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,
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.