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:
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.