I am trying to create polygons around rivers that extend all the way up to the top of the surrounding slopes.

I am currently able to do this using stream data and contour data, but I would have to digitize it by hand. Because this would be a very time consuming process, I was wondering if there was an automated process of identifying ridges within ArcGIS or QGIS. I have attached two figures to show what I mean.

In Figure 1, you can see the contour lines surrounding the river. From this, it is very clear from the contours that there is a steep slope on either side of the river.

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In Figure 2, I have filled in the area with red showing the desired polygon I would like to make.

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Does anyone know if the software can identify these areas of steep elevation change using contour data and create a polygon of the area between them? Or, would my only choice be to digitize?

1 Answer 1


In ArcGIS, you could make a raster surface model using topoToRaster, than use Slope to represent your raster, then use RasterToPolygon to make your steep areas and your relatively flat areas into polygons and then delete the relatively flat areas that do not have a watercourse in their midst or that are not circumscribed between steep slopes. There will be manual editing, but it might save you some time!

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