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I am hoping to identify sections of a trail system which are close enough to potentially add sediments to wetlands downhill.

To this end, I would like to select polygons (wetlands) that are within a given distance (say 50m) below/downhill of polylines (trails) based on a 10m DEM raster.

Is there a way to buffer a line downhill a given distance? Or alternatively, is there a way to select by location in 3D? Any thoughts on potential workflows?

I am hoping that once I find a solution, I would be able to incorporate this process into model using ModelBuilder.

I am using ArcMap 10.2, have access to Spatial Analyst, 3D Analyst and most other tools.

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    This type of thing is easier if you use raster analysis, try to find a DEM of the area (a raster) to use as your elevation data, rather than contour lines (vector). – Dan C May 29 '16 at 21:55
  • Yes, I should have added that I have a 10m DEM that I intend to use. – aemdschej May 30 '16 at 0:27
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    There is an edit button beneath your question that you can use to revise it with requested and any other details at any time. – PolyGeo May 30 '16 at 7:06
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My first thoughts when reading this would be to maybe incorporate some kind of conditional cost distance calculation from the trails with the inverse altitude change as being the cost. This would mean that you would have to convert the trails to a raster and from each cell of the trail raster, calculate the distance which can be traveled without reaching a maximum cost (maximum negative altitude change) of over 50m.

I then found following link, which seems far simpler, explaining how to create vertical buffers, which is, as I understand, what you would like to accomplish. : http://www.donmeltz.com/generating-vertical-buffers/

Once you have the buffers in raster format, you can check for overlaps with your wetland polygons (maybe by converting the wetland polygons to a raster and checking for overlaps cell by cell or converting the buffer raster to polygons and doing a "touch" or "in" or "overlap" query with the buffer polygons and the wetland polygons).

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