Inaccuracy between lines

[irneb]

I've come across this scenario: when offsetting a line (while in a rotated UCS) the distance between the "parallel" endpoints differ by fractions of a unit (I'm working in millimeters so don't draw in fractions).

This seems to happen throughout all the drawings, only when operating in WCS does this not occur. Usually it's not a problem, as the variation is very slight (something like 0.01%) - but the error gets compounded over large buildings where you've offset many times. For example setting out a grid at 6000 intervals sometimes gives an overall dimension (over 18 lines) of 108003. Then I have to adjust each line slightly so their dims don't show something other than 6000, while the overall should show 108000. I know that you can't find a contractor to build at that accuracy, but I'd at least want my drawings to appear as if I've had a calculator at hand.

Is there some error in AutoCAD? I know that you get inaccuracies when using the Copy-n-Paste functions, as well as using the Express Tool's Flatten command, but these commands were not used in the problem drawings - exactly for the inaccuracy it creates.
The only thing I can think of is that there's a slight loss in accuracy when converting between WCS & UCS. Possibly because of a different floating point variable type used - something like single precision instead of double (or extended).

Has anyone come across this before, if so have you found a solution?

[rkmcswain]

I know that you get inaccuracies when using the Copy-n-Paste functions

What leads you to believe that?

Copy+Paste is no different than Wblock+Insert.
No accuracy is lost in either case.

The exception to this is if you are working in an area with very large coordinates since AutoCAD is only accurate to 15 significant digits.

Ref URL: http://www.intelcad.com/pages/autocad/index.htm

[arshiel88]

Flatten does give you inaccurate 'recreated' objects but not the copy and paste. My suspicion is that the error is when you rotate coordinate system and 99%, its a user error not the program itself.
I have a colleague that produced a very much inaccurate drawings, endpoints are not to endpoints, lines are overlapping, angles supposed to be the same but with little differences.
To escape from the blame, he said he used Educational version of AutoCAD. Can somebody please confirm this statement if this version really produce inaccurate drawings.

[jaberwok]

........ To escape from the blame, he said he used Educational version of AutoCAD. Can somebody please confirm this statement if this version really produce inaccurate drawings.

I've seen no evidence to support the claim.

[ekubaskie]

Plot the drawing. If he used the educational version it will have a watermark that says so.

If you DO get the watermark, slap the guy around, because that watermark will find it's way into any drawing touched by the ed version, and to any drawing that has content created in the ed version inserted, pasted, etc.

[david.scheu]

I can't reproduce the inaccuracy you're getting. Even at maximum UNITS precision, each endpoint of my 31st line is 180,000.00000000 from the corresponding endpoint of the first. UCS rotation doesn't seem to matter.

Are you seeing the discrepancy in a DIST command, or just when you dimension?

Do you get the same result if you use ARRAY, rather than repeated offsets?

[pertti.ahjokivi]

Same problems here.

Getting quite fed up with inaccuracies of a program, which is supposed to be accurate to 16 decimals...

As most of us in metric environment, I design in full millimeters, and seeing decimals in coordinates usually means that your model is screwed...

Apparently, one cause of inaccuracy can be found in AutoCad not having accurate angles, so when you copy something to an angle (ie. 90deg, 180deg, 270deg, etc...), the copied object goes slightly off the intended coordinates.

Discussing the problem with people using imperial system is also quite frustrating, because they simply cannot understand the problem (due to them having "infinite" number of decimals all the time...)

[michael.12445]

Actually I'm using Imperial units and also having accuracy problems - see my post(s) under the "Strange inaccuracies" thread.

In particular, I've noticed odd behavior in connection with osnaps - AutoCAD "finds" intersections, etc., that are very slightly off where the geometry actually intersects. (Yes, I know about how the limitations of display hardware can make snapping to arcs and circles appear to be off the geometry, but I'm seeing these inaccuracies with lines only.)

Michael Evans
Togawa Smith Martin Residential, Inc.

[rkmcswain]

Discussing the problem with people using imperial system is also quite frustrating, because they simply cannot understand the problem

I'd like to see this. Can you provide a step-by-step procedure to illustrate this? Thanks.

[irneb]

I've noticed odd behavior in connection with osnaps - AutoCAD "finds" intersections, etc., that are very slightly off where the geometry actually intersects.

That could be due to the "Alignment Point Acquisition" feature in AutoCAD (I think from 2006 onwards - could be wrong). What I've found is if you wait just slightly to long when hovering the cross-hairs over say an endpoint, you get a very slight distance away from that endpoint.

AutoCAD does provide 2 ways of circumventing this problem: 1 turn the Object Snap Tracking ON/OFF (F11 key). 2 Or rather - what I prefer is change the Alignment Point Acquisition from Automatic to Shift to Acquire. This you change in the Options dialog (Drafting page). The that tracking vector only shows after you've hovered over the relevant point & pressed the Shift key, otherwise you will always just select the endpoint, intersection, etc - not some vector distance away from the point you want.