Re: Garmin Geko - Accuracy of conversion to Bristish National Grid
From: Paul Saunders (pvs1_at_wildwales.fsnet.co.uk)
Date: 07/26/04
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Date: Mon, 26 Jul 2004 18:01:01 +0100
Michael Chare wrote:
> What I am pointing out is that once EGNOS starts my (and many other)
> GPSs will not give accurate Grid References not because the GPS can
> not determine its true position, but because it can not calculate the
> Grid Reference accurately enough from a known position.
>
> In the example I quote the calculation error is 7m which when added
> to the positon error of 3m means that I can only be confident that
> the GPS is somewhere in a circle 20m in diameter. (Ignoring the fact
> that the Grid calculation error is constant rather than random in
> error).
True, but I ask again, what do you need this positional accuracy for?
If you have a specific need for high accuracy then fair enough, but if
you simply want to use the GPS for navigation, a 10m error is really no
problem. Given the low resolution, feature exaggeration and feature
distortions present in typical maps, better datum accuracy won't make
any practical difference to conventional navigation.
What will make far more difference is bad signal reception, such as you
often get in forests or deep valleys, or near large obstacles. Even
when EGNOS comes online, these situations will still be common, and
there'll be plenty of places where you won't be able to receive EGNOS
corrections anyway, even if you're getting an otherwise good signal.
So unless you have a specific reason for needing high accuracy, I
woudn't get too hung up on it.
>> SS 62168 90898 - This is the correct OS grid ref.
>> SS 62171 90903 - This is the WGS84 grid ref converted to OS BNG
>
> Now that I have learnt about the BNG position calculation errors, I
> understand the significance of your use of OziExplorer to calcuate
> the a Grid Reference which you use for your comparison!
Yes, I wasn't aware of datum conversion errors until I decided to try
checking the accuracy of my GPS against published trig point
coordinates. Then I was a bit baffled by the consistent errors, always
in the same direction.
Whilst a more accurate datum conversion would obviously be the ideal
solution, the fact is that there are already large numbers of GPS users
out there who are using the "simple" datum conversion in consumer grade
GPS receivers. In spite of its inaccuracies, the errors are at least
consistent, so there exists a "relative accuracy" between users. Thus
user waypoints are compatible with one another.
Most of these users are not even aware of the datum error.
> 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? In practice it
wouldn't. A very large error could lead to navigation problems, but a
10m error won't.
>> Were you using an extremely large scale map?
>
> Yes, 1:2,500 (as I said).
Sorry, I missed that. I must have misread it as 1:25,000.
> 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?
>> I mentioned earlier that I'm in the process of marking summit
>> waypoints (and averaging them) for publishing on my website.
> I would suggest that any sharing of such data should include WGS 84 -
> can also make it easier for PC up load to GPS.
Well since the GPS uses WGS84 internally, all PC software should be set
to upload and download in that datum anyway. Other datums and display
formats are used for display purposes only. Since we use OS BNG in this
country, that's what we have our software set to display. I doubt that
many users would feel comfortable entering waypoint coordinates in an
unfamiliar format. There's also a lot of potential for errors if you
ask people to start changing their datums. Much simpler just to give
the coordinates in the datum that the majority are using (I will of
course point out that the grid refs relate specifically to the standard
OS datum used in consumer grade GPS receivers).
Having said that, I'll probably provide files which can be loaded
straight into the software, which will avoid any potential conversion
errors. OziExplorer files save all the data in WGS84 format anyway.
It might interest you to know that there are already a number of
websites providing GPS coordinates for various things. How many of
these point out the datum conversion issue I don't know. Perhaps many
aren't even aware of it themselves.
> In principle I would like the Geko (and other GPSs) to do a better
> job of calculating BNG references,
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.
> 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. 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.
Paul
-- http://www.wilderness-wales.co.uk http://www.wildwales.fsnet.co.uk http://www.photosig.com/go/users/userphotos?id=118749
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