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]> 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]> > 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] <[email protected]>] > *Sent:* Friday, June 17, 2016 12:13 PM > *To:* Mailing list to discuss Project OSRM <[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]> > 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] > 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] > https://lists.openstreetmap.org/listinfo/osrm-talk > > > > _______________________________________________ > OSRM-talk mailing list > [email protected] > https://lists.openstreetmap.org/listinfo/osrm-talk > > -- - Kerrick
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