Information courtesy of USGS

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    Additional information:

    ASTER Coverage

      SRTM 3-Arc Second arrow

    Description:

    The Shuttle Radar Topography Mission (SRTM) is a partnership between NASA and the National Geospatial-Intelligence Agency (NGA). Flown aboard the space shuttle Endeavour (February 11–22, 2000), SRTM fulfilled its mission to map the world in three dimensions. The USGS is under agreement with NGA and NASA’s Jet Propulsion Laboratory to distribute the data. SRTM used dual Spaceborne Imaging Radar (SIR-C) and dual X-band Synthetic Aperture Radar (X-SAR) configured as a baseline interferometer to successfully collect data over 80 percent of the Earth’s land surface, everything between 60 degrees North and 56 degrees South latitude.

    SRTM data is being used to generate a digital topographic map of the Earth’s land surface with data points spaced every 1 arc second of latitude and longitude (about 30 meters) for the United States. The SRTM "finished" data meet the absolute horizontal and vertical accuracies of 20 meters (circular error at 90 percent confidence) and 16 meters (linear error at 90 percent confidence), respectively, as specified for the mission.

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    Additional information:

    SRTM Coverage

      SRTM Derivatives arrow
          Drainage Basins arrow

      Description:

      The Drainage Basins data set was derived using the Shuttle Radar Topography Mission (SRTM) Level 2 (30-m) resolution digital elevation model (DEM). The basins were seeded based on the basin delineation from HYDRO1k Elevation Derivative Database. These seed points can also be referred to as the lowest points in the drainage basin. There are six levels of basin delineation, and each polygon is attributed with a Pfafstetter code that uniquely identifies each sub-basin (HYDRO1k Documentation). The Pfafstetter codes in each sub-basin reflect the code in its respective HYDRO1k basin. Some basins may not have the same spatial extent or shape as the HYDRO1k basins, but the code was assigned because there were similar stream flow characteristics. An additional attribute was included that further identifies smaller sub-basins that and are most prevalent along the coast and were assigned arbitrary numbers to hold a unique identity.

      Data Access:

      Drainage Basins

          Pour Points arrow

          Synthetic Streams arrow

      Description:

      The stream line data layer is derived from the flow accumulation and flow direction layers. Cells with upstream drainage areas greater than 1.8 km2 are selected from the flow accumulation layer and processed through the STREAMLINK function, then followed by the STREAMORDER function. The resulting output from the STREAMORDER function assigns a numeric order to the grid representing branches of a linear network. The greater the value the higher the stream order or the greater the volume of water flowing down the stream. This linear network does not represent the true physical stream network; it is only a representation of the stream network based on the STRM Level 2 (30-meter) DEM. This difference is seen in areas where there is little or no slope. The data layers are often referred to as "synthetic streams" because of this fact.

      Data Access:

      Synthetic Streams

          Flow Accumulation arrow

      Description:

      The flow accumulation data layer defines the amount of upstream area draining into each cell. It is a measurement of the upstream catchment area. The flow direction layer is used to define which cells flow into the target cell. Since the cell size of the data set is 30 meters, the flow accumulation values can be translated into drainage areas in square meters. Values range from 0 at topographic highs to very large numbers (on the order of millions of cells) at the mouths of large rivers.

      Data Access:

      Flow Accumulation

          Aspect arrow

      Description:

      The aspect data set describes the direction of maximum rate of change in the elevations between each cell and its eight neighbors. It can be thought of as the slope direction. It is measured in positive integer degrees from 0 to 360, measured clockwise from the north. Aspects of cells of zero slope (flat areas) are assigned values of –1. Aspect data was derived from Shuttle Radar Topography Mission (SRTM) Level 2 (30-m) resolution.

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      Aspect

          Shaded Relief arrow

          Flow Direction arrow

      Description:

      The flow direction data layer defines the direction of flow from each cell in the DEM to its steepest down-slope neighbor. Values of flow direction vary from 1 to 255. Defined flow directions follow the convention adopted by ArcInfo’s flow direction implementation: 1, east; 2, southeast; 4, south; 8, southwest; 16, west; 32, northwest; 64, north; and 128, northeast. Cells with undefined direction of flow represent sinks and have flow directions that are simple combinations of its neighbors’ flow direction values. The Flow Direction data layer was derived from the Shuttle Radar Topography Mission (SRTM) Level 2 (30-m) resolution.

      Data Access:

      Flow Direction

          Hillshade arrow

          Slope arrow
      Slope Degree

      Description:

      The degree slope data layers describe the maximum change in the elevations between each cell and its eight neighbors. The slope is expressed in integer degrees of slope between 0 and 90. Slope data was derived from Shuttle Radar Topography Mission (SRTM) Level 2 (30-m) resolution.

      The percentage slope data layer describes the maximum change in the elevations between each cell and its eight neighbors. The slope is expressed in percent values and is a function of the rise divided by the run multiplied by 100 [(rise/run) * 100]. With a value of 0 for flat surfaces, the value increases as terrain becomes steeper, approaching infinity as the slope gets closer to a vertical line. Slope data was derived from Shuttle Radar Topography Mission (SRTM) Level 2 (30-m) resolution.

      Data Access:


Related Information
http://edcintl.cr.usgs.gov/iabin_datadownload.html

Tappan, G.G., Sall, M., Wood, E.C., Cushing, M, 2004. Ecoregions and land cover trends in Senegal. Journal of Arid Environments 59 (3), 427-462. Click here to read the article.

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