[OSRM-talk] calculation of jump distances

[Sayer, Bryan]

Hi Daniel,

Thanks for the advice. We use the Stata implementation on a secure network so I can’t try any of the usual options that are available over the internet. I will double check that I have the order (longitude, latitude) but I have over 450 million routes all over the USA (every populated tract to every hospital, with certain restrictions in Hawaii and Alaska), so I imagine if I had them reversed I would have really strange results.

I can check the area size of the tracts. I suppose some tracts might not have any roads, but generally tracts are defined by roads or natural elements like rivers. I only use tracts with non-zero population, so it seems like every tract I use should have a road in it. It seems to me that it should never be the case that the jump distance from the tract centroid to the starting point should exceed the largest dimension of the tract, or really one-half of that distance.

It is not a large number of routes with these large jump distances, just a few.

When you say “the first thing that happens is that the nearest point on the road network is found” how is the nearest road network found? That is, what is the algorithm? Does it spiral out from the point until a road is hit?


Bryan Sayer
Statistician
Social & Scientific Systems, Inc.
Monday-Friday 9:30 to 5:30
(301) 628-1576
https://www.s-3.com/


From: Daniel Patterson [mailto:daniel at mapbox.com]
Sent: Tuesday, January 16, 2018 5:24 PM
To: Mailing list to discuss Project OSRM <osrm-talk at openstreetmap.org>
Subject: Re: [OSRM-talk] calculation of jump distances

Hi Bryan,

  OSRM stores the road network in memory.  When you supply a coordinate to start/finish a route, the first thing that happens is that the nearest point on the road network is found.  Routing then happens from those "snapped" points.

  If you've got big rural areas, and you're using large regional centroids, then I suspect the snapping is starting or finishing your routes on roads you don't expect.

  Several hundred KM is pretty weird though, unless your road network is *really* sparse.  Do you have your coordinate the correct way around in your requests?  The order should be <longitude>,<latitude> for every pair - getting this wrong is often the source of really weird results.

  Try making a single request with `overview=full&geometries=geojson`, then plot the full route geometry on http://geojson.io/ or something to see if it even looks reasonable.

daniel

On Tue, Jan 16, 2018 at 1:43 PM, Sayer, Bryan <BSayer at s-3.com<mailto:BSayer at s-3.com>> wrote:

Hi,



We are calculating distances between an U.S. census tract centroid and hospitals. A tract averages about 4,000 people, but can vary in area. Obviously, the centroid is likely to not be on a street, and thus a jump distance has to be calculated to get to a street.



Our question is what is the general algorithm for getting to the starting point? We definitely end up with some very large numbers (several hundred kilometers) on some jump distances, which seems incorrect.



Bryan Sayer

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