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Coordinate Systems

No matter what kind of drawing you do in AutoCAD, you need a systematic method of specifying points. Points define the beginnings and endpoints of lines, the centers of circles and arcs, the axis points of ellipses, and so on. The capability to place points accurately is important. When an AutoCAD command prompts you for a point, you can either specify a point on the screen with the mouse or pointing device, or enter coordinates at the command line. When entering points, AutoCAD uses a three-dimensional Cartesian, or rectangular, coordinate system. Using this standard system, you locate a point in 3D space by specifying its distance and direction from an established origin measured along three mutually perpendicular axes: the X, Y, and Z axes. The origin is considered to be at 0,0,0. Figure 6.1 illustrates such a coordinate system. Only two dimensions are depicted with the Z axis projecting up, perpendicular to the page. If you are concerned only with two-dimensional drawings, this is the presentation of AutoCAD’s coordinate system that will be seen.

Figure 6.1. The X and Y axes in a 2D coordinate system.



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