NeutralAxis

Its never too late to learn something new.  I’ve used AutoCAD professionally since the early 1990s and just today learned how to take advantage of a bug, or perhaps it would be more accurate to simply call it a limitation, that has helped to save me from some tedious work.

We get drawings from surveyors who commonly prepare their topographic drawings in 3D.  My firm, and most other architects I’ve dealt with, still primarily work in 2D despite all efforts by AutoCAD to stuff 3D down our throats.  If the architect or engineer doesn’t pay attention to the fact that they’ve been given a 3D drawing when they begin using that survey base, the drawing becomes a mix of 2D and 3D that can create complications.  For example, my most recent experience with this was today when trying to make revisions to a sidewalk ramp on a drawing prepared by someone else.  I measured what should have been about a 12″ distance and it reported that it was 103 feet.  Anticipating the problem, I tilted the drawing from a flat view to an isometric view and saw that the drawing had this mishmash of 2D and 3D.  This happens when somebody draws a line and isn’t careful about how they pick the endpoints of that line.  You can get one endpoint at 0 elevation and the other at some other elevation on the Z axis.  Once the mistake is made one time it starts to snowball as more and more elements of the drawing start being inserted relative to misplaced points.  In a standard flat view, it looks correct and you’re not aware of the problem.

Drawing Viewed Flat

Isometric View

I attempted to fix that problem years ago with a macro that simply selected everything in the drawing and moved it to 0 elevation.  If you do that from the beginning when everything is flat, but just at different elevations, it works fine.  But once a few things are tilted into 3D where different endpoints or nodes of an object have different Z elevations, it no longer works because the object moves as a whole, and not the individual endpoints or other nodes within the object.

Enter the bug.  AutoCAD doesn’t know that space is infinite and has a limit on how big a number can be.  Thus if you take a whole drawing and move everything in it up in the Z direction a distance of 1e99, or 1×10^99 (how does one do superscripts in html?), it flattens everything against that imaginary ceiling.  You then move it back down again -1e99 and it comes to rest with every node of every object at 0 elevation.  If there were -Z elements in the drawing you’d also have to do the same procedure in reverse.

Isometric View of Flattened Drawing