LT On-line: Lesson 12

28 Feb, 2001 By: Mark Middlebrook

Why Line when you can Pline?

Page 1: A polyline primer

After all of the conceptual "heavy lifting" in my previous sets of LT Online lessons on blocks and xrefs, we'll take a breather this month and consider when and how to use the Pline command. If you're one of those LT or AutoCAD users who doesn't fully appreciate why Pline is better than the Line command for many drawing tasks, then use this lesson to develop your polyline prowess.

The procedures described here work with AutoCAD LT 98-2000i and AutoCAD Release 14-2000i.

A polyline primer

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Figure 1.
By now, you're familiar with using the Line command to draw straight line segments-either a single segment or a series of segments that lie end to end. The Line command is fine for some drawing tasks, but the Pline (or Polyline) command is a better, more flexible choice in many situations. The most important differences between these two commands are:

The Pline command draws curved segments as well as straight ones (figure 1). Curved polyline segments are circular arcs, of the sort that you can draw with the Arc command.

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Figure 2.
With the Pline command, you can add width (either uniform or tapered) to straight and curved segments (figure 2). This feature is less important in AutoCAD 2000, which has object lineweights and plot styles for controlling on-screen and plotted lineweights.

The Line command draws a series of single line segment objects. Even though they appear to be linked onscreen, each segment is a separate object. The Pline command, on the other hand, draws a single, connected, multi-segment object. Figure
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Figure 3.
3 shows how the same sketch drawn with the Line and the Pline commands responds when you select one of the segments.

The last difference is the most important. When you use the Line command, AutoCAD draws a type of object called, appropriately enough, a line. AutoCAD stores each segment as a separate line object. The Pline command draws a special kind of multi-segment object called a polyline (sometimes referred to as a pline because of the command name). AutoCAD stores all of the segments together as one object.

Why is this difference important? With the Pline command, you can group together segments that belong together. If you draw a series of end-to-end segments, there's a good chance that those segments are logically connected. For example, they might represent the outline of a single object or a continuous pathway. If the segments are connected logically, then it makes sense to keep them connected in AutoCAD.

The most obvious practical benefit of grouping segments together into a polyline is that many editing operations are more efficient when you use polylines. If you move one line segment, other segments that you drew at the same time don't move with it. The same is true for other common editing operations, such as copying, erasing, rotating, and mirroring. When you select any segment in a polyline for editing, the entire polyline is affected.

In short, you can get the same plotted results by using the Line and Arc commands or the Pline command, but Pline results in objects that are more logically organized and that respond more sensibly to editing operations.

Why Line when you can Pline?
  Page 1: A polyline primer
  Page 2: Follow the Pline prompts
  Page 3: Polyline prowess

About the Author: Mark Middlebrook

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