Clipping Rasters - Part C - Reverse-Clipping on a raster (aka Masking)

NOTE: you may want to read and understand the previous 2 clipping blogs before reading this one.
Raster Clipping - Part A - Better Raster Clipping options in ArcGIS
Raster Clipping – Part B - Using an irregular shape to clip a raster dataset

At the 2009 ESRI User Conference, many users were asking if it's possible to do a “reverse-clip” on a raster dataset.  So instead of clipping out the data that you want to keep, you take the middle out of the raster dataset. 

There is no tool that actually can perform this at the moment, but we can use a workflow to achieve the same result.

At ArcGIS 9.3, a raster dataset can be clipped based on the geometry of any feature or graphic.  That means if your feature or graphic has a “hole” in the middle, then the raster will follow suit and ignore that area in your output clip.

Therefore, if you have a feature or graphic with the area already excluded, then you are set to perform your clip.  Chances are that you do not have such a feature already created, so we will cover the steps to create such a graphic.  Once the graphic is created, it can then be converted to a feature.

Creating a graphic with a hole in the middle

Most users that want to perform such a task usually want to exclude some data from the middle of the raster.  In our example we will exclude the lake area in the middle of the raster dataset.

1. In ArcMap, display the raster dataset.
2. Make sure that the Draw toolbar is turned on.
3. Use the Polygon tool to create the area that you would like excluded from your output.



4. Use the Rectangle or Polygon tool to outline the area that you want included in your output.
5. Since your second graphic is overlapping the original graphic you used, right-click on your graphic, point to Order, and click on Send to Back.



6. Now that you can see both of your graphics, use the Select Elements tool to select both of them. The Shift key will allow you to select more than one element at a time.



7. Click the Drawing menu, point to Graphics Operation, and then click Remove Overlap.

Voila! You have a graphic that covers the area you need. Now you can convert this graphic to a feature so that you can use it to clip the original raster.

Convert your graphic to a feature

1. With your graphic selected, click on the Drawing menu, and click on Convert Graphics to Features.
2. Navigate to the folder or geodatabase that you would like to store the feature in, and give it a name.
3. Click OK, to persist the feature.

And finally, you are able to use this feature within the Clip tool.  Remember to check the “Use input features for clipping geometry” checkbox.

Here is the result of clipping the orginal raster using that feature: