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# How to clip polyline with z value where it would intersect DEM value?

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04-05-2015 12:56 PM
New Contributor

I hope the question is pretty straightforward.  What would be the workflow if I had a polyline with a certain z value that represents an elevation and I wanted to clip it where it's value is greater than the underlying DEM?   Imagine, if you will, a polyline drawn across a river valley, for instance.  If it had an value of 800 ft, I would like to clip it to length where the DEM values are less than the polyline z value....essentially creating an elevation top width.

Thanks,

Gene

6 Replies
by
Frequent Contributor

Here's one idea: If it's not a very large analysis area, convert your DEM to an integer, and then use raster to polygon conversion, using the value field. Intersect these polygons with your polyline. Once you have the polygon elevation values in your polyline table, you can add a field called "elev_diff" perhaps, and then calculate that field by subtracting the polyline elevation from the polygon elevation. After this, select all polylines where that value is negative, and delete! Viola!

New Contributor

Thank you for your idea Sephe.  This seems like a good approach and I will investigate it.

Occasional Contributor III

Try following steps:

1. use Interpolate shape tool to extract the values of dem to polyline

2. Split the output polyline Useing split line at vertices tool

3. Use calculate geometry to have dem values in a field.

4. Use spatial join to join the elevation values from original polyline for comparison

5. Use field calculater to compare the extracted dem values and joined elevation from polyline

Or use them for further analysis.

Hope this helps

Kishor

New Contributor

Kishor,

I am also going to look at your approach.  If the workflow seems plausible, I should be able to do this with python scripting.

New Contributor

To expand upon this.  I have a DEM of a river valley and I know the depth of water at many locations on the river (decreasing as I go downstream).  These would be point values on the river.  What I would like to do is come up with a polygon of depth of water that would represent the area under water, based off the DEM and know water depth.  Essentially this would be a very crude inundation map, without advanced hydraulic analysis.  The polygon is a hatched area representing the inundation area.

by
Frequent Contributor

I'm not sure how feasible your idea is, just because DEMs don't usually contain bathymetry information, ie. the elevation under the water. Some sort of spatial analysis of your DEM might work better, perhaps looking for the breakpoint where the DEM becomes flat at the surface of the water, and then using Con, or a Reclass to separate out just those water areas, and converting those to polygons. Just a thought.