GPS Problems? | Forum — The Modern Antiquarian

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FourWinds Feb 17, 2008 09:17:42

Replying to gjrk

gjrk wrote:

I wonder if anyone out there can throw any light on this problem:

At any of the sites I visit I take a GPS reading - from the centre of the circle, if it's a stone circle, at a rough mid-point of a row, if it's a stone row, and so on. These sites would all be less than ten metres in maximum length, or diameter.
I noticed recently that about half of my readings are out, by maybe ten to forty metres, from the coordinates given in the Archaeological Inventory (of Cork). The remainder agree perfectly.
Is my GPS giving the wrong coordinates, and how would that be possible?
Could the Archaeological Inventory be incorrect?

Thanks,
g.

Irish OS maps are out of synch compared with the GPS system, that's why google maps gets things wrong some of the time when plotting a GPS recorded point.

Perhaps some of the Cork Inventory readings are adjusted to the OS maps. Perhaps some of them are simply crap. I know some inventories have their points taken from the old 6-inch maps, which would give errors.

T

tiompan Feb 17, 2008 09:39:16

Replying to gjrk

gjrk wrote:

I wonder if anyone out there can throw any light on this problem:

At any of the sites I visit I take a GPS reading - from the centre of the circle, if it's a stone circle, at a rough mid-point of a row, if it's a stone row, and so on. These sites would all be less than ten metres in maximum length, or diameter.
I noticed recently that about half of my readings are out, by maybe ten to forty metres, from the coordinates given in the Archaeological Inventory (of Cork). The remainder agree perfectly.
Is my GPS giving the wrong coordinates, and how would that be possible?
Could the Archaeological Inventory be incorrect?

Thanks,
g.

Very often SMR/inventories grid refs are way out for various reasons . But the GPS can be be at fault often by far greater margins than is suggested in the manual . By the look of your problem I would go for the inventory being out but don't necessarilly believe the gps .

G

gjrk Feb 17, 2008 10:58:59

Replying to gjrk

Thanks very much for your prompt replies 4W and Tiompan. I suppose it must be the heavily drilled respect for authority in me, but I found it difficult to believe that the Inventory could be wrong!
To check the GPS readings I'd have to compare them with someone else's and I just had a quick glance at your website 4W. I'd rechecked Drombeg yesterday (It's a handy place for my small boy to play around, except for that mound of rusty offerings in the centre) and my circle reading corresponds with your Fulacht Fiadh?
Sorry for getting so bloody obsessive about a few metres but I'm trying to sort something out.
g

P

postman Feb 17, 2008 23:09:14

Replying to gjrk

I used to have a working GPS once, and tried to use it to find Rhos y Beddau stone circle, I could get the GPS to correspond exactly with Aubrey Burls ref point ,but still failed to find the circle , there was high bracken though, and I did find the circle next time without a GPS, I just approached from the opposite direction.
No help though is it,
Postie.

R

Rupert Soskin Feb 22, 2008 17:53:10

Replying to gjrk

I've been away this week and would have posted sooner but it may depend on your grid set-up.
A number of the mapping options have different base grids and can give variations of up to a couple of hundred metres. The standard for the UK is 'British Grid' in the position format option, and 'Ord Srvy GB' in Map Datum. Have a look in your Map Datum options and you should have the option to change it to "Irish Grid'.

Hope this helps

Rupert