3D polyline in AutoCAD: 3DPOLY, 3DFACE and THICKNESS — three ways to take linework into space

A pipe run that rises and falls, the edge of an embankment whose level changes at every point, a sloping roof plane: none of these can be drawn with an ordinary polyline, because an ordinary polyline lies entirely in one plane. A 3D polyline in AutoCAD handles routes through space, and two older tools, 3DFACE and THICKNESS, handle faces and height.

This article tries all three in AutoCAD 2027 with the acadiso.dwt template. Ordinary polylines and the PEDIT command are covered in polylines in AutoCAD. True solids are covered in 3D solids in AutoCAD. For drafting taught properly, the Institute runs an AutoCAD course for construction.

Starting the three commands

Locations and aliases come from the 2027 acad.cuix and acad.pgp files. The Autodesk documentation on 3DPOLY describes it as creating a connected sequence of straight segments as one object, where the segments need not lie in one plane.

The 3DPOLY command: an elevation for every vertex

Type 3P, enter four points, each at a different elevation, and close with C:

Specify start point of polyline: 0,0,0
Specify endpoint of line or [Undo]: 1000,0,500
Specify endpoint of line or [Undo]: 1000,1000,0
Specify endpoint of line or [Close/Undo]: 0,1000,300
Specify endpoint of line or [Close/Undo]: C

The result is one closed AcDb3dPolyline. Reading its vertices back, each keeps exactly the elevation typed: Z = 0, 500, 0 and 300.

The options are only Close and Undo. There is no Arc and no Width as in PLINE: a 3D polyline is made of straight segments only and has no width. The object also lacks the Elevation, ConstantWidth, Thickness and Area properties an ordinary polyline has.

Length is measured in space. In plan the four vertices form a 1 000 square with a perimeter of 4 000. The 3D polyline is 4 324.129 long, 324.129 more, because every side climbs or drops. Taken side by side, the two sides that rise or fall 500 are 1 118.034 each and the two that rise or fall 300 are 1 044.031 each, which adds up to exactly 4 324.129. For the true length of a sloping pipe run, this is the figure to read, not a length measured on the plan.

Control: PLINE refuses 3D points

Run PLINE with the same four points. The first point 0,0,0 is accepted. The second, 1000,0,500, is refused:

Specify next point or [Arc/Halfwidth/Length/Undo/Width]: 1000,0,500
2D point or option keyword required.

Every later point with a Z is refused the same way. Even C is refused at that stage with the same message, since after a single point the prompt offers no Close option. PLINE accepts an elevation only at its first point. Draw a PLINE starting at 0,0,300 and continue with 2D points, and you get an AcDbPolyline with Elevation = 300: the whole line lies flat at 300, and its length is 4 000, exactly the square's perimeter.

The way each object stores its vertices shows the difference plainly. The 3D polyline returns its vertices as X, Y, Z triples, twelve numbers for four corners. The ordinary polyline returns X, Y pairs, eight numbers for the same four corners, plus a single Elevation value that applies to all of them.

In short, an ordinary polyline has one elevation for the whole line; a 3D polyline has its own elevation at every vertex.

The 3DFACE command: three- or four-sided faces

3DFACE creates a thin face with three or four corners, each at its own elevation. AutoCAD asks for the first, second, third and fourth points in turn:

Specify first point or [Invisible]: 0,0,0
Specify second point or [Invisible]: 1000,0,0
Specify third point or [Invisible] <exit>: 1000,1000,400
Specify fourth point or [Invisible] <create three-sided face>: 0,1000,0
Specify third point or [Invisible] <exit>:

The four corners need not lie in one plane: a third corner raised by 400 is accepted, and the AcDbFace keeps all four elevations. The face has no Thickness or Area property. Its four corners do not lie in one plane, yet AutoCAD stores it as a single object with four corners rather than splitting it into two triangles: the drawing count goes up by exactly one.

Press Enter at the fourth-point prompt and AutoCAD makes a triangle. Reading the coordinates back, the face still has four corners, but the fourth repeats the third exactly.

Chaining 3DFACE: point order decides everything

After the first face, AutoCAD does not stop; it asks Specify third point again. Each new pair of points adds a face, reusing the previous face's third and fourth points as its first two corners. Six points make two faces. Press Enter at Specify third point to end the command.

The easy mistake is the order of the new pair. The first face has corners 6000,0, 7000,0, 7000,1000, 6000,1000. The second face starts at corner 3 and then corner 4, that is 7000,1000 then 6000,1000:

In both cases the two faces share corners 7000,1000 and 6000,1000 exactly, so they meet with no gap; only the order of the two far corners differs. Point 5 must go next to point 4, not next to point 3. In other words, when chaining, the pairs zigzag: left to right this time, right to left the next.

Hiding 3DFACE edges with Invisible

Type I before a point to hide the edge that starts at that point. DXF code 70 records which edges are hidden:

So I before the first point hides edge 1–2, and before the fourth point hides edge 4–1, closing back to the start. I does not create a point: AutoCAD repeats the same prompt, Specify second point or [Invisible]:, and waits for coordinates.

Thickness in AutoCAD: giving 2D linework a height

The THICKNESS variable gives 2D objects drawn afterwards a height along Z. Set THICKNESS to 500, then draw:

Set THICKNESS back to 0 and draw a LINE: the new line has Thickness 0. The variable affects only objects drawn after it is set, never existing ones.

To change the thickness of existing objects, use CHPROP and its Thickness option:

Enter property to change [Color/LAyer/LType/ltScale/LWeight/Thickness/TRansparency/Material/Annotative]: T

Enter 250 and the selected line has Thickness 250.

The ELEV command: default elevation and thickness

ELEV sets both defaults for new linework at once:

Specify new default elevation <0.0000>: 300
Specify new default thickness <0.0000>: 200

The values in angle brackets are the current ones; in a new drawing from acadiso.dwt both are 0. Afterwards ELEVATION = 300 and THICKNESS = 200. Draw a LINE from 0,0 to 1000,0, typing 2D coordinates, and the line sits at Z = 300 and is 200 thick. Type an explicit Z, such as 0,500,50, and the line sits at Z = 50: a typed elevation beats the default, while the thickness stays 200.

Remember to set both back to 0 when you are done. Otherwise every line you draw with 2D coordinates afterwards sits at elevation 300 with a thickness of 200.

Which tool for which job

Linework with a thickness is still 2D linework, not a solid. For a real solid to cut, subtract or measure, use the extrude command.

Three things often repeated that are not true

Frequently asked questions

Which command draws a 3D polyline in AutoCAD?

3DPOLY, alias 3P, or Home → Draw → 3D Polyline. Each point is typed with its own Z.

Why does PLINE refuse points with an elevation?

PLINE takes a Z only at the first point and uses it as the elevation of the whole line. Later points with a Z are refused.

Can a 3D polyline have arcs or a width?

No. Its only options are Close and Undo.

My chained 3DFACE crosses like a bow tie. How do I fix it?

Enter the new pair in the opposite order: put point 5 next to point 4 of the previous face.

How do THICKNESS and ELEV differ?

THICKNESS sets only the thickness. ELEV sets both the default elevation and the thickness in one command.