Hi Daniel,
Thanks for the link to the documentation! I had indeed not seen that -
it was very helpful.
I'm a little unclear though on what the difference between "weight" and
speed and length. Is "weight" equivalent to the "cost" of traversing an
edge, which would presumably be a function of speed and length? Or does
a higher weight give preference to an edge over others?
And what happens if I have an edge with a high weight and a low speed or
vice versa? i.e. how do weight and speed (and duration?) interact in the
selection of a shortest path? Are these all names for the same thing?
Thanks,
Nate Wessel
Jack of all trades, Master of Geography
SAUSy Lab <http://sausy.ca>, Sid Smith Hall
On 05/22/2017 08:28 AM, Daniel Hofmann wrote:
Actually we base map matching on the distance of _the fastest path_ here:
https://github.com/Project-OSRM/osrm-backend/blob/a63b43c259d49fc4915810eb4bfdd86bc78bce93/src/engine/routing_algorithms/map_matching.cpp#L220-L226
I got confused a bit this morning because `getNetworkDistance` does
this two levels of indirection approach.
So if you set edge weights in the profiles accordingly (based on if a
way is a transit-relevant way or not) orsm will find the fastest route
and then does map matching on it based on its distance.
Hope that helps
On Mon, May 22, 2017 at 10:38 AM, Daniel Hofmann <hofm...@mapbox.com
<mailto:hofm...@mapbox.com>> wrote:
Did you see the profile docs here? Granted they could be more
detailed but should get you started. If you have anything to add
we're always happy for pull requests :)
https://github.com/Project-OSRM/osrm-backend/blob/master/docs/profiles.md
<https://github.com/Project-OSRM/osrm-backend/blob/master/docs/profiles.md>
What I can suggest is to
- always provide timestamps otherwise defaults will be used internally
- investigate the map matching's tidy option for blob removal
(think: waiting at a traffic signal)
Unfortunately map matching is distance based so modifying edge
weights or speeds in the profiles won't help.
And completely discarding ways won't help you either since you
specifically want to capture detours.
Maybe have a look at Figure 7 in the map matching paper "Hidden
Markov Map Matching Through Noise and Sparseness. And then just
give it a try with your data and see how it goes.
There are probably other ways to do your analysis, e.g. map match
your traces on a regular profile and then compare the results
against a osrm route query on a transit-ish profile.
For this transit-ish profile you could set low weights (note:
modify weights, keep speeds for correct ETAs) except for actual
transit routes. I don't know if the transit data is good enough
for this, though.
Interested in what you find out.
On Mon, May 22, 2017 at 2:02 AM, Nate Wessel
<nate.wes...@mail.utoronto.ca
<mailto:nate.wes...@mail.utoronto.ca>> wrote:
Hi all,
I have recently been using OSRM to do map-matching for transit
vehicles, with locations from historical GPS data. The
documentation on creating profiles is pretty minimal from what
I can tell, so I've just been able to make a few modifications
to the car.lua profile to allow vehicles to go on streetcar
tracks, agency service roads, etc.
The data I've been playing with so far has a ~20second
sampling frequency, which has worked well, but I'm about to
start working with data that has a lower sampling rate
(30-60secs) and I'm worried that I'll get a lot of spurious
matches. I was wondering if it is possible to modify the
profile such that a strong preference is given to known
transit routes (which are tagged with route relations in OSM).
Transit almost always follows a known route, but sometimes it
detours, and it's important to my research to be able to
capture that in matched GPS data.
What might be a good way to add this to a routing profile?
Does everything in the profile come down to speed, access, and
node-related penalties? Would I want to artificially inflate
the "speed" of transit-served streets? Is there more thorough
documentation on profiles somewhere than what is provided here
<https://github.com/Project-OSRM/osrm-backend/wiki/Profiles>?
The profile I've been using is:
https://github.com/SAUSy-Lab/retro-gtfs/blob/master/etc/ttc.lua
<https://github.com/SAUSy-Lab/retro-gtfs/blob/master/etc/ttc.lua>
Any help would be greatly appreciated!
--
Nate Wessel
Jack of all trades, Master of Geography
SAUSy Lab <http://sausy.ca>, Sid Smith Hall
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