Comments on: When AutoCAD is not Accurate

By: Edwin Prakoso

Edwin Prakoso — Thu, 01 Jul 2010 00:23:21 +0000

Thank you Mike.
Hatches just ‘trigger an alarm’ that something is going wrong. If I go further, my drawing will loose the accuracy. And it’s not just hatches. I’ll try your suggestion if it can fix the problem.

By: Mike Croftcheck

Mike Croftcheck — Wed, 30 Jun 2010 13:56:57 +0000

I have worked in the Civil/Surveying industry for 10+ years and there is another quick fix for hatches far away from 0,0 if you need to stay in the World Coordinate System. Use the SETVAR “SNAPBASE” and pick a coordinate position that is closer to where you are working in the WCS. This will also affect where the snap and grid origin is, so if you need to use snap/grid make sure you key in even coordinates, 3000000,10000000 for example if you were working in Texas State Plane Central Zone.

By: Scott Campbell

Scott Campbell — Tue, 30 Mar 2010 11:00:40 +0000

We have a similar problem where our client insists that we deliver CAD model data at real world coordinates – which is 10,000,000 units away from the origin. I refuse to do this in house as I have seen the crazy problems that occur with hatches and particularly wall cleanups in ACA etc. We therefore model at a local building grid and then run a custom aplication that transforms the model data to real world coordinates. This means that the client must battle with the problems but our data maintains its integrity internally.

By: Edo

Edo — Mon, 06 Jul 2009 00:07:06 +0000

Mike,
I won't suggest anyone to scale drawing in model space. Always draw real measurement on your model.
Any modification can produce this error. I tried to rotate my drawings with reference, and rotate them back by keying in the angle… it won't come back to the original location. You might want to try to draw lines, and measure them. Try to draw them in millions unit away from 0,0.

By: Mike

Mike — Sat, 04 Jul 2009 19:21:13 +0000

Currently finishing a Drafting degree with CAD specialization and we are taught to scale drawings so problems like this don't occur. An interesting point though. What is the exact error, or process that create these distortions?

By: Edo

Edo — Thu, 04 Jun 2009 01:33:31 +0000

Thank you Eric. That's also what we do here. We tried to create a local coordinate to avoid this. But there are some architectural firm refuse to do this. They prefer to draw in mm for the building design in local coordinate, and then reference it to the whole site which is drawn in larger unit, ie. m or km.

By: Eric Colburn

Eric Colburn — Wed, 03 Jun 2009 20:53:44 +0000

You have touched on an important point by drawing attention to using large coordinates in an AutoCAD file (or for any other calculations, for that matter). Land surveyors sometimes work in assumed coordinate systems basing their work in low numbers, like N 5000, E 5000, and calculation errors will be smaller for this coordinate system, than say, for a state plane coordinate system where the points and drawing elements are, instead, located in the hundreds of thousands. i.e. N 3000000 E 300000.

By: Eric Colburn

Eric Colburn — Wed, 03 Jun 2009 16:53:44 +0000