Different isochrones when specifying multiple travel time ranges vs just one range

openrouteservice isochrones
1

Hi,

I’m using the QGIS ORS tools plugin via docker. Running into a strange issue where when I use the isochrones from point layer I’m getting different isochrones results when I run the plugin with “foot-walking” & times of “15, 30, 45, 60” vs just “foot-walking” & one time of “15” (this happens for other times as well). Is the algorithm doing something differently if it gets multiple ranges vs just one time to calculate? It seems to only happen in places where there’s multiple points close together causing overlapping isochrones.

Here is 30 min from (“foot-walking” & “30 min” only)

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image1155×878 88.3 KB

Here is 30 min from ("foot-walking & “15, 30, 45, 60”)

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image1160×883 96.2 KB

*marked places where there’s a little bit extra; also the lower boundary of the top point almost touches the middle point (other discrepancies obviously but these are the clearest to see)

2

Hey,

there’s another (hard to find) thread with a similar problem, and some explanation in this and this issue.

In short, this is not a problem with the reachability analysis, but with the fact that these results (basically a point cloud) have to be translated to polygons somehow.
This will for example lead to deadlocks being seemingly excluded from the isochrone, e.g. in the northwest corner of your example.

If you want more detailed isochrones, have a look at the isochrone smoothing factor (not available via qgis).

Best regards

1 Like
3

Thank you so much for the prompt and detailed response @jschnell