How to Select Ridgelines From a DEM Using Spatial Analyst

Caitlin Dempsey


Selecting out ridgelines from a DEM using ArcView 3x’s Spatial Analyst extension is pretty straightforward. These steps assume the initial DEM is called “elev”.

Time Required: 10 minutes

Here’s How:

  • Compute a flow direction grid – a grid with codes indicating for each cell which of the eight possible directions the flow would go in from that cell: ([elev].flowdirection(false))
  • Compute a flow accumulation grid from the flow direction grid; a flow accumulation grid has values in each cell showing how many cells are upstream from it: ([Map Calculation 1].flowaccumulation(nil))
  • Find all the places where the flow accumulation is zero – these will be ridgelines and hilltops – places with no other points upstream of them; use the Map Query tool: ([Map Calculation 2] = 0.asgrid)
  • The resulting grid, automatically named ‘Map Query 1’, will have ones (trues) wherever the flow accumulation is zero – these are your ridgelines – and zeros (falses) elsewhere.


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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.