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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.

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you should have scanned them as 1 bit, bw. your gray background is probably not one pixel color. If it is then you just need to set that color as transparent. otherwise you should convert them to 1bit bw. right click, export data, choose the color depth, you will probably need to experiment and find a contrast setting that will keep data but remove the gray. –  Brad Nesom Mar 1 '13 at 17:46
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Thank you, your response led me to finding the Image Analysis Window. I've used Arc for other things for over a year, and never knew it was there. What I wanted to do is easier than I thought. –  cndnflyr Mar 1 '13 at 21:48
    
please add an answer with your solution and mark it correct –  Brad Nesom Mar 1 '13 at 21:51
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@Brad had the best idea if I was starting out from the beginning, scanning the maps as a 1 bit image. However, as they had already been scanned, the easiest solution was to bring them into Arc (I'm using 10.0) and use the Image Analysis Window to adjust the contrast. Pushing the contrast up to 80 or 90 gave the dark writing and symbols a value of 0 and the background a value of 255 (for the most part, because it was a scanned image, there were a few blemishes, but very few). Then save the image with the Image Analysis window. Once the image is polarized like that, when you bring it into Arc, go into the layer properties and set "Display background data: (255) as (NoData)". That way almost everything on this overlay is completely transparent, without washing the color out on the symbols and writing. Looks just like the originals.

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