Colormaps and non supported raster types?

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07-28-2020 10:12 AM
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JamesAmato
New Contributor III

Downloading GeoTIFF topo maps from: http://libremap.org. When I open these raster's in ArcMap they have a colormap under the Symbology tab in Properties that I use to make the colored image black and white (not grey scale) for the purpose of a basemap. The site doesn't have a specific 24k quad that I needed, so I downloaded the topo from The National Map Data Download. I was able to find the same year topo that I was looking for (1992) that uses NAD27 GCS but when I open it in ArcMap, there is no colormap option under the Symbology tab (just Vector Field, Stretched, and RGB Composite).

I understand that colormaps are only supported on some 1-band raster types. Wondering if there is a way to convert a raster to a file type that supports colormaps or is there a way to convert from RGB Composite to a colormap? I know I can add 1-band of a 3-band raster which allows more options under the Symbology tab (Vector Field, Unique Values, Classified, Stretched, and Discrete Color), but I am still not able to have the functionality of a colormap. I know there is an Add Colormap tool, but that doesn't work. 

Using ArcGIS Desktop 10.7.1. I have attached low res screenshots of what I start with and what I am trying to end up with. Links to topos below. Any input would be greatly appreciated. Thanks!

Link to GeoTIFF 24k topo that has a colormap and functions as needed

Link to GeoTIFF 24k topo that does not have a colormap that I need to somehow convert 

 

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PeterBecker
Esri Regular Contributor

Suggest you use Raster functions. First use Grayscale to (virtually) convert the 24bit image to an 8bit image. You can define different weights for each color. If this does not provide the required separation of colors consider also adding other functions such as 'Color Model Conversion' before Greyscale. Then add Colormap raster function in which you can define the required colormap. Leave as a layer, else export.  I can't confirm that this all works on 10.7.1, but it should. 

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JamesAmato
New Contributor III

Thank you for your help, using raster functions did the trick. First used the grayscale function, and then add colormap function, exported the map, and then symbolized the raster using classified with two classes with the break between black and white somewhere in the 215-220.

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