Thank you. This is very helpful. Artur…
From: Kerrick Staley [mailto:[email protected]] Sent: Monday, June 20, 2016 3:17 PM To: Mailing list to discuss Project OSRM <[email protected]> Subject: Re: [OSRM-talk] GPS Accuracy for match service The map-match algorithm's running time is O(radius^4 * samples), because the number of states (candidate segments) is proportional to radius^2 and the Viterbi running time is O(states^2 * samples). It's definitely a tricky balance between (really) poor performance and poor matching. On Mon, Jun 20, 2016 at 12:12 PM, Kerrick Staley <[email protected]<mailto:[email protected]>> wrote: FWIW we (Lyft) have found that using a query radius of 3x gps_precision is insufficient and causes bad matches; we use either 10x (which is what 4.x uses) or 5x depending on how big gps_precision is in the first place. (We patched 4.x to make the multiplier configurable by URL param but can't upstream because we're not on 5.x yet :( ). The change going from 10x to 3x is here<https://github.com/Project-OSRM/osrm-backend/commit/2ce74c05e16cd178a38c08080568b47b9ae4f08c>. On Mon, Jun 20, 2016 at 11:41 AM, Daniel Patterson <[email protected]<mailto:[email protected]>> wrote: Artur, The meaning is the same in both versions, "gps_precision" and "radiuses" are "the size of 1 standard deviation of accuracy". However, between 4.x and 5.x, the range that we check is a lot narrower. It turns out that in 4.x, the default was about 10x too large, which makes map-matching very slow because of the increased number of candidates that it needed to check. The smaller the radius you can use, the faster map-matching will be. There's no way to change the setting globally on the URL. You can change the code and re-compile OSRM if you want to modify the default. Not the best way to do it, but all we've got at the moment. daniel On Jun 20, 2016, at 11:24 AM, Artur Bialecki <[email protected]<mailto:[email protected]>> wrote: Hello, Thank you for the answer. Did the match algorithm change between V4.8.1 and V5.2.2? Given the default settings for GPS accuracy (5), V4 seems to match roads in larger radius then version V5. Also, is there a way to globally change the default GPS accuracy instead of having to specify it for every point? Thanks you, Artur… From: Daniel Patterson [mailto:[email protected]] Sent: Friday, June 17, 2016 12:13 PM To: Mailing list to discuss Project OSRM <[email protected]<mailto:[email protected]>> Subject: Re: [OSRM-talk] GPS Accuracy for match service Hi Artur, TL;DR - there's no direct conversion from HDOP to radius, that's not what HDOP is. Just knowing HDOP isn't enough. HDOP is based on satellite position and basically tells you "if you had perfect reception right now, the best accuracy you could achieve would be X". Less-than-perfect reception will also affect accuracy, and isn't part of the HDOP calculation. Number of satellites in view, multi-path-error, etc, all contribute to inaccuracy and aren't part of HDOP. Things like iPhones can give a reasonable estimate because they have a known GPS device with known characteristics, and they're possibly monitoring satellite count, signal-to-noise ratio, etc and doing a fancier calculation than you get from simple NMEA sentences. Cheap GPS devices sometimes do something naive like 3-5m * HDOP ~= 95% radius (2 standard deviations). It's not really correct, but if that's all you've got, run with it. daniel On Jun 17, 2016, at 8:36 AM, Artur Bialecki <[email protected]<mailto:[email protected]>> wrote: Hello, In the documentation of the V5 match service it states that radiuses are “Standard deviation of GPS precision used for map matching. If applicable use GPS accuracy”. If I have HDOP, how would I convert it to the radius value. Thank you. Artur… This e-mail message is confidential, may be privileged and is intended for the exclusive use of the addressee. Any other person is strictly prohibited from disclosing, distributing or reproducing it. If the addressee cannot be reached or is unknown to you, please inform us immediately and delete this e-mail message and destroy all copies. Thank you. _______________________________________________ OSRM-talk mailing list [email protected]<mailto:[email protected]> https://lists.openstreetmap.org/listinfo/osrm-talk This e-mail message is confidential, may be privileged and is intended for the exclusive use of the addressee. Any other person is strictly prohibited from disclosing, distributing or reproducing it. If the addressee cannot be reached or is unknown to you, please inform us immediately and delete this e-mail message and destroy all copies. Thank you. _______________________________________________ OSRM-talk mailing list [email protected]<mailto:[email protected]> https://lists.openstreetmap.org/listinfo/osrm-talk _______________________________________________ OSRM-talk mailing list [email protected]<mailto:[email protected]> https://lists.openstreetmap.org/listinfo/osrm-talk -- - Kerrick -- - Kerrick This e-mail message is confidential, may be privileged and is intended for the exclusive use of the addressee. 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