NavList:

David Pike, you asked:
"could I just pick a point on max scale Google Earth, e.g. the corner of a patch of concrete, and pin that.  It appears to give a lat & long to 6 decimal places of degrees or about four inches, but how accurate is the lat & long really likely to be. "

No, not that accurate. I would say that Google Earth is as accurate as the market demands and no better --typically a few meters. Greater errors are relatively rare, but you will enounter them. How could you detect small errors anyway? One way is to wait for a good smartphone GPS to build a position from a really large number of satellites. If you have four or five satellites contributing to the fix, the accuracy is considerably lower than when you have ten or fifteen. It's also useful to check several days in a row.

You started off your post explaining:
"I’m working on a learning game for students, but I need to know the lat & long of at least one point to decimetre accuracy. "

But are you sure you really need that? How would you use that point? If they have no means of acquiring positional data at that accuracy, how could they use such data? Have you read about any geo-caching procedures? That community has been working on these gaming issues for many years.

Frank Reed