Geographic Information in Satellite Images – Where and What Kind of Geographic Information Is Stored in Satellite Images?

geotiff-tifflandsatqgis

I downloaded the landsat8 image tif file, and I tried to overlap it with Google map in QGIS.

The match between landsat and Google map means that the landsat image contains geographic information, but does it contain coordinate values for all pixels? If not, what geographic information is stored somewhere?

Best Answer

The definition of the TIFF file format allows for the storage of metadata in the file with the actual image data. This could be used by photographers for example, to store the exposure and aperture settings for every picture taken by a camera.

The GeoTIFF standard extends this to geographic metadata, and defines metadata chunks that specify the origin point, pixel size, and coordinate system. So for example it might say the origin is at 2.4 degrees N, 54 degrees W, the pixels are 0.02 degrees square, and the coordinate system is WGS84 lat-long degrees.

So each individual pixel coordinate isn't there, just enough to layout the grid.

If you have the gdalinfo command you can view this metadata, or load into a GIS and use some sort of "properties" dialog.

Here's the info on a simple 1 degree global grid:

$ gdalinfo r.tif 
Driver: GTiff/GeoTIFF
Files: r.tif
Size is 360, 180
Coordinate System is:
GEOGCRS["WGS 84",
    DATUM["World Geodetic System 1984",
[etc]
    ID["EPSG",4326]]
Data axis to CRS axis mapping: 2,1
Origin = (-180.000000000000000,90.000000000000000)
Pixel Size = (1.000000000000000,-1.000000000000000)
Metadata:
  AREA_OR_POINT=Area
Image Structure Metadata:
  COMPRESSION=LZW
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (-180.0000000,  90.0000000) (180d 0' 0.00"W, 90d 0' 0.00"N)
Lower Left  (-180.0000000, -90.0000000) (180d 0' 0.00"W, 90d 0' 0.00"S)
Upper Right ( 180.0000000,  90.0000000) (180d 0' 0.00"E, 90d 0' 0.00"N)
Lower Right ( 180.0000000, -90.0000000) (180d 0' 0.00"E, 90d 0' 0.00"S)
Center      (   0.0000000,   0.0000000) (  0d 0' 0.01"E,  0d 0' 0.01"N)
Band 1 Block=360x5 Type=Float32, ColorInterp=Gray
  Min=1.000 Max=64800.000 
  Minimum=1.000, Maximum=64800.000, Mean=32400.500, StdDev=18706.293
  NoData Value=-3.39999999999999996e+38
  Metadata:
    STATISTICS_MAXIMUM=64800
    STATISTICS_MEAN=32400.5
    STATISTICS_MINIMUM=1
    STATISTICS_STDDEV=18706.293058754

Satellite data on non-regular grids can't be stored in GeoTIFF files (I think) and other formats are used, such as NetCDF, where every pixel can have its own X and Y coordinate. This is often seen in unprocessed satellite data where the satellite's sensor has scanned a grid of pixels, but its not a rectangle on the earth's surface. Imagery providers will warp images like this onto regular grids to provide users with easier-to-use data.

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