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I have thought for a while that my GPS was telling lies when it gives the accuracy of the reading.

It most often says that the accuracy is 6M, well last night whilst out walking I was able to work out that the actual accuracy was only 15M, assuming that the OS map is accurate.

What is interesting is that at times the stated accuracy can be a lot lower, the GPS maybe says that acuracy is 35M, now if that reading is out by a similar ratio your GPS might only be accurate to 90M, and that could be quite a problem in the mountains.

Before SA was removed, I was walking along Crib Goch (a very narrow ridge) in Snowdonia and a GPS was telling me that I was about 1km away in the middle of  Glasyn Lake. In this case the error in the GPS was obvious, I knew I was not standing in the middle of a lake more than a 1000' below me since my feet were not wet, but there was no way of telling from the GPS that the reading was so much in error.  

Supposedly WAAS (or EGNOS for Europe) will improve accuracy to 8ft and this project was supposed to have been completed in Spring this year. I think it's delayed though; I can occasionally lock a satellite but as yet have received no differential fix. Best accuracy I can get according to the display is 16ft.

I'm not entirely convinced that OS maps are always 100% accurate however. I suppose the only way of really checking is to calibrate against a known reference point. Remind me to put my anorak on one day and try it out...

We all neglect to mention one of the most useful functions of the Garmin range over an Ordnance Survey map and that is the best hunting/fishing times function

Quote: Originally posted by halfpiste on 23/09/2004

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I'm not entirely convinced that OS maps are always 100% accurate however. I suppose the only way of really checking is to calibrate against a known reference point. Remind me to put my anorak on one day and try it out...

The position of trig points should be narked very accuratly on the map  .......

We all neglect to mention one of the most useful functions of the Garmin range over an Ordnance Survey map and that is the best hunting/fishing times function

I neglect to mention it because I have no idea what the hunting/fishing times function is or does.  

Indeed, trig points would be an ideal calibration point.

Supposedly uses position, and sunset/sunrise times to work out the best times for fishing and hunting. Doesn't say if there are any fish there though. Another example of bloatware gone mad.

Quote: Originally posted by halfpiste on 23/09/2004

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Indeed, trig points would be an ideal calibration point.

Here is a tiny pdf showing where my track last night stopped at a trig point, where the two red tracks cross is where the GPS thought we were when we were leaning on the trig point, its 15M away from where it should have been even though the GPSs (both of them) were convinced the position error was 6M.

http://homepages.nildram.co.uk/~srnet/Navex/Printing%20Map%20-%20tigpoint.pdf

Supposedly uses position, and sunset/sunrise times to work out the best times for fishing and hunting. Doesn't say if there are any fish there though. Another example of bloatware gone mad.

How not useful.

Quote: Originally posted by halfpiste on 23/09/2004

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Aaahhh, Wales. Can't wait to get back into the mountains... 3 weeks to go. A different world up there.

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Indeed.

And the weather was wonderful last night too, 30mph winds, 3C windchill, torrential rain, 5M visibility ...... I bet you cant wait.

Although jesting aside I am fortunate to have the Brecon Beacons about 30 mins drive away.

Ever seen Hertfordshire from an office window on a dreary Thursday? Mountains win hands down for me.

Beacons top on to visit list next year, but in the meantime roughing it in a cottage near Lampeter in October.

Soft, I know.

Quote: Originally posted by halfpiste on 23/09/2004

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Beacons top on to visit list next year, but in the meantime roughing it in a cottage near Lampeter in October.

Soft, I know.

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Nowt wrong with staying in cottages, and if you dont wan to pay full wack, then there are a lot of YHAs in remote areas, old farmers cottages etc.

Our climbing club have a barn\cottage in Snowdonia, that costs me £3.50 a night, now whilst I like camping ...........

This weekend I am walking on the pembroke caost path, staying in a flat thats part of a rather grand country house just outside  Newport, Pembrokshire.  

Quote: Originally posted by srnet on 23/09/2004

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I have thought for a while that my GPS was telling lies when it gives the accuracy of the reading.

It most often says that the accuracy is 6M, well last night whilst out walking I was able to work out that the actual accuracy was only 15M, assuming that the OS map is accurate.

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I have discovered that when the GPS is telling you its level of accuracy (say 6M or less) it is not telling you the facts.

In reality the quoted accuracy is just a statistical possibility that the accuracy is within that figure. i.e. if the GPS says the reading is accurate to 6M, it is likley that that is the accuracy but the GPS has no way of knowing that the accuracy is indeed 6M. So the accuracy could be 30M or 100M, you have no way of knowing.

The worst I have come across so far on a moderm GPS is it telling me it was accurate to 6M, when in reality it was 28M out.

Was it WASS ?

GPS WAAS ACCURACY

The Wide Area Augmentation System (WAAS) is a form of differential GPS (DGPS) giving enhanced position accuracy developed primarily for aeronautical navigation but usable by other users.  Each Wide Area Reference Station (WRS) provides correction data to a Wide Area Master Station (WMS), which computes a grid of correction data to be uplinked to a geostationary satellite (GEO) via a Ground Earth Station (GES) in the Ground Uplink System (GUS).  The geostationary satellite transmits the correction data (and also navigation data) to the user on the L1 GPS navigation frequency (1575.42 MHz).  The user GPS receiver uses the downlink WAAS data to correct received navigation data.  The goal of WAAS is to obtain at least a 7-meter horizontal and vertical accuracy.

Quote: Originally posted by delta9 on 12/10/2004

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Was it WASS ?

No.

That would potentially make it more accurate ..........

WAAS is the US version and is operative, and usable if you are in America (WAAS is aimed more at Aircraft navigation than ground navagation). EGNOS is the european version, and is not due to go live until some time in 05 (or 06/07/08) the date keeps being put back. When it goes live, from test acuracy should be up to 3m. Have to wait and see if thats true.

Dave