It interests me to to know about measuring things. I like the fact that my clock is is synchronized to an atomic clock via a radio signal MSF broadcast by the UK's National Physical Laboratory, NPL. The geek in me takes an interest in that the time signal is sent on 60 KHz and is now broadcast not from Rugby but Anthorn in Cumbria. I do not know why I am interested in such things but I am.

So what about the accuracy of the GPS in the N95? I have compared it to my Garmin GPS receiver but which one is right? What is the uncertainty of the reported positions? When the GPS reports an accuracy of so many meters to what extent can that be trusted?

Researching measurement uncertainty of GPS soon gets very heavy going and I am not that interested so I decided to take a more practical approach.

Trig Point

I found a Trig Point, I know that trig points have been made have been made obsolete by high accuracy professional GPS measurement, but continuing my pactical experiment I placed the N95 and the Garmin GPS on top of the trig point and recorded the positions reported. I can now compute the differences between the recorded GPS poistions and the published corordinates of the Trig Point.

 I also wanted to get a feel as to the effect of EGNOS which is a correction system that can be enabled in the Garmin.

With EGNOS on the difference between the Trig points recorded position and the Garmin measurements was just 3.6ft, with the Garmin reporting an accuracy of 7ft. Disabling EGNOS and the agreement was 9.4ft. 

The N95 agreed to the co-ordiantes to 12.17ft. 

Viewranger is my all time favourite mobile application. with it I can see my current position plotted on an OS map. By adding a waypoint to the map with Viewranger I can compare the newly added way point to the postion of the trig point on the map. Hoping the waypoint is added "exactly" on top of the map legend.

POI on TP

The OS map shows the trig point as a small blue dot in a blue triangle.

Viewranger added the POI as gold colour circle, directly on top the centre of the OS symbol, bullseye! I am impressed.

The small icon at the lower right shows a Geocache location, an interesting one at the site of an old ungerground nuclear bunker. 

So in conclusion the N95 GPS compares to a Trig Point to 12ft, and agrees with my Garmin to better than this. In practical terms with Viewranger on the N95 it puts my postion on the map exactly where I would expect.

I am still thinking of pursuing uncertainty sources for the Trig Point, the conversion from OS grid to WGS84 and possibly more but I am happy, if reminded of some words by Robert Frost,

“We dance round in a ring and suppose, While the secret sits in the middle and knows”