I'm trying to merge some river bathymetry with LiDAR from the surrounding riverbed using QGIS 2.14.0 Essen. The river bathymetric data is a GeoTIFF, and it comes with a separate scale associating the colors with elevations in feet using the vertical datum NGVD 1929.
If the values in the Band Rendering window corresponded to the elevations associated with each color on the scale included with the bathymetry data (see below) instead of hues, I would simply use the Raster Calculator to add up the 3 bands and divide the sum by 3. The problem is that I have no idea how to associate the color values on the scale with their corresponding elevations.
I am seeking guidance about associating color values with the corresponding elevations on the scale. My ultimate goal is to get this bathymetry data in single-band greyscale like the DEM I intend to merge it with. Once the two sets are merged, I should have a DEM that provides elevation data above and below the river surface.