[OSRM-talk] GPS Accuracy for match service

Daniel Patterson daniel at mapbox.com
Fri Jun 17 16:13:18 UTC 2016

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.

> 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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