The Landsat program, started in 1972, is one of the mainstays of NASA's earth-observation program. Landsat 7 is the most recent satellite of the Landsat mission and was launched on April 15, 1999. The Landsat satellites were designed to be of use to a variety of fields like forestry, agriculture, geology, and land-use planning, and the choice of spectral bands for the Landsat satellites was geared toward discriminating different types and amounts of vegetation. Landsat's strengths are generally seen to be it's regular acquisition schedule (revisits each spot on the earth every 16 days), long-term data archive (image with comparable specifications is available from 1982), and relatively rich spectral information (not as rich as hyperspectral data, but more bands than most high-resolution satellites like Ikonos or Quickbird). Limitations of Landsat data (excluding the problem with the scan-line corrector described below) are that it is only a moderate-resolution image source (30m multispectral data, 15m panchromatic), and the fixed acquisition schedule makes it sometimes difficult to acquire imagery for a particular place at a particular time (especially important if clouds or smoke are frequent).
Landsat 7 continues to operate and provide imagery, however, on May 31, 2003, the satellite's scan-line corrector failed. The scan-line corrector is a device on the satellite that keeps the scan lines parallel to each other. Without the scan-line corrector, the scan lines are mis-aligned and there are wedge-shaped data gaps in the image (see sample image below). USGS offers a number of different procedures for filling-in the data gaps, but each amounts to using data from good images prior to 2003 to do so. Obviously the further one gets from 2003, the less valid this approach will be. Landsat Thematic Mapper 4 and 5 (TM4 and TM5) continues to operate and collect imagery with the same specifications as Landsat 7 (minus the panchromatic band), and it is a good alternative to Landsat 7 at this point. Because Landsat data are integral to many mapping and planning projects and one of the most popular image sources, NASA has plans to launch a new Landsat satellite in 2012 under the Landsat Data Continuity Mission Operational Land Imager.
Landsat 7, like the Landsat 4 and 5 satellites, has six 30m bands that cover the visible and near infrared and short-wave infrared regions of the electromagnetic spectrum and one 60m thermal infrared band. Landsat 7, however, added an 8th, higher-resolution panchromatic band. The Landsat satellites were designed to be broadly applicable to fields like forestry, geology, agriculture, and regional planning, so the band locations and widths were picked to be sensitive to changes in vegetation and land cover.
Landsat 7 data is delivered in scenes that measure 115mi (185km ) by 106mi (170km).
Landsat 7 is on the WRS-2 orbit path and revisits the same spot on the earth every 16 days. Because Landsat 7 is in a sun-synchronous orbit, it crosses the equator between 9:30am and 10:00am each day.
Landsat 7 was launched in April 1999 and continues to operate. Images from 1999 to May 31, 2003 (when the SLC failed) are called SLC-on images. SLC-off images are available from May 31, 2003 to present, and can be ordered “as-is” or with an option to “fill” the SLC data gaps with pre-2003 imagery.
As of October 2008, all Landsat 7 archived imagery and new acquisitions are free. You can search for, order, and download Landsat 7 data from a number of sources including: the USGS Global Visualization Viewer GLOVIS, the USGS http://earthexplorer.usgs.gov/, or Landsat.org
Format and delivery options for Landsat 7 imagery varies with where you order and download the imagery. Images are commonly shipped as TIFF image files - one image for each band. Alternatively, data may come in USGS' HDF format.
Landsat imagery is one of the most commonly used remote-sensing products for rangeland applications. These are just a few of the many examples:
Landsat imagery is one of the most ubiquitous satellite image types, and a lot of effort has gone into making it easy to access and use. For the most part, Landsat imagery can be used in ArcGIS or other GIS applications without any special processing. Most Landsat images obtained either through USGS or another provider are distributed in TIFF image format with one band per file. For the purposes of making it easier to handle, manipulate, and display the imagery, most people combine all of the separate image bands into a single, multi-band image file using an image-processing package like ENVI or ERDAS Imagine.
Landsat images are not terribly big or difficult to process by today's computing standards. In a TIFF image format, the file for a single 30m band is approximately 53MB. As long as you are dealing with a study area that is contained within one or a few Landsat scenes, you should not need any special computer hardware to be able to use Landsat imagery.
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