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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. Bathymetric_GeoTIFFBathymetric_GeoTIFF_scale

The RGB color scheme loads into QGIS without any associated elevations, just values that seem to represent the hues used in each of the 3 bands of data. Band Rendering window for river bathymetry data

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.

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    I know of no way to reverse-render a DEM, asking the agency the data came from for a copy of the original is the only way I know of. – Michael Stimson Mar 17 '16 at 21:57
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    The only way I can think to do this is by working through creating layers for each elevation class one elevation class at a time and then add the resulting layers together at the end.You would have to separate the bands out as individual files.For class 23 (for example) the level of yellow may be unique to that class and you can just do a reclass or you might need to use the values in 2 or 3 of the bands and perform some masking to get the common areas. – Sethinacan Mar 21 '16 at 16:37

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