Isochrone layers are not accurate


#1

Hi all,

i’m new to this forum…
I’ve been a QGIS user for a while and I regularly use the ORS Tools. However recently the output looks somewhat funny. As shown in the photo below the 10 minutes cycle catchment extent is somewhat bigger than the 20 minutes cycle catchment. Has this happened to anyone before?


#2

That is to do with the geometry generation, which is dependent on the cost limit. In this case, apparently there is no cycling possible in this dent you’re describing. It’s a little confusing and we’re aiming to get rid of this behavior, but it’s not worrying too much. Just ignore these little issues for now.


#3

Thank you for responding so swiftly - much appreciated.

The dent I have highlighted does show that cycling is possible (i.e. see the 10 mins cycle catchment) within the area (circled in blue) albeit GIS does suggest that cycling on water is possible… I fail to understand how it is possible that you could reach a greater distance within a 10 minute cycle journey time, than within a longer cycle journey time of 20 minutes?

These little issues with the cycling catchment isochrones display incorrect information that is misleading and contradictory to what is actually possible.


#4

albeit GIS does suggest that cycling on water is possible

What does that mean? Apparently bikes are not allowed to go over the water (in OSM!). You can find out on https://openstreetmap.org

Do you know how isochrones are calculated? If not, I recommend reading up a little. Basically, it gives you a point cloud from routing calculations and to get a polygon you have to create a Concave Hull. The latter is dependent on a few parameters, one of them being a minimum distance below which it doesn’t look for nearer nodes (very abstracted explanation). So, for different ranges of isochrones in our current implementation this parameter changes. That leads rarely to more or less inconsistent behaviour. That’s all there is. The routing is of course fine, it’s just the polygon/concave hull generation. Not a thing to worry about, it’s rare and obviously not very impactful.