I am having issues getting things to be perfectly flat with respect to the world top view in Rhino, I don’t know if I just have a precision setting wrong or what’s going on. Here is a simple example that demonstrates what is happening. I have a plane that I originally drew at some funny compound angle… but it is a FLAT plane. Then I try to make a copy of this plane that is flat to the “World Top” CPlane using Orient3Pt. I select what I want to become the lower left corner first by its endpoint, then the lower right, then the upper left as shown:

then to get it perfectly flat, I don’t select anything, instead I type in coordinates… for point 1, I enter 0,0,0 for point 2 I enter 100,0,0 and for point 3 I enter 0,100,0

So I have specified Z=0 for all 3 points.

This puts the plane somewhat where I expected, but it is NOT perfectly flat, and NOT at Z=0

I fully expected it to be precisely at Z=0.0000000 as if I drew it there.

If I put a bounding box around it, the bounding box has the following:

min = -0.0000003,0.0000000,0.0000000

max = 70.4130707,37.9674815,0.0000006

dimensions = 70.4130710, 37.9674815, 0.0000006 inches

where did the 0.0000006 thickness come from? how can I get this FLAT?

if I draw a plane specifying its coordinates to be flat, then it really is flat as in this example of another plane I drew by coordinates.

min = -72.0000000,0.0000000,0.0000000

max = 0.0000000,48.0000000,0.0000000

dimensions = 72.0000000, 48.0000000, 0.0000000 inches

the plane re-oriented with Orient3Pt is also proven to be out of flat by getting the distance from 0 to the lower right corner:

Command: Distance

First point for distance ( Units=Model_Units ): 0

Second point for distance ( Units=Model_Units Undo )

CPlane angles and deltas: xy = 0.0000000 elevation = 0.0000005 dx = 70.4130707 dy = 0.0000000 dz = 0.0000006

World angles and deltas: xy = 0.0000000 elevation = 0.0000005 dx = 70.4130707 dy = 0.0000000 dz = 0.0000006

Distance = 70.4130707 inches

I can’t see any reason why any of the plane should be above 0 because I specified Z=0 for every coordinate of the Orient3Pt.

I know I can force things flat with Project to CPlane, but that will not work because the things I am trying to get to be flat are complicated blocks… I’m just using the plane as an example that demonstrates the issue and hopefully whatever solution I find for the plane would work for the block as well.

Another thing I have tried was to set a CPlane to the diagonal plane, then copy it to the clipboard then set it back to World Top and paste it… but it just pastes it back where it was diagonally… apparently copy and paste do not take the CPlane into account… but I do notice that when I set the CPlane this way and then check the 4 corners of my plane with Distance that all 4 corners report Z=0.0000000 so if I could somehow copy it with respect to this CPlane then paste it to the “World Top” CPlane it should be right. but I don’t see how to copy with respect to CPlane.

Here is the sample drawing in case anyone sees where I have a setting wrong or something

Flat Planes.3dm (186.1 KB)