5.1 Polylines & Polyline Editing

5.1 Polylines & Polyline Editing

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1. The PLINE Command Architecture & Creation Options

Executing PLINE (or PL) prompts for a starting point. Once the initial vertex is placed, AutoCAD opens a rich set of geometric sub-options at the command line:

Specify next point or [Arc/Halfwidth/Length/Undo/Width]:

Polyline Width vs. Halfwidth

AutoCAD allows drafters to assign physical thickness to polyline segments directly within the vector geometry, completely independent of layer lineweights or plot styles:

• Width (W): Prompts first for Starting width <0.0000> and then Ending width <0.0000>. Entering matching values yields a uniform wide segment (such as a 4-inch wall or a printed circuit trace). Entering differing values generates a linearly tapered segment. Specifying a starting width of 10 and an ending width of 0 creates a sharp arrow head.
• Halfwidth (H): Functions identically to Width, but the numerical value represents the distance from the centerline to either side. For example, entering a halfwidth of 3.0 yields a total polyline segment width of 6.0 drawing units. Halfwidth is frequently employed in civil road design where roadway lanes are dimensioned from a surveyed centerline.

Uniform Width:   Start Width = 4.0 ══════════════════════ End Width = 4.0

Tapered Arrow:   Start Width = 12.0 ╲                    
                                     ════════════►        End Width = 0.0
                                    ╱

Polyline Arc Mode Sub-Options

Typing A at the PLINE prompt switches into arc construction mode. By default, AutoCAD draws an arc segment tangent to the preceding polyline segment. However, entering specialized sub-options provides full geometric control:

• Angle (A): Prompts for the included angle of the arc segment. Positive values draw counterclockwise; negative values draw clockwise.
• Center (CE): Specifies the center point of the arc circle.
• Direction (D): Overrides default tangency by allowing the user to specify a custom tangent direction vector for the starting point of the arc segment.
• Radius (R): Prompts for the radius of the arc segment.
• Second pt (S): Initiates a 3-point arc segment, prompting for an intermediate point on the circumference followed by the endpoint.
• Line (L): Exits arc mode and returns the PLINE command back to standard straight line mode.

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2. Lightweight Polylines (LWPOLYLINE) vs. Heavy Polylines (2DPOLYLINE)

AutoCAD maintains two distinct internal database representations for 2D polylines:

Database Architecture Comparison:

Legacy Heavyweight (2DPOLYLINE):
[POLYLINE Header] ──► [VERTEX 1] ──► [VERTEX 2] ──► [VERTEX 3] ──► [SEQEND]
(Multiple database entity headers, unique entity handles per vertex, high memory overhead)

Modern Lightweight (LWPOLYLINE):
[LWPOLYLINE Entity]
  ├─ Number of Vertices: 3
  ├─ Coordinate Array: [(X1,Y1), (X2,Y2), (X3,Y3)]
  ├─ Width Array: [(SW1,EW1), (SW2,EW2), (SW3,EW3)]
  └─ Bulge Array: [B1, B2, B3]
(Single compact record, single entity handle, smaller file footprint)

The PLINETYPE System Variable

The PLINETYPE system variable dictates how AutoCAD creates new polylines and how it handles legacy polylines from older DWG files:

• 0: Polylines in older drawings are not converted when opened; new polylines created with PLINE are written as legacy heavyweight 2DPOLYLINE objects.
• 1: Polylines in older drawings are not converted when opened; new polylines are created as lightweight LWPOLYLINE objects.
• 2 (Default Standard): Polylines in older drawings are automatically converted to lightweight LWPOLYLINE objects when the file is opened; new polylines are created as lightweight.

The CONVERTPOLY Command

To manually optimize drawings containing thousands of legacy heavyweight polylines, use the CONVERTPOLY command:

Command: CONVERTPOLY
Convert polyline type [Heavy/Light] <Light>:

Selecting Light converts selected or drawing-wide heavyweight polylines into optimized lightweight polylines. Selecting Heavy converts lightweight polylines back into traditional heavyweight entities (required by certain legacy CAM post-processors or third-party LISP routines).

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3. Polyline Editing with PEDIT

The PEDIT (or PE) command modifies existing 2D polylines, 3D polylines, and converts independent lines and arcs into polyline structures.

