Removing Null Values

Question

I want to mosaic Landsat 8 scenes but the final product has dark areas on overlaps and on the background of each scene. Help to remove the dark areas.

Answer 1

You can use the open-source GIS Whitebox GAT (http://www.uoguelph.ca/~hydrogeo/Whitebox/) to do this. Import the bands of Landsat data using the 'Import GeoTIFF' tool then use the 'Set NoData Value' tool to change the background to no-data. You can create a colour composite and then I recommend using the Balance Contrast Enhancement with Landsat imagery to improve the image quality. You can then mosaic the various images using either the Mosaic tool or the Mosaic With Feathering tool. (Full disclosure, I am the lead developer of Whitebox.)

Here is an example of a band 7-6-4 colour composite of Landsat 8 data created using this method:

Answer 2

You need to assign the dark color (value) to the NO DATA / TRANSPARENT value/flag.

What you need can be done in various GIS applications using either classify functions (which change the back value to a new one) or by assigning a different color in the color table (which will not change the value).

In my opinion is better to use the reclassify functions. For that you need to identify the color/value with the info tool, and pass it to the NO DATA value.

In ArcGIS you need Spatial Analyst / Reclass / Reclassify, in GRASS GIS you need r.mapcalc, in QuantumGIS you need Raster Calculator (see here) and in SAGA GIS you need the Grid - Tools / Reclassify.

For color table solution see here.

Cheers, Mihai