Polylines — Why Never Simple Lines
The LINE command exists. That doesn't mean you should use it.
Why this matters
A simple line (L) is two endpoints connected by a segment. That's it. It has no area, no continuity with adjacent segments, no ability to define a closed boundary, and no single-object editability. In a production drawing with dozens of intersecting site elements, individual line segments become liabilities — hard to hatch, hard to move as a unit, hard to clean up.
A polyline (PL) is a connected sequence of line and arc segments that behaves as a single intelligent object. It can be closed, measured for area and perimeter, used as a hatch boundary, exported as a clean geometry definition for 3D, and edited as a whole or vertex by vertex. Every piece of site geometry you draw in this course — every path, edge, boundary, planting bed, and paving area — will be a polyline. Splines are evil, Lines lack intelligence, Polylines are your friend.
Key polyline behaviors
| Option | How to invoke | What it does |
|---|---|---|
| Arc mode | During PLINE, type A | Switches to arc segment drawing. Type L to return to straight segments. |
| Close | During PLINE, type C | Closes the polyline back to the start point with a final segment. Creates a true closed polyline — not a gap. |
| Width | During PLINE, type W | Assigns a drawn width to the polyline segment — useful for section arrows, graphic symbols. |
| PEDIT (PE) | Type PE, select objects | The polyline editor. Converts lines to polylines, joins adjacent segments, closes open shapes, edits individual vertices. |
| Join | PEDIT → J | Combines multiple line segments and polylines into a single polyline. The repair tool for linework that comes in as individual segments. |
| Close (PEDIT) | PEDIT → C | Closes an open polyline by adding a segment from the last vertex to the first — essential for hatch boundaries. |
Closed polylines and why they matter downstream
A closed polyline defines an area. This is not just useful — it is required for several downstream operations. Hatches need closed boundaries. Area calculations (LIST command) work on closed polylines. When your CAD drawing is imported into Rhino, closed polylines become the geometry from which surfaces and volumes are built. A planting bed that is almost-closed but not-quite will produce hatching failures in CAD and geometry errors in Rhino.
Use the C option within the PLINE command to close shapes. Do not manually snap the last point to the first — it looks closed but usually isn't, leaving a gap too small to see but large enough to break everything that depends on it.
Try this
Draw the same simple irregular path with one curved section twice: once using the L and A commands as separate segments, and once as a single PL with an arc segment. Then try to hatch the interior of both shapes. Notice the difference in how many clicks and corrections the hatch process requires. Then try selecting both shapes and observe how many objects you've selected in each case.
What breaks
Open polylines used as hatch boundaries cause hatch failures. The gap is almost always invisible at normal zoom. Use PEDIT → C on any boundary before hatching, and use the Z - Hatchline layer to draw explicit closed boundary polylines rather than relying on AutoCAD to detect boundaries from overlapping geometry.
Snapping the last point to the first manually appears to close the shape but typically leaves a microscopic gap at the endpoint. The C option closes mathematically. Always use it.
Exploding polylines to edit a segment returns you to individual lines. Use PEDIT → Edit Vertex to modify individual vertices without destroying the polyline. You can also select a polyline, then click on a vertex to STRETCH the point to a new location.