[talk-au] GPS accuracy

[Brett Russell]

Hi all
 
Interesting discussion and thanks for confirming that WAAS is not an option in Australia so I will switch it off.  Subtle details like this get missed in various reviews and also means that USA and now likely European reviews with claims of accruacy need have factored in that WAAS might be enhancing accuracy that will not happen in Australia.  Also now better placed to identify "inexpert" "expert" sales people.
 
Point taken on absolute accurary considerations with survey marker location.  My goal is pratical accuracy.  By that, avoiding my GPS complaining with OSM that I am heading into oncomming traffic.  The issue with that was the particular road was aligned to native Bing so was say five to ten metres out.  If I can establish the correct alignment of Bing for the area then I can have the road in the correct place.  Now of course if my GPS can not resolve to the necessary accuracy then I still will have the issue but that is between my GPS and OSM rather than OSM and the real position.
 
I have found my Garmin 62s is remarkably consistent with traces as I have walked the same tracks now a few times and generally it is within 5 metres at least and more often than not within two metres.  In the car it is readly aparent what side I am on with and out and back trips, until I reach a cutting, in that case the traces very rarely cross but do tend to broaden out.  Courners do catch it out but then I have my recoding interval set to 10 metres as I am told that gives the best result for my unit.   Now using aligned Bing to clean up and remove unnecessary points for roads but leaving more points in for finer detail for walking tracks as five metres off track can me rather painful scrub bash.
 
My reading of various information suggests that the Garmin maps themselves, due to using 24 bit reference number, have at best a theortical accuracy of 2.5 metres so even with professional grade surveying the maps in the unit will not be better than 2.5 metres.  Someone might have different or better information so more than happy to have this point challenged.  All still very much a learning curve for me.
 
Actually GPS accuracy needs to be remarkable with complicated intersections as often one lane out can mean a massive side trip.  Spent a lot of time figuring out how to get to Ballarat from the Melboure Airport as for a Tasmanian we are not use to such massive intersections.  I case where I should have been the navigator not the driver.  Was pre OSM for me so was usuing Google Maps on the iPhone.
 
Gradually working my way around Tassie by lake tracing and that is where I am understanding the challenges Bing gives you.  Its image quality ranges from top class to murky but still great to have it as the alternative would be walking around every lake, tarn and pool.
 
Anyway thanks for the feedback.  Hopefullty it will result in better maps.
 
Cheers Brett
 

> Date: Fri, 21 Sep 2012 06:23:13 +1000
> From: snowgum at gmx.com
> To: talk-au at openstreetmap.org
> Subject: Re: [talk-au] GPS accuracy
> 
> On 20/09/12 22:27, Peter Hoban wrote:
> 
> > Discovering the accuracy of a unit is easy. Find a convenient spot near
> > your house and with your GPS record its position. Come back next day
> > (or at least a few hours later) and do it again. Repeat daily until you
> > are sick of it and you will then have a good idea of how accurate any
> > particular observation is likely to be. No technical expertise required.
> >
> > The question of absolute accuracy is complex. Survey marks mostly were
> > placed before the current modelling of the earth was developed. While
> > these may now have GDA coordinates (typically about 100 mm different
> > from WGS in Australia) there are complexities that arise (eg from
> > continental drift and the instability of the earth's axis of rotation)
> > which are significant variables. There are many assumptions in the
> > modelling.
> >
> > WAAS also works in Europe and Japan. There is no likelihood of it being
> > implemented in Australia as our population density is too low. Switch
> > it off. If it is left switched on there is some risk that spurious
> > signals from other systems may degrade the accuracy of your device.
> 
> I've been using a Garmin GPSMAP 60CSx for a number of years. Normally 
> for logging GPS tracks by car for use in OSM, I use it in conjunction 
> with an external antenna (mounted above the driver's seat so it's closer 
> to the centre of the road).
> 
> With a good view of the sky, this GPS unit usually claims its accuracy 
> to be ± 3m, with one important exception. And that's when cornering.
> 
> If I superimpose track logs from several days in JOSM, I do see 
> incredible consistency in the tracks. The exception is in the corners, 
> where there's distinct variation.
> 
> The solution is to drive around corners more slowly (where safety 
> considerations permit). Then the GPS seems more inclined to accept 
> cornering as the reason for the deviation from going straight ahead 
> (rather than its interpreting the change in direction as resulting from 
> a noisy or degraded satellite signal instead).
> 
> I've struck another situation where the GPS reports significant 
> uncertainty about its position. That's when bushwalking with thick tree 
> cover and especially with cliffs or hills to one or both sides. It's 
> clear to me that the signal is weak, with echoes only making the 
> situation worse. Here the GPS might report an accuracy in the order of 
> ±15m, and superimposed logs confirm this variation.
> 
> John
> 
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