Dennis Pogson wrote:

G S wrote:
Is his any good?

William the Conqueror used one of these to navigate from Calais to
Hastings and thought it was a super instrument!

It's a little-known fact that he was actually bound for Bournemouth
for a relaxing seaside holiday, but ended up in Hastings as a result
of a navigation error.

This was because his own GPS was literally stabbed to death in an accident
at an informal darts match in a Calais vinotheque, so a replacement was
hurriedly procured locally. His fleet was already more than half way to
Bournemouth by the time his navigator had come to grips with the instruction
manual for the unfamiliar Garmin (he was used to a Magellan), and so when
they switched it on, the position displayed told them they'd gone too far.

They duly turned back and the rest, as they say, is history.

What went wrong?

Because the navigator was busy frantically studying the manual, he didn't
personally supervise the DR navigation, which he had delegated to an
apprentice. Once the GPS was up and running, the apprentice's EPs, which
were in fact accurate, were dismissed as obviously erroneous.

Unbeknownst to the navigator, who hadn't quite gotten round to digesting the
manual's "advanced features" section, the GPS receiver, having been supplied
for the French market, had been factory-pre-programmed to default to French
datum, which of course back then was based on the zero meridian going
through Paris.