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Angle: This option specifies the included angle that the arc sweeps out. A 180-degree angle, for example, is a semicircle.
Length of chord: This option specifies the length of an imaginary straight line connecting the endpoints of the arc. Most people use this option seldom or never.
Endpoint: This option specifies where the arc ends. It’s the default option and is often the easiest to use.
If you specify the start point as the first option, you can choose among the following three command line options as well:
Center: This option prompts you for the arc’s center point and then finishes with the three options listed previously.
End: This option specifies the endpoint of the arc. You then need to define the angle the arc covers, its direction, its radius, or its center point.
Second point: This is the default option. The second point you choose is not the endpoint; instead, it’s a point on the arc that, along with the start and endpoints, defines how much the arc curves. After you enter the second point, you must enter an endpoint to complete the arc.
To get a feel for how these permutations can be strung together to create different arc-drawing methods, choose DrawOArc and look at the impressive submenu that unfurls, as shown in Figure 6-7.
The following example shows how you draw an arc with the default start point/second point/endpoint method:
1. Set the desired layer current, and set other object properties that you want applied to the arc that you’ll draw.
2. Click the Arc button on the Draw toolbar.
AutoCAD starts the ARC command and prompts you to specify the first endpoint of the arc. The command line shows:
Specify start point of arc or [Center]:
150 Part II: Let There Be Lines
A deluge of Arc options, with the results of using some of those options.
3. Specify the start point by clicking a point or typing coordinates.
AutoCAD prompts you to specify a second point on the arc.
Specify second point of arc or [Center/End]:
4. Specify a second point on the arc by clicking a point or typing coordinates.
The second point lies somewhere along the curve of the arc. AutoCAD determines the exact curvature of the arc after you choose the final endpoint in the following step. To align the second point with an existing object, use an object snap mode.
AutoCAD prompts you to specify the other endpoint of the arc; as you move the cursor around, AutoCAD shows how the arc will look.
Specify end point of arc:
5. Specify the other endpoint of the arc by clicking a point or typing coordinates.
AutoCAD draws the arc, as shown in Figure 6-7.
Chapter 6: Where to Draw the Line
As you may recall, pressing Enter repeats the last command. What often throws new AutoCAD users is that Enter does not repeat the options of the last command. If you go through the command prompts or the Draw menu to draw an arc using the Center, Start, End option, for example, pressing Enter is not going to repeat that method — it’s going to repeat the ARC command in its default form, and the three points you pick will probably not give you the arc you meant to draw.
An ellipse is like a warped circle with a major (long) axis and a minor (short) axis. These axes determine the ellipse’s length, width, and degree of curvature. An elliptical arc is an arc cut from an ellipse.
The AutoCAD ELLIPSE command provides a straightforward way of drawing an ellipse: You specify the two endpoints of one of its axes and then specify an endpoint on the other axis. But like the ARC command, the ELLIPSE command offers a bunch of other options:
Arc: This option generates an elliptical arc, not a full ellipse. You define an elliptical arc just as you do a full ellipse. The following methods for creating an ellipse apply to either.
Center: This option requires that you define the center of the ellipse and then the endpoint of an axis. You can then either enter the distance of the other axis or specify that a rotation around the major axis defines the ellipse. If you choose the latter, you can enter (or drag the ellipse to) a specific rotation for the second axis that, in turn, completely defines the ellipse.
Rotation: With this option, you specify an angle, which defines the curvature of the ellipse — small angles make fat ellipses (0 degrees creates a circle, in fact), and large angles make skinny ellipses. The name of the option, Rotation, has something to do with rotating an imaginary circle around the first axis. If you can figure out the imaginary circle business, then you have a better imagination than we do.
The following command line example creates an ellipse by using the default endpoints of the axes method. Figure 6-8 shows an ellipse and an elliptical arc.
Specify axis endpoint of ellipse or [Arc/Center]: pick or type the first endpoint of one axis Specify other endpoint of axis: pick or type the other endpoint of one axis Specify distance to other axis or [Rotation]: pick or type the endpoint of the other axis