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I'm using QGIS 1.8 and i need to analyze possible routes for roman roads. Is there a Possibility to show routes which comply special criteria, e.g. straightest way between two points, or path which is not higher than 50m... Since i search for possible roman roads, there are no streets with which QGIS can work. My idea is, it should analyze the map by my own criteria to find the routes. Is there any Plugin or function to do that?

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This sounds like a job for raster cost path analysis. I must admit I am more familiar with doing this sort of analysis in Arc (and here is a link to describe the process in generic enough terms so you can apply the knowledge to QGIS.)

I believe that in QGIS you will have to use GRASS. There are a selection of tools that look promising including r.cost, r.walk and r.spreaddistance depending on your exact use-case.

Having quantified the "cost" of getting from A to B you can then determine the likelihood of the road following your route and it would be especially useful to discuss your findings with an archaeologist

I recommend that you read up on cost path analysis first as you have quite a few criteria to consider and you will need to develop your cost surface(s).

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