We received this reply today. Confirms my last post but with all the geeky bits.
Hi Andy,
hmm not as far as I was expecting. I have seen a similar problem when using the OS grid for the UK (which is off the south west coast of cornwall) as the drawing origin. This places objects over 100 million units from the origin. Here is what I found about the problem.
<http://autodesk.blogs.com/between_the_lines/2004/01/more_on_autocad.html>
<http://autodesk.blogs.com/between_the_lines/2003/10/64_bit_computer.html>
<http://autodesk.blogs.com/between_the_lines/2004/01/more_on_64bit_p.html>
This page says "The current 64-bit Precision Limit: Double-precision format - 64-bit data width, 15 to 16 digit precision, 1,000,000,000,000 to 0.0000000000000001"
<http://www.intelcad.com/pages/autocad/>
<http://www.intelcad.com/pages/dwfin/main.htm>
This page has the following quote "AutoCAD standard accuracy is 16 significant digits."
The bottom line is that you get 15/16 decimal points of accuracy. With our big drawings (OS UK 0,0 datum point) you are >100,000,000 mm from the origin. so that's 9dp in front of the decimal point and you have 8 after of which only 6 or 7 are accurate. So the last two digits may change, and obviously this gets worse if the UCS is rotated or the walls are rotated. A wall end point is calculated from its start point, length and angle, as soon as we get further than 1*10^9 from the origin you cant be 100% sure that the least significant digit(s) is/are accurate. And this isn't limited to Autodesk software, most other applications would have the same problem if not worse. This is probably why the Z value is fluctuating, we dont see it with the X+Y as they arent at integer values but at large coords I would expect them all to fluctuate as the "noise" becomes an issue for the last two digits.
Hope this helps
BearSummer