Nokia N95, Internal GPS - Accuracy

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”

5 comments Tags: nokia, gps, geocache, garmin, n95, trig point, accuarcy …

Shipwreck: Atlantic (TBC)

I think this is the Atlantic - close to the Star of Hope at Ainsdale

Located Close to Start of Hope: Pic Taken 2nd June 2007

Viewranger Review - on the N95

A Geocahce on a Public Footpath

What is it?
Viewranger is an S60 application that plots your position on an British OS Map and provides a variety of related services and tools. It supports the internal GPS of the Nokia N95 or external bluetooth receivers.

I had first seen this some time ago for sale at an online GPS shop, I was put off by what appeared to be a high price tag, £150 for Maps to cover the whole of Great Britain or £25 for coverage of the national parks. The first price was too high for my casual use and the second OK but didn't cover an area of interest to me. Yet in fact, you can select your own maps from around GB and pay just for those, you can get a sizeable chunk of maps (10,000 square km) for £20 and that includes the application itself! You also get a map of the whole of Britain but at a miniscale rather than 1:25000 or 1:50000 of the paid for maps. Its not obvious at first glance you can get this, look here.

What Does it Do
Well Nokia Maps shows you where you are on the road - this shows you were you are an OS Map, its for off road use. Walking along a public footpath in the Lakes? Not sure where you are on the map? Well with Viewfinder you can see exactly where you are - its really amazing to see on your phone a high resolution map with public footpaths and your position shown on the map.

Navigating To

You can add your own points of interest, you can download POI sets, say a walking route and have Viewranger show them on the map. You can choose to navigate to the POI. I am a Geocacher, you can simply download a GPX file of the caches and they appear on the map. I hadn't realised GPX files include the URL of the cache. So when stuck looking for a cache I could highlight the POI, click details and then open the URL to read the details on Geocaching.com and decrypt the clue all seamlessly on the N95. That's exciting because you can't do that on the usual GPS receivers - its an example of convergence bringing something new.

You can also capture images and add notes to way points and upload that to the Viewranger server, you can also have Viewranger send your location as a "Buddy Beacon" and have your position shown on a Viewranger web portal, PIN protected.

It is an S60 application - and it feels like one. It is very configurable too, you can tweak a lot of the settings. I have enjoyed other mapping products on the N95, most leave the feeling I wish I could change that, with Viewranger you are left thinking hey that's great I can set it just the way I want it. Tip for Geocaching in options set "GPS" - "Show Next Waypoint Distance" to 1.

It does loads more too, you can get it draw a panoramic view, it draws the peaks of hills for you from a configurable viewpoint, you can even tell it how high your eyes are from the floor to help it compute what you can see from where you are. The is well worth a look.

Support
After downloading on a Saturday AM I was initially disappointed to learn that I had to send my serial number for the application to be signed, apparently that's only while its undergoing some sort of formal Symbian signing whatever that means. As the trial was for 10 days I was thinking I was going to loose a couple of days. Worry not, the application came back in 20 mins. Since I downloaded it now has official signed status and it is no longer necessary to get the application signed individually. The Viewranger team don't seem to sleep or have weekends off because I had a couple of queries which was answered almost immediately - and get this, a phone number to ring (or Skype) if you want to talk to someone.

Conclusion
Unless you know otherwise you have to dig deep to learn that for £20 you can have 10,000 square km of OS Maps for anywhere in Britain along with the slickest application I have seen on the N95 backed by the best support I have come across for any software product. If you like maps this is truly awesome. Downside, just one it uses licensed OS data so Great Britain only.

5 comments Tags: map, gps, os, s60, geocache, n95, view ranger …