How to export a polygon based on elevations using a DTM or a DEM

How to export a polygon based on elevations using a DTM or a DEM

I have a LiDAR generated DTM and a DEM (accurate to 10x10 feet). I am trying to export a polygon within a boundary which includes all elevations below a specific number. Seems like this should be simple but I am having a hard time. Ideally, I would like this to be accurate to 0.5 ft.

Also, I no longer have a 3D analyst license. If that is the only way to deal with the DTM, I may procure a new license. I'm using ArcMap 10.2.2.

Note that both methods below will export non smoothed polygons boundaries. You may have to resample your DEM file to get smoother lines or use further smoothing techniques.

Step 1: Spatial Analyst (Classification)

You could use Spatial Analyst to classify your data in 2 classes: 1 = data within the boundary values (example : elevation < X meter) 0 = other data

Step 1 (alternative): Spatial Analyst (Conditional tool)

You could also use the Con Tool (Conditional)

Step 2: Spatial Analyst (Raster to polygon)

You will then have an integer raster you can convert to polygon with the Raster to Polygon Tool.

Step 1: QGIS (Classification)

If you cannot have ESRI's extension, you can alternatively do a classification opération with a formula in QGIS:

  • Load your raster layer
  • Click on the Raster menu/Raster Calculator
  • Type in the following formula replacing MyRaster by your data and ElevationMax by the maximum value):

    ([email protected] < ElevationMax ) * 1 + ([email protected] >= ElevationMax )*0

This codes allow you to create a new raster with 2 values (0/1).

Step 2: QGIS (Raster to polygon)

With that, you can convert it to polygon by going in the Processing panel:

  • Click on GDAL/OGR, then Conversion, and use thePolygonize (raster to vector)tool.

Convex HULL

Last method I can suggest :

  • Filter your source LIDAR file to keep only points of interest.
  • Create a Convex Hull for the remaining points (lastools, 3D analyst LIDAR tools, etc)

f I understand correctly you only want to create polygon boundary that surrounds elevations that are below a certain value…

Assuming you also have 3D analyst/ Spatial Analyst extensions, there are probably several different ways to do this. Here is the Cut/Fill Tool technique but it involves several steps: I

(The Cut Fill tool but this requires raster as input and also outputs raster.)

  1. Create a raster with a constant elevation of your desired "below"

  2. Run the Cut Fill tool

  3. Convert resulting raster to polygons

  4. Net Gain (Fill) is your polygons "below" and Net Loss (Cut) is your polygons "above"

  5. Clip Fill polygons with your boundary.

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