I am working at mobile app that uses external GPS module. What I need is to calculate accuracy for the fix from GPGGA command to be able to compare it with my mobile platform's GPS accuracy. Mobile platform uses the radius of 68% confidence measured in meters as an accuracy.

My question is: what is Horizontal dilution of position in GPGGA command? How can I convert it to the radius of 68% confidence? I used this description for my parser implementation.


It is useless to compare GPS modules by HDOP. HDOP is calculated mathematically from the positions of the satellites and at the same time, at the same place HDOP is the same for all receivers. They may be some difference if the receivers have decided to use different satellites for computing the position but anyways DOP values indicate only what accuracy is possible to achieve with current satellite positions. You must do a real study and measure positions of known ground control points with both devices. It is not easy to achieve reliable results but fortunately it is not an uncommon task and you will find references and tools from the net.

  • Thanks for the answer. In fact I just need to "convert" HDOP to meters. Is there some relatively easy way to do this? – Alex May 23 '14 at 7:32
  • Yes, by finding examples from the net. For example gps-forums.net/roughtly-converting-dop-metric-error-t40105.html. But because HDOP is used to multiply the other errors you should also know the technical accuracy of your receiver. – user30184 May 23 '14 at 7:41
  • Thanks. It seems that you have showed me the correct way, though it is not as easy as sinply applying some kind of formula) Will accept your post as an answer) – Alex May 23 '14 at 7:44
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    I looks that for the cheap USB/Bluetooth/built-in GPS units the manufacturers simply use 3-5 m as the accuracy of the device and then multiply it with HDOP but they may also fine tune the result with an unknown formula. – user30184 May 23 '14 at 7:55
  • That is the solution I have ended up previosly) I just whanted to improve it – Alex May 23 '14 at 8:00

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