[OSRM-talk] GPS Accuracy for match service

Daniel Patterson daniel at mapbox.com
Mon Jun 20 18:41:41 UTC 2016


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 <abialecki at intellimec.com> 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:daniel at mapbox.com] 
> Sent: Friday, June 17, 2016 12:13 PM
> To: Mailing list to discuss Project OSRM <osrm-talk at openstreetmap.org>
> 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 <abialecki at intellimec.com <mailto:abialecki at intellimec.com>> 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…
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