# [OSRM-talk] 'penalty' for snapping

Steven M. Ottens steven at minst.net
Tue Apr 19 22:00:55 UTC 2016

```Hi Daniel,

Your idea is pretty much what I used for my travel time map:
http://stvno.github.io/page/bartholomap/ though N was easier to
determine: nearest non-stop airport, nearest one stop airport etc.
For overland speed: I like the wiggle factor Bjorn was suggesting,
however it supposes we know the landuse, which as far as I know OSRM
doesn't take into account
For N if we can have a default of 4 on separate roads that would
probably help a lot already without too high a cost. (separate roads
being at least one fork in the road away) Does that sound reasonable?

I guess the strategy to determine the N points could be made
configurable, different use cases require different strategies I'd say.

Steven

On 4/19/2016 4:44 PM, Daniel Patterson wrote:
> Hi Steve,
>
>   I was thinking about exactly this just the other day when
> considering https://github.com/Project-OSRM/osrm-backend/issues/2277
>
>   What I came up with was roughly:
>
>     Find N nearest neighbours, and calculate the walking duration to each
>     Insert all neighbours into the search heap, and set the initial
> cost to the walk duration
>     Route as normal, but include the walk duration in the total time
> at the end
>
>   The question is, how big do we make N, and how do we determine the
> overland speed?  Overland speed I suppose could be a profile
> parameter, and it feels like walking speed would usually be the most
> appropriate thing.  But I'm not sure how many possible start points we
> should reasonably consider. Everything within a radius?  Closest N points?
>
> daniel
>
> On Tue, Apr 19, 2016 at 12:39 PM, Steven M. Ottens <steven at minst.net
> <mailto:steven at minst.net>> wrote:
>
>     Hi all,
>
>     I am working on a tool that will calculate the travel time from
>     villages to for instance the nearest hospital for rural
>     accessibility analysis. So far I have build a tool which will
>     calculate this for 215.000 villages in a few minutes, thanks to
>     the help of Daniel P. However I realized that not all villages are
>     on a road, actually quite a few are at least an hour walk from the
>     http://stvno.github.io/img/OSRM-snapping.png
>     The villages are typically connected to the roads by informal
>     trails, which I do not have as a road network. So I was wondering
>     if it would be possible to add a basic penalty to the distance of
>     the point to the nearest road.
>     I am aware that the nearest service will provide me with the
>     distance:
>     https://github.com/Project-OSRM/osrm-backend/wiki/New-Server-api#service-nearest
>     but before I am going to calculate the extra traveltime for each
>     village, I was wondering if it is possible to integrate this with
>     an osrm.table request. I am aware this opens quite a can of worms:
>     if there is a penalty to snapping, it might be worth it to snap to
>     a road which is further away, but in the end quicker to get to the
>     destination. But for the moment I'm happy with just the added
>     traveltime for nearest road snapping.
>
>     Any suggestions on how to handle this?
>
>     Steven
>
>
>
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>
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