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I have a tif image of soil thorium content (ppm) measured by gamma aerial spectroscopy. The original file is a 8 bit RGB image set in a rainbow-similiar pallet (blue>green>yellow>red>magenta). I have a jpg file about the legend, which was attached with the original file. The thorium content ranges from 2 (blue) to 36 ppm (Fig. 1).

enter image description here

For modelling purposes, I want to convert this RGB file to a single raster of thorium content. I have already tested the a processing module of QGIS:

Raster > Conversion > RGB to PCT

The module transformed to a single band, but it produced a palleted band containing a unsorted color ramp (Fig. 2)

Fig. 2

Could I sort the color ramp and reclassify the raster values to match with the original values from the jpg legend? If it is possible, how could I sort the colors?

Is there another way to transform the RGB image to a gray-scale single band?

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