Selecting Single vs. Multiple Objects

When invoking PEDIT, AutoCAD prompts:

Select polyline or [Multiple]:

If you select an individual line or arc rather than a polyline, AutoCAD displays: Object selected is not a polyline. Do you want to turn it into one? <Y>: Pressing Enter converts the object into a single-segment polyline.

Typing M (Multiple) allows window or crossing selection of hundreds of lines, arcs, or polylines simultaneously, batch converting them without individual confirmation prompts.

Core PEDIT Operations

• Close / Open: Toggles the topological closure flag. If a polyline is open, Close connects the last vertex to the first vertex with a straight or curved segment. If already closed, the option changes to Open, which removes the closing segment without deleting vertices.
• Join (J): Merges contiguous lines, arcs, or polylines whose endpoints touch (or, when PEDIT was started with Multiple, lie within a specified fuzz distance) into a single continuous polyline.
• Width (W): Assigns a single uniform width across all segments of the polyline, instantly stripping any individual segment taperings.
• Fit (F): Constructs a smooth curve consisting of pairs of tangent circular arcs passing through every polyline vertex.
• Spline (S): Uses the polyline vertices as a control frame to compute a cubic or quadratic B-spline approximation curve. The curve does not pass through interior vertices.
• Decurve (D): Strips all curve-fit or B-spline calculations, reverting every segment back to straight linear segments connecting the original defining vertices.
• Reverse (R): Reverses the vertex order of the polyline from end to start. This is vital when working with directional linetypes (such as gas lines with embedded text ---GAS--- or fence lines) where text or symbology appears upside down or points backwards.

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4. Deep Dive: PEDIT Edit Vertex Operations

Selecting the Edit Vertex (E) option in PEDIT places an interactive marker (X) on the first vertex of the polyline and opens the vertex editing sub-menu:

Enter a vertex editing option
[Next/Previous/Break/Insert/Move/Regen/Straighten/Tangent/Width/eXit] <N>:

Vertex Editing Navigation:
  [V1 (X)] ──Next──► [V2] ──Next──► [V3] ──Next──► [V4]
     ▲                 │
     └────Previous─────┘

Step-by-Step Procedure: Straightening a Polyline Section

1. Type PEDIT and select the polyline.
2. Enter E for Edit Vertex.
3. Type N (Next) or P (Previous) until the X marker rests on the beginning of the section to be straightened.
4. Type S for Straighten.
5. AutoCAD prompts: Enter next vertex <N>:. Type N to navigate the marker to the ending vertex of the target section.
6. Type G for Go. AutoCAD instantly eliminates all intermediate vertices and connects the two marked vertices with a clean straight line segment.

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5. Continuous Linetype Generation: PLINEGEN and Ltype Gen

When non-continuous linetypes (such as HIDDEN, CENTER, or DASHED) are applied to polylines with short segment lengths, unexpected graphical errors frequently occur:

PLINEGEN = 0 (Off):  [Dash-Gap-Dash] ──► Vertex ──► [Dash-Gap-Dash] ──► Vertex
(Linetype restarts at each vertex; short segments lack space and appear solid!)

PLINEGEN = 1 (On):   ───  ───  ───  ───  ───  ───  ───  ───  ───  ───  ───
(Continuous linetype generated across the entire length, ignoring vertex spacing)

The PLINEGEN System Variable

• 0 (Disabled / Off): AutoCAD calculates the linetype pattern independently for each segment. It forces a full dash at the beginning and end of each segment. If a segment is shorter than the dash-gap cycle length of the assigned linetype, the entire segment appears as an unbroken solid line.
• 1 (Enabled / On): AutoCAD calculates the linetype pattern continuously across the entire polyline path, treating the geometry as one seamless entity. Dash-gap cycles flow smoothly around corners and across short segments without interruption.

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6. The JOIN Command

Objective 3.2.b names JOIN alongside PEDIT, and it is usually the faster way to combine objects that already meet end to end. Select a source object (or select several objects at once), and AutoCAD joins them into one object whose type depends on what was selected:

Rules that decide many exam items:

• Objects joined into a polyline must touch end to end; JOIN does not close gaps between mixed lines and arcs. Use PEDIT > Multiple > Join with a fuzz distance when small gaps must be bridged.
• Arcs are joined counterclockwise starting from the source arc.
• The joined object takes its layer and other properties from the source object.

After joining, set PLINEGEN or the polyline's Linetype generation property so dashed linetypes run continuously across the new vertices (section 5 above).