Creating Polylines

Polylines are created using the PLINE command. The PLINE command enables you to draw two basic kinds of polyline segments: straight lines and arcs. Some PLINE command prompts, therefore, are the same as those you find in the LINE and ARC command prompts. If, for example, you draw straight polyline segments, you will find options such as Specify Next Point, Close, and Undo. You see these options in the standard polyline prompt for line segments:

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

In addition, certain prompt options, such as Width, are specific to polylines. Those prompt options include the following:

• Arc. Switches from drawing polylines to drawing polyarcs and issues the polyarc options prompt.

• Close. Closes the polyline by drawing a segment from the last endpoint picked to the initial start point and then exits the PLINE command.

• Halfwidth. Prompts for the distance from the center to the polyline’s edge (half the actual width). For more information, read about the Width option.

• Length. Prompts for the length of a new polyline segment. AutoCAD draws the segment at the same angle as the last line segment or tangent to the last arc segment. The last line or arc segment can be that of a previous polyline, line, or arc object.

• Undo. Undoes the last drawn segment. It also undoes any arc or line option that immediately preceded drawing the segment. It does not undo width options.

• Width. Prompts you to enter a width (default 0) for the next segment. Enables you to taper a segment by defining different starting and ending widths. AutoCAD draws the next segment with the ending width of the previous segment. Unless you cancel the PLINE command prior to drawing a segment, the ending width is stored as the new default width.

• Specify Start Point / Specify Next Point. Prompts for the starting point or the next point of the current line segment. After you begin a new polyline, Specify Next Point is the default option.

If you select the Arc option, the Arc mode options prompt appears:

[Angle/Center/Close/Direction/Halfwidth/Line/Radius/Second pt/Undo/Width] 

This prompt contains some of the same options as the ARC command and others specific to the drawing of polylines. The Arc options include the following:

• Angle. Prompts for an included angle. A negative angle draws the arc clockwise.

• Center. Prompts you to specify the arc’s center.

• Close. Closes the polyline by using an arc segment to connect the initial start point to the last endpoint and then exits the PLINE command.

• Direction. Prompts you to specify a tangent direction for the segment.

• Halfwidth. Prompts for a halfwidth, the same as for the Line option.

• Line. Switches back to the line mode.

• Radius. Prompts for the polyarc’s radius.

• Second pt. Selects the second point of a three-point polyarc.

• Undo. Undoes the last drawn segment.

• Width. Prompts you to enter a width, the same as in line mode.

• Specify Endpoint of Arc. Prompts for the endpoint of the current arc segment. This is the default option.

Although using the PLINE command to draw lines and arcs is similar to using the LINE and ARC commands to draw similar objects, there are several important differences:

• You get all the Line or Arc mode prompts every time you enter a new polyline vertex.

• Prompt options for the Halfwidth and Width control the width of the segment.

• You can switch back and forth from line segments to arc segments as you add segments to the polyline.

• You can apply linetypes continuously across vertices.

• You can apply the MEASURE and DIVIDE commands to polylines.

• You can use the AREA command to report the total length of a polyline and to calculate the area enclosed by certain polylines.

In the following exercise, you practice drawing a single polyline using many of the options for both the Line and Arc mode options of the POLYLINE command.

