I have a series of adjacent rasters which comprise a digital elevation model. Unfortunately they have been given to me as PNG files, rather than Esri Grids. The only key that I have to the actual elevation values is a separate legend PNG.

Each PNG file has an inbuilt colormap file, and they display correctly when drawn in ArcMap using the automated Colormap symbology option. The red line in this screenshot shows the boundary between two adjacent images:

enter image description here

However, Identifying within the dark green sections of each image shows that while the colors match (a,r,g,b = 255,0,225,0 on both the upper and lower images) the Color Index values do not match.

This means that if I try to Mosaic these images, there are discrepancies between the values. I'm using the Mosaic To New Raster tool with the Match Colormap option, but this seems to match the Color Index values rather than the colors(?).

Exporting the colormaps and comparing them in a text editor confirms that the Color Index vs a,r,g,b values do not match between the images, and that there isn't a simple shift between the values in each image:

enter image description here

Does anyone have any suggestions on how I can normalize (if that's the correct term) these adjacent images so they are using the same values? I can then mosaic and reclassify this single image to match with the legend.

(A cumbersome workaround would be to reclassify each individual image based on the legend, but I'd like to avoid this as it'll be hard to scale).


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