U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3095

November 2007

Land Remote Sensing

Ghana Watershed Prototype Products

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 Products

Landsat products include recently acquired Landsat 7 

(http://landsat.usgs.gov) satellite data as well as data from the 

Tri-Decadal Global Landsat Orthorectified (http://eros.usgs.

gov/products/satellite/landsat_ortho_overview.html) data collection (fig. 1).

Landsat 7 developed problems with the sensor’s scan line 

corrector, a system designed to collect data while compensating for the forward motion of the satellite. Gaps in the resulting images were “filled” for the Ghana Watershed products by 

special processing using data values from previously acquired 

images over the area. For more information, please see the 

Landsat 7 SLC-off Gap-filled product description (http://

landsat.usgs.gov/data_products/slc_off_data_products/l1g_

products/gap-filled_l1g.php).

The Tri-Decadal Global Landsat Orthorectified data collection consists of approximately 7,500 Landsat 1–5 Multispectral 

Scanner (MSS) images, 7,461 Landsat 4–5 Thematic Mapper 

(TM) images, and approximately 8,500 Landsat 7 Enhanced 

Thematic Mapper Plus (ETM+) images, which were selected 

to provide full sets of global coverage over three time periods: 

circa 1975, circa 1990, and circa 2000. All selected images 

are either cloud-free or contain minimal cloud cover and have 

a high quality ranking with regard to the possible presence of 

errors such as missing scans or saturated bands.

Landsat MSS, TM, and ETM+ data were orthorectified by 

using geodetic and elevation control data to ensure positional 

accuracy and correct for relief displacement.

ASTER Products

The processed ASTER radiometrically and geometrically 

corrected (Level-1B) product contains co-registered data for the 

acquired channels of the three different telescopes of unprocessed (Level-1A) data (fig. 2).

Figure 1.  Tri-Decadal data for southern Ghana from 1970,

1990, and 2000.

Figure 2.  ASTER image of southern Ghana.

SRTM 90-Meter Products

Digital topographic data were acquired by the Shuttle 

Radar Topography Mission (SRTM) over 80 percent of the 

Earth’s land surface (all land areas between 60 degrees north 

and 56 degrees south latitude). From that data, SRTM Digital 

Terrain Elevation Data (DTED) were processed to represent 

elevation values indexed to specific points on the ground 

(fig. 3).

Figure 3.  SRTM 90-meter data for southern Ghana.
Aspect

Shaded Relief

Flow Accumulation

Flow Direction

Hillshade

Slope Degree

Slope Percent

Drainage Basins

Pour Points

Synthetic Streams

SRTM 30-Meter Derivative Products

SRTM data at 1 arc-second (30 meters) are currently available for the United States and its territories. SRTM data at 3 arcsecond (90 meters) are currently available for global coverage 

between 60 degrees north and 56 degrees south latitude. The 

data are expressed in geographic coordinates (latitude/

longitude) and are horizontally and vertically referenced to 

the Earth Gravitational Model 1996 (EGM96) geoid. Several 

30-meter derivative products (fig. 4), made available through an 

arrangement with a USGS science project, are produced from 

the SRTM 30-meter data set.

Figure 4.  SRTM 30-meter derivative data for southern Ghana.

For More Information

For additional information on Ghana Watershed Prototype 

Products and ordering procedures, visit http://gisdata.usgs.

gov/gwpp/.

Landsat Products

http://landsat.usgs.gov

http://landsat.usgs.gov/data_products/slc_off_data_products/

l1g_products/gap-filled_l1g.php

Tri-Decadal Global Landsat Orthorectified

http://eros.usgs.gov/products/satellite/landsat_ortho_overview.

html

ASTER

http://LPDAAC.usgs.gov

For additional information on USGS Center for 

Earth Resources Observation and Science (EROS) 

products and ordering procedures, visit 

http://eros.usgs.gov/ or contact USGS EROS at:

Customer Services

U.S. Geological Survey

Center for Earth Resources Observation and Science 

(EROS)

47914 252nd Street

Sioux Falls, SD 57198-0001

E-mail: lta@usgs.gov