Positional accuracy of TomTom GO?

[JohnGray]

I haven't seen this matter dealt with on this BBS, so apologies if I've just missed it.

Somewhere, I found that the positional accuracy of the TomTom GO system is given as "around 10 metres". I presume that means that it does not use WAAS/EGNOS unlike (say) the Garmin 2620, which quotes accuracies of "about 3 metres".

Assuming I'm correct, my question is -- does having or not having WAAS/EGNOS in your SatNav box make any practical difference to the average driver? The downside of having it is presumably that fixes/redirections/recalculations are slower; is the upside a bigger chance that you'll go down the correct turning, if there are several quite close together?

Thanks!

(In passing I would say that I've been severely discouraged in my original intention of buying a TomTom GO by some of the "which version of maps should we have" debate, which seem to indicate that TomTom customer service could Do With Improvement -- but please don't let me hijack my own thread!!)

[DavidW]

SBAS systems, such as WAAS and EGNOS, aren't needed for in-car navigation. The accuracy of GPS without SBAS is good enough in almost every case.

The navigation software uses 'snap to road' techniques, where it uses your direction of travel and your GPS position to place you on the most likely road. This will deal with all but the very worst of GPS positioning errors.


Further, where SBAS could potentially help the most, which is in city centres and similar dense areas, it's unlikely that your GPS receiver will have a view of the geostationary SBAS satellites. They sit over the equator, meaning that you need a good view to the south for a satellite that's often no more than 15 degrees above the horizon. The chances of that in a city centre is extremely remote.


Finally, the European SBAS system, EGNOS, has been very delayed; it's not likely to start operation in any real way for another six months and it won't be certified for 'safety of life' applications such as aviation until 2006.



David

[JohnGray]

David

Thanks for your response, from which I conclude that my assumption that the Garmin 2620 uses EGNOS to improve its positional accuracy beyond the TomTom GO is incorrect.

That raises the further question -- why is the TomTom GO less accurate than some of the Garmin equipment? Better algorithms using a faster processor? Some other form of Differential GPS? A "snap to road" mechanism is obvious a Good Thing -- but only if it is the correct road!

Thanks!

[DavidW]

A lot of Garmin navigation kit - though not so much the 2600 series - can be used as both general purpose GPSes and in-car navigation systems. For general purpose use, SBAS support is of value - for the latter it's of very dubious value.

The accuracy figures quoted for either device bear no resemblance to the official GPS SPS specification, which firstly talks about confidence intervals (if I remember rightly, the key specification is a 95% confidence interval one, which means that 95 in 100 times, the true horizontal position will fall within a given radius of the GPS position), then goes on to give a figure that's usually greatly exceeded these days.


Go is not a general purpose device; its only purpose is street navigation. TomTom software on the Pocket PC doesn't support configuration of SBAS on GPS hardware.



David

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