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I need to generate isochrones considering emergency vehicle’s speed. I have checked several packages in R and different plugins in QGIS. I found QNEAT3 (Iso-Area as polygons) as the only tool that the speed can be configured in but I am not sure if this plugin is the appropriate one in my case or not.

Selecting optimization criteria is mandatory in QNEAT3. It means that I have to choose shortest path or fastest path, while I just need to generate isochrones and it should be created using a feasible path for an emergency vehicle and I am not looking for routing optimization. My question here is that how does QNEAT3 works concerning one of these optimization criteria? Does it consider road network for creating iso-Area polygons or the iso-area polygons which QNEAT3 creates maybe be created through paths which a car could not traverse?

Another question is what is the difference between default speed and speed field?

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Yes, QNEAT3 calculates isochrones based on the underlying input street network. The isochrones are based on shortest paths from the source point to all vertices (nodes) of the graph that can be reached in the defined size value of the iso area (either defined as distance or time (seconds) value). You can lock streets for the routing by applying a definition query in QGIS (eg. filter the streets) or setting their speed value to 0.

Regarding your question on default speed and speed field. You can either define a default speed that is used for all graph edges (eg. streets) the time based routing (fastest path) or you can add edge specific speed values inside a field in your graph (measured in KM/H if your coordinate system is in Meters, otherwise MPH, etc.) and use this field for the fastest path calculation.

  • Thanks for your answer. I am new to QGIS. Could you please explain more about how to lock streets for routing in QGIS? or if you know a QGIS documentation including this explanation could you please share it? – Elham Nov 4 '20 at 17:29
  • You can exclude them via definition query (select the layer with a mouse click and press ctrl+f) then enter your SQL selection string (eg. "your_speed_field" != 0). This selects streets with a non zero speed (eg excludes locked streets from routing). – root676 Nov 5 '20 at 8:21
  • How the algorithm identify to use which of the columns data from the attribute table as the maximum speed? Should the dataset has a specific column name? – Elham Nov 13 '20 at 13:43
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    In the Advanced Parameters section you can find a parameter called Speed field [optional]. Use this parameter to supply the algorithm with the speed values stored in your dataset for each street. – root676 Nov 14 '20 at 14:03

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