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GPS Problems?

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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.