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For my Bachelor's thesis I try to reconstruct a road betweeen two ancient cities.

One method I'd like to try is least cost path analysis with QGIS. I've worked with QGIS before, but mostly only basic stuff. Can someone explain how least cost path analysis works? I know there are already a lot of threads on this, but I don't really understand.

I downloaded a DEM layer (SRTM satellite data) and transformed it into a slope layer and an elevation layer. Then I used the plugin called "Least Cost Path" on both, but got different results (which isn't suprising I think).

I'd like to take both, slope and elevation, into account for my least cost analysis. How can I combine these two layers? Is there anything else I have to do or any other layers I need?

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It seems you're mostly confused about how to define cost. I highly recommend this post by Amanda Briney on GIS Lounge, especially the Requirements for Least Cost Path Analysis section.

Cost can be a function of time, distance or other criteria that is defined by the user.

"The user" in this context is you. You need to figure out your criteria for cost. Start by answering this question:

How will elevation and slope interact to determine how difficult it is to travel across each grid square?

Once you answer that question in a conceptual way, you need to put numbers to it. Create a cost raster, where each raster cell has a value representing the cost of travelling across that cell.

The cost raster is the raster that defines the cost or other impedance (defined by the user) to move through each raster cell. A cost raster has three characteristics:

  • The first is that the cost of each cell is the sum of the different costs while
  • the second is that the cost can represent either the actual or the relative cost (Chang).
  • The third characteristic is that the cost factors can be weighted depending on the importance of each factor (Chang). For example if travel time is considered a more important factor than the monetary cost, it can be it can be weighted more in the analysis.

Once you figure out how to calculate cost, use the raster calculator to combine your slope and elevation rasters.

(Quotes are from the linked post, with emphasis and punctuation added by me.)

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