‘Unboxing’ Landsat L1 image archive

[rahmansunbeam]

We have all seen, downloaded and used Landsat satellite imageries. Landsat, starting from its journey on july 23, 1972 is the longest running enterprise for collecting satellite images of the earth. Researchers around the globe know Landsat for its vast pool of information zipped in an archive and free to download anytime from internet. In this post let me go through the contents inside a level 1 image archive when you download one.

 

When we download an image archive from USGS or USGS Glovis website from level 1 section, we use to download small section of its image archive stretching 170 km north-south by 183 km east-west.

 

In the box

Landsat images we download are not the direct product from the satellites, these images use to go through several revision, rectification and reorganization took place. In a archive you will meet these files,

 

 

Naming convention

The product name for different Landsat product varies for each set. For ETM+ scenes the names are like this,

 

[Landsat-7 mission][ETM+ data format][path] [starting row]_[ending row][year][month][day]_B[bandnumber].TIF.gz

 

Example: L71129032_03220060815_B10.TIF.gz, where

• L7 refers to the Landsat-7 mission, followed by “1” which refers to the ETM+ data format.
• 129 is the path of the scene, followed by 032_032 which are the starting and ending rows of the scene.
• The acquisition data is 2006-08-15, represented as 20060815.
• The band number for this file is B10 (band1). Other bands in the file may be referred to as B20 (band2), B30 (band3) and so on. B61 means band 6 low gain, B62 is band 6 high gain.

For TM product it will be TM in the beginning, for OLI it is L8.The metadata for ETM+ scenes collection ends with ‘_MTL.TIF’ or ‘_MTL.txt’ extensions.

 

The Metadata file

Opening the _MTL.txt file with a text editor will show you all the information you need to start processing image. Within the group PRODUCT_METADATA you will find several declarations,

• The group of product origin, creation and initialization information
• The group of image metadata, within this group we find,
  • PRODUCT_TYPE = “L1T”, this is the level 1 terrain corrected product
  • ELEVATION_SOURCE = “GLS2000″, source of elevation data
  • PROCESSING_SOFTWARE = “LPGS_11.1.0″, software used to process
  • EPHEMERIS_TYPE = “DEFINITIVE”, Definitive Ephemeris is used for geometrically correcting Landsat data, and provides improved accuracy over predicted ephemeris
  • SPACECRAFT_ID = “Landsat5″, of course…
  • SENSOR_ID = “TM”, the Landsat thematic mapper sensor
  • SENSOR_MODE = “BUMPER”, in early 2002, the TM instrument onboard Landsat 5 lost synchronization between the scan mirror and calibration shutter, resulting in “caterpillar tracks” on imagery. In order to fix this problem, the USGS switched the TM instrument from the scan angle monitor (SAM) mode to backup “bumper” mirror mode in order to extend the useful life of the TM instrument.
  • ACQUISITION_DATE = 2010-01-30, the data the image was took
  • SCENE_CENTER_SCAN_TIME = 04:15:40.9110630Z, time associated with the center of a scene center scan in World Reference System
  • WRS_PATH = 137, path in World Reference System
  • STARTING_ROW = 44, the row where it starts
  • ENDING_ROW = 44, the row where it ends
  • BAND_COMBINATION = “1234567”, combination of bands with numbers
  • Upper-left, upper-right, lower-left, lower-right latitude and longitude for georeferencing
  • Designation of other file name in the same archive
• Group of minimum and maximum radiance
• Group on minimum and maximum pixel value in the range of 0-255
• Group of product parameter with gain and bias information for each band, sun azimuth, sun elevation and output format
• Group of correction information for different technical and physical errors
• Group of projection parameter

This metadata format of Landsat 7 image of 2010, but if was not always like this. Here are changes you will see in the _MTL file time to time.

 

Landsat processing standards in brief

There are two type of processing system to shape the standard products,

Level 1 Product Generation System (LPGS) – currently, all Landsat data is processed through LPGS. LPGS metadata is contained in a MTL.txt file and have these parameters,

• GeoTIFF output format
• Cubic Convolution (CC) resampling method
• 30-meter (TM, ETM+) and 60-meter (MSS) pixel size (reflective bands)
• Universal Transverse Mercator (UTM) map projection (Polar Stereographic projection for scenes with a center latitude greater than or equal to -63.0 degrees)
• World Geodetic System (WGS) 84 datum
• MAP (North-up) image orientation

There are three types of Level 1 processing

• Standard Terrain Correction (Level 1T) – provides systematic radiometric and geometric accuracy by incorporating ground control points while employing a Digital Elevation Model (DEM) for topographic accuracy.
• Systematic Terrain Correction (Level 1Gt) – provides systematic, radiometric, and geometric accuracy, while employing a Digital Elevation Model (DEM) for topographic accuracy.
• Systematic Correction (Level 1G) – provides systematic radiometric and geometric accuracy, which is derived from data collected by the sensor and spacecraft.

Another processing standard is the National Land Archive Production System (NLAPS). There are some available Landsat 4 and Landsat 5 data were processed through NLAPS. NLAPS metadata is contained in a .WO file, which accompanies all data files.

While the products generated by the LPGS and NLAPS systems are similar, there are geometric, radiometric and data format differences.

• Geometric differences, where both systems align the bands to the center of each pixel. Before December 2008, LPGS aligned the bands to the center of each pixel, and NLAPS aligned bands to the edge of each pixel.
• Radiometric Differences, where both system scale Level 1 products to a range of 1-254. DN values of 0 are reserved for scan gap and flag fill. DN values of 255 are reserved for saturation.
• Data Format Differences, where both system uses GeoTIFF format. But The 60 meter and 15 meter NLAPS and LPGS products may have different image sizes.
• A very small number of Landsat TM scenes are processed using the National Land Archive Production System (NLAPS).

Other files

Some Landsat TM scenes include a Work Order (.WO) file that contains the metadata information about scenes processed on the National Land Archive Production System (NLAPS), common for Landsat 5 archives.

 

A file containing the ground control points (GCP) used during image processing is also included with Landsat MSS and TM data for controlled georeferencing.

 

Landsat 8 scenes include a Quality Assessment (QA) band file. Used effectively, the bits of the Quality Band improve the integrity of science investigations by indicating which pixels might be affected by instrument artifacts or subject to cloud contamination.

 

Landsat MSS and TM scenes also include a Verify Image File (VER.jpg), which displays a grid of verification points in various colors that represent the accuracy of geometric correction. Cross-correlation techniques based on the GLS 2000 dataset are used as reference. This graphic representation of the Geometric Verify Report (VER.txt) assists users in determining the geometric accuracy of each MSS and TM scene.

 

Landsat ETM+ SLC-off scenes also include Gap Mask files for each band. These ancillary data allow users identify the location of all pixels affected by the original data gaps in the primary SLC-off scene. Gap mask is a set of flat binary scan gap mask file which comes one per band.

 

The README file contains a summary and brief description of the file contents and naming convention. This file contains further clarification of files and metadata of the archive contents, a general documentation and contact information.

 

Also published here, 

https://clubgis.net/unboxing-landsat-l1-image-archive/