Geographic Location

Hi, I gave this a go in AutoCAD 2014, just revisiting it in 2015 to see if it got any better.

What I don't understand is -

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If my drawing is in a known grid co-ordinate system (Ordnance survey UK) and I select this grid co-ordinate system under "geographic location" then why do I need to give it a geographic location point in both systems?

I have tried doing the conversion manually and giving it one point in both systems. I have also tried picking a feature and locating it this way.

Both ways the bing aerial photos only fit in one place - a few km away and the are significantly out.

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