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Crop Agriculture

Landsat Background

Related pages: Landsat bands    Field location relative to Landsat scene extent    About Crop Imaging

 

Landsat imagery is free. Currently, Landsat 8 and Landsat 7 ETM+ are operational and acquiring imagery; Landsat 7, however, has stripings of missing data due to a scan line corrector (SLC) instrument problem in May of 2003. Landsat 8 had successful launch in February of 2013. Landsat 8 data are based on a 12-bit dynamic range but are delivered as 16-bit images; Landsat 8 DNs have a maximum value of 65,535 (however, 55,000 corresponds to one hundred percent reflectance without factoring in a cosine of the solar zenith correction) as opposed to 255 for previous Landsat satellites. Landsat imagery has 30 x 30 meter resolution in most bands (4.5 pixels per acre), and a 16-day revisit cycle. Landsat imagery dates back to July of 1982 (Landsat 4 TM) so historical assessments of a field can be made. The amount of imagery available is a function of whether or not a particular field is in a Landsat scene overlap zone and how much clear imagery there is of a particular field at the correct time of the growing season to sense crop condition and predict yield well enough.

Current and historical images are shown below from Landsat 5 Thematic Mapper (TM) or Landsat 7 Enhanced Thematic Mapper Plus (ETM+) satellites. Landsat 8 imagery can be viewed here. Landsat imagery is free, dates back to July of 1982 (Landsat 4 TM) so historical assessments can be made. Landsat has 30 x 30 meter pixels which equates to 4.5 pixels per acre and a 16-day revisit cycle. A Landsat image (scene) is about 185 (e-w) x 175 km (n-s) (115 x 106 miles) and has overlap areas, particularly on the east and west sides, so fields fortuitously located in an overlap area have more imagery available. Presently, Landsat 8 (February 2013 launch) and 7 ETM+ are operational. In 2011, Landsat 5 TM (the longest operating Earth-orbiting satellite) experienced a problem and stopped providing imagery; the satellite provided data for 27 years which was 24 years more than it was designed for. Although Landsat 5 TM imagery has been suspended, past data is available and has many uses. Landsat 8 data is based on a 12-bit dynamic range but are delivered as 16-bit images and have a maximum digital number (DN) of 65,535; DNs are calibrated radiance values (radiance is the amount of radiation emitted from a surface). Landsat 5 and 7 DNs have an 8-bit range (maximum value of 255). Reflectance (total radiation emitted from a surface / total incoming radiation that strikes a surface) can be calculated from DNs; depending on the application, reflectance may or may not be necessary. See the Atmospheric Correction Guide and Landsat 8 Atmospheric Correction pages for conversion to reflectance information.

Below is Landsat 7 ETM+ NIR imagery on 7/31/05 with missing data shown. Landsat 7 developed a problem in 2003 that causes striping of missing data; the stripings starts near the middle and get wider towards the sides.

Landsat 7 satellite imagery missing data stripings

 

Below is eastern edge of Landsat 7 NIR image (same image as above)

Landsat 7 satellite imagery missing data stripings

 

The boundaries of pixels for Landsat 4, 5, 7, and 8 are in the same location and do not change. The positional accuracy of the data can be off somewhat and is inconsistent in the amount and direction of the error, so the data within the pixel boundary may represent an area that is not precisely within the extent of the pixel. For example, a pixel may actually represent data starting 10 meters in and direction. Sometimes there are obvious situations where Landsat imagery needs to be moved to improve positional accuracy, but this uncommon.