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I have a floodplain layer with elevations assigned to it. Per scope of the project, I need to create a safety setback. The flood elevation is raised by 2 ft. and when the raised point extended horizontally to the ground becomes the setback. How do I create this setback/buffer?

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    can you define 'setback' nearest point to dry land from the edge of the flood plain? – Mapperz♦ Dec 11 '14 at 21:54
  • Welcome to GIS SE! Can you use the edit button beneath your question to do the above but also to include the version of ArcGIS for Desktop that you are using, please? – PolyGeo♦ Dec 11 '14 at 22:28
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You didn't specify a software requirement for your solution (although there is an ArcGIS tag on the question) so I've provided a method below based on the free and open-source GIS Whitebox Geospatial Analysis Tools, for which I am a developer.

Whitebox GAT has a tool called Elevation Above Stream (see image below):

enter image description here

For a given DEM and rasterized stream network...

enter image description here

...the tool will calculate the elevation difference between each location and its nearest downslope stream cell:

enter image description here

You can then threshold using the Raster Calculator, Reclass tool, Less Than tool, or Conditional Evaluation tool. In this example I've simply pulled out each cell in the elevation above stream raster with a value less than 2 ft and vectorized the resulting polygons:

enter image description here

There are your setbacks! I should also note that while I used a stream network as the target in this example, you could easily replace the required input streams file to the tool with a rasterized version of a floodplain polygon. It would work equally well, but would redefine the output as elevation above the floodplain. In fact, this is nearly the exact application that inspired me to develop this tool in the first place. Good luck and let me know if you need any further details.

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    +1. Great answer. I wouldn't delete your answer even if an Arc solution is required. This may be useful to other users who are trying to do the same thing. And WhiteboxGAT is a great program. – Fezter♦ Dec 12 '14 at 0:52
  • @Fezter Thanks for the feedback. It's well appreciated. – WhiteboxDev Dec 12 '14 at 12:36

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