masking a polygon to mask everything outside the polygon hidden...!

12-13-2012 06:49 AM
New Contributor
I am a student.  I am trying to create a mask of a county, so that it will mask all other features including rasters, points and polygons that are not within the internal boundary area of the county.  I have tried most of the masking tools and what ends ups happening is that the masked polygon hides the area within and not the outside area.  I can get the extraction masking tool to work but when I    do this tool it does not mask the other polygons, lines and points.
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2 Replies
MVP Honored Contributor
This might be what you're looking for.
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New Contributor

The easiest way I've found is this:

1) Draw a white (or gray) colored square larger than the area you'll be mapping. The county that you are interested in should be beneath this square.

2) With the white square selected, go to 'Drawing' menu on drawing toolbar and choose 'Convert Graphics To Features'.

3) From analysis toolbox, search for 'Symmetrical Difference' then apply the tool to the county layer and your newly made square-shaped layer. This will create a new layer that is your white square layer minus the shape of the county; ie. a square mask with a hole that is exactly the shape of the county. If you move this square mask to be on top of all other layers, everything will be covered except features within the county that will show through the hole.

4) With this new mask layer placed on top of all other layers, right click on the mask layer and choose Properties > Display. Adjust transparency to reveal the layers underneath.

5) Adjust the transparency, and also adjust the colors of other layers to achieve the effect you want.

6) If you have labels, they will show above the mask. The only way for labels to go under the mask is to change them into annotations.

This is a neat trick to give emphasis to features within a county, city or other polygon, while allowing features outside to still come through. I like using this to give context of surrounding area and road network.

(I know this is replying to an old thread, but might be useful to someone else. I'm also posting here as a requirement of a MS-GIST course...)