We have a flat file of 50 Geology maps that have a transparent overlay with faults and drill holes. I have them scanned and georeferenced as TIF images and they will be used in ArcGIS 10.0, 10.1 and Geosoft Oasis Montaj.
To scan the overlay (which is mostly transparent with black circles and lines depicting the features) I needed to tape it to a white sheet, otherwise the scanner wouldn't recognize that there was anything there. As a result, the image has a light gray background. To see the Geology map and the overlay together, the overlay can be displayed as transparent, but that washes out the color of the features too (they turn a lighter grey instead of black).
I don't konw how to work well with raster images, so could someone tell me how I can go about turning light gray background of the overlay to a different value? I want it to show up as nodata in Arc, but I'm not sure how to grab a range of cell values and change them to a different value. Thanks.
btw, I have access to Spatial Analyst as well, but I don't know enough about rasters to know where to start.