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Question:

What could be causing the "seamline" in a mosaic of multiple images?

Answer:

In order to obtain a "seamless" mosaic, radiance (and preferably reflectance) may need to be calculated before performing the mosaic, because gain changes may occur in one or more of the scenes. When two scenes have different gain states, they have different dynamic ranges, and there will be a shift in Digital Number (DN) values from one scene to the next. This can be determined by reviewing the included metadata file for each scene.

Some imaging software packages have a utility that can automatically create the radiance image, based on the gain/bias in the header file. If this is not available, a manual "band math function" may be necessary.

Visit the Landsat Level-1 Conversion Equations web page for more information, or access the following documents: 

Lansdat 7 Data Users Handbook

Landsat 8 Data Users Handbook