Exercise 8.1 Practicing with the POLYLINE Command Options

1.	Open a drawing named 08DWG01.dwg found on the accompanying CD-ROM. This blank drawing will be used to show the power of polylines.
2.	Start the POLYLINE command by typing PL and pressing Enter. The initial polyline prompt Specify start point: appears. Respond by entering the point 2.5,1.0 . The Line mode options prompt appears.
3.	Use the default endpoint option by entering the relative polar coordinate @5<90 . AutoCAD draws the first segment of the polyline from to in Figure 8.2. The Line mode options prompt reappears. Figure 8.2. Drawing a multisegment polyline.
4.	Switch to the Polyarc mode by typing A and pressing Enter. The Arc mode options prompt appears.
5.	Use the default Specify Endpoint of Arc: option by typing @.25<45 and pressing Enter. AutoCAD draws the polyarc to . The Arc mode options prompt reappears.
6.	Switch back to the Line mode by choosing the Line option. Type L and press Enter. The Line mode options appear. Choose the Length option by entering L . Then, at the Specify length of line: prompt, type @3<0 and press Enter. AutoCAD draws the next segment three units long, to . Now switch back to the Arc mode by entering A . The Arc mode options prompt appears.
7.	Choose the angle option by typing A and pressing Enter. At the Specify included angle: prompt, type -90 and press Enter. This specifies a 90-degree clockwise included angle to the point at . The Specify endpoint of arc or [Center/Radius]: prompt appears.
8.	Choose the Radius option by typing R and pressing Enter. When you’re prompted to specify the radius of the arc, enter the value 0.125 . Respond to the Specify direction of chord for arc: prompt by typing 315 and pressing Enter.
9.	Switch to the line mode by entering L . Choose the Width option by entering W .
10.	Respond to the Specify starting width: prompt by typing 0.1 and pressing Enter. Accept the Ending Width <0.1>: value by pressing Enter. Respond to the Line mode’s Specify next point: prompt by entering @1<-90 . AutoCAD draws the next segment with a uniform width of 0.1 units (see ).
11.	Now at the Specify next point: prompt, enter the relative coordinate @2,0 . AutoCAD draws the next segment at the current width of 0.1 units.
12.	Again, switch to the Arc mode by entering A . Respond to the Arc mode prompt by typing CE and pressing Enter. At the Specify center point of arc: prompt, enter @0.25,-0.25 to specify the center of the next arc section.
13.	Respond to the Specify endpoint of arc or [Angle/Length]: prompt by entering A. Specify an included angle of –90 (see ).
14.	Switch to the Line mode (type L and press Enter) and enter the relative polar coordinate @4.052<270 . AutoCAD draws the next line segment at the current 0.1 width (see ).
15.	Now change the width for the next segment by typing W and pressing Enter. Respond to the Specify starting width: prompt by entering 0 . Then accept the default ending width by pressing Enter.
16.	The standard Specify next point: or [Arc/Close/Halfwidth/Length/Undo/Width]: prompt appears. Choose the Close option by entering C . AutoCAD draws the next segment to the start point of the first segment of the series with a 0 width (back to ).
17.	Your drawing should now resemble Figure 8.2. You will continue using this drawing in the next exercise.

As you saw in the preceding exercise, you can easily switch back and forth between the Line mode and the Arc (Polyarc) mode within the PLINE command. Each mode has its own set of prompts that repeat after you draw each segment. You also saw that both line and arc segments can have widths other than zero and that when you specify a new width, it becomes the default width for the next segment. In other words, both the mode (Line versus Polyarc) and the width remain in effect unless and until you explicitly change them. This facilitates drawing a long series of either straight line or curved arc segments.

In the following exercise, you construct an arc leader using both the Line and Arc modes. You will also take advantage of the capability to vary the width of a single polyline segment.

Exercise 8.2 Drawing an Arc Leader with a Polyline

1.	Continue in the drawing from the preceding exercise. Refer to Figure 8.3 for this exercise. Figure 8.3. Picking the points for a PLINE arc leader.
2.	Begin the PLINE command by choosing the Polyline option from the Draw menu. The Specify start point: prompt appears.
3.	Pick a point near in Figure 8.3. At the Specify next point: prompt, choose the Width option by typing W and pressing Enter.
4.	Specify a starting width of 0.0 and an ending width of 0.1 .
5.	Respond to the Specify next point: prompt by entering @.2<115 . AutoCAD draws the first segment with a tapered width. You may not see the taper until the next step is complete.
6.	At the Specify next point: prompt, enter W again.
7.	Specify a starting width of 0 and an ending width of 0 .
8.	Respond to the Line mode prompt by choosing the Arc option.
9.	The Arc mode prompts appear. Note that the arc drags as you move the cursor near . Pick a point near , and AutoCAD draws the arc. Press Enter to exit the PLINE command.
10.	Your drawing should resemble Figure 8.4. Please close the drawing without saving. Figure 8.4. The completed polyline arc leader.

The preceding exercise demonstrates the versatility of polylines. Not only can polylines have varying widths, but each segment also can have different starting and ending widths. The current polyline width is stored in the system variable PLINEWID.

Controlling Polyline Appearance with FILL

A drawing that consists of a large number of polylines with widths other than zero can significantly increase the time it takes AutoCAD to redraw the screen or to plot the drawing. AutoCAD provides the FILL command to enable you to control the visibility of the filled portion of wide polylines. When you turn FILL off, AutoCAD displays or plots only the outline of filled polylines. You must regenerate the drawing in order to see the effect of the FILL command. Figure 8.5 shows the effect of having turned FILL on and off. The FILL command also controls the appearance of all hatches and of objects created using the SOLID, TRACE, and MLINE commands.