[OSRM-talk] GPS Accuracy for match service

Kerrick Staley kstaley at lyft.com
Mon Jun 20 19:12:55 UTC 2016


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 <daniel at mapbox.com>
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 <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 <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>
> 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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-- 
- Kerrick
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