Re: Garmin Geko - Accuracy of conversion to Bristish National Grid

From: Michael Chare (Michael_Chare_at_deletethis.btinternet.com)
Date: 07/26/04 

Date: Mon, 26 Jul 2004 19:52:53 +0000 (UTC)

"Paul Saunders" <pvs1@wildwales.fsnet.co.uk> wrote in message
news:ce3del$sdu$1@news8.svr.pol.co.uk...
> Michael Chare wrote:
>
>
> True, but I ask again, what do you need this positional accuracy for?

I dont have a particular problem that I am trying to solve where such accuracy
is essential. It is more a case that a feel a bit let down from the expectations
created by Garmins adverts.

Understanding the problem is a help, since the errors are consistent I can
compensate for them if I want.

> Most of these users are not even aware of the datum error.

I have a theory that once EGNOS goes live more people will start to question why
their GPSs are not giving precise answers.

>
> > My point would be that the GPS is not giving a true Grid reference
> > for the reasons identified.
>
> True. My question is whether or not you really need that degree of
> accuracy. Certainly it would be nice to have it. It would be great to
> think that your GPS is reporting your position accurate to 1m as you
> walk along, but how would that affect your walk?

Generally I agree

> In practice it
> wouldn't. A very large error could lead to navigation problems, but a
> 10m error won't.
>

I recall traversing the side of a mountain. The proper path was about 10m above
me, but I could not see it. That is the sort of error which I would lile to be
able to avoid when EGNOS starts.

> > The Geko kept giving the wrong answer for a
> > point in my garden and I now understand why.
> > I used the Excel Macros from the OS Web site to convert the WGS 84
> > position and this gave me a better answer.
>
> Do you have a link for those?

http://www.gps.gov.uk/additionalInfo/gpsSpread***.asp

>
> I agree, in principle. But in practice I doubt it would make much, if
> any, difference to the average user. The best option at the moment
> would be to get a Lowrance or a Magellan which offer multiple OS datums
> for different parts of the country, and to use the appropriate one of
> those.

The Geko does not make this easy, but I have now been told how to do this with a
User Grid.

>
> > I don't know what the implication
> > would be for manufacturing costs, or what can be done retrospectively
> > by upgrading the firmware.
>
> It's unlikely to be done retrospectively, if they ever bother to do it
> at all. The least Garmin could do would be to provide the other OS
> datum variants that other makes of GPS already provide.
>
> > The Geko supports quite a large number of
> > other reference systems which may contain similar errors.
>
> Indeed. Someone said that it would require a 10Mb look up table to
> convert to the OS datum accurately.

That gives 2cm accuracy. http://www.gps.gov.uk/accuracy.asp

My thought is that we are reaching a time when a more accurate solution to what
we have now is both feasible and economically pratical.

> If a similar situation applies to
> many other datums then a GPS might require hundreds of megabytes for the
> conversions alone. This is quite impractical for consumer grade GPS
> units, and most users wouldn't notice the difference anyway.

Some sort of PC upload to enable the GPS to a accurately convert one particular
Grid system might be one solution.

Michael Chare

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