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I am working on a project where I need to identify the area of a hilltop that is relatively flat within which stone structures could be built before the slope becomes too great to build without having to modify/terrace the hill.

Specifically, I am doing a project with known pre-historic settlements, all of which are located on hilltops or spurs. I am trying to get an estimate of settlement size, assuming the inhabitants only built structures following the natural topography. This isn't always the case - there is evidence of intensive terracing in some areas, but this is relatively uncommon and only exists in the largest settlements. In all others (following physical visits to 10% of the total number of settlements I am working with), the limits of the settlement follow the topography - once it starts to become too steep to build houses, a wall was built around the settlement.

I have been trying to think through how I could use GIS to identify the settlement extent for settlements that have never been formally studied following what I have seen. I have a relatively accurate DEM with 1 arc-second horizontal resolution. I am using ArcGIS 10.5.

I am thinking Topographic Position Index will be my best bet, but have had issues that I think are connected with neighborhood size and have never quite figured this out, despite trying to use various TPI tools.

Or perhaps TPI is more complex than what I need? Perhaps I could just generate slope lines and identify the line where the slope reaches a certain point and identify the point at which the slope goes over a certain value (e.g. 10%)?

  • Fellow archaeologist checking in. What software are you using? QGIS, ArcGIS, etc? I think you're right that slope is the simplest way to get at what you want, and it's relatively straightforward to identify areas with slope below a certain threshold and create outlines of those zones. If you give a little more info, I can help you get pointed in the right direction. – lambertj Apr 22 at 15:45
  • I am using ArcGIS Desktop Advanced 10.5. I have a license for all the extensions included in the All Extension Bundle – Jackson Apr 25 at 16:24
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    I would also suggest running a fill on your DEM to remove any sinks and peaks which may be artifacts in your DEM. You can set a z- value threshold. This essentially smooths your data. – AWGIS Apr 26 at 15:07
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Here is one potential approach to identifying potential settlement limits based on slope:

  1. Run Spatial Analyst > Slope tool, using your DEM as input.
  2. Use Spatial Analyst > Reclassify, with your slope raster as input. Classify the raster into two new classes, setting areas with slope > 10% to one value and < 10% slope to a second value (e.g., 0, 1).
  3. Run Conversion > Raster to Polygon to extract boundaries for your potential settlement areas that have slope < 10% (or whatever threshold you set in the Reclassify step). Use your reclassified slope raster as the input. It's up to you whether you check Simplify Polygons. The results will look cleaner (i.e., polys won't have jagged edges from the input raster cells), so try it both ways and see what you like better.

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