I need to export color orthophoto map to gray scale geotiff file using free tools (like qgis) any idea?
I tried qgis preview but this does not produce new geotiff file
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See also http://www.johndcook.com/blog/2009/08/24/algorithms-convert-color-grayscale/ – AndreJ Jan 08 '15 at 06:52
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In QGIS you could use the r.composite module from GRASS in the Processing toolbox. Alternatively you could use the raster calculator to average across the bands, emphasis a particular band over the others, or just export a single band.
You specifically mention GeoTiff, but if you have a world file, you could simply open the tiff in Photoshop or GIMP and change the mode to grey-scale and save as a new file (to retain the original), then copy the tfw file and rename to match the new grey-scale tiff.
MappaGnosis
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How can I do it with command line tool so I can run a script on bunch of maps – Ala Mar 10 '15 at 21:35
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Read the documentation. Also have a look here and here. A quick search will yield many other examples. – MappaGnosis Mar 11 '15 at 07:13