Once you understand the coordinate system used by AutoCAD, you can draw
lines to any length and in any direction. Look at the shape shown previously
in Figure 2.1. Because you know the dimensions, you can calculate (by adding and subtracting) the absolute coordinates for each vertex—the connecting point between two line segments—and then use the LINE command to draw the shape by entering these coordinates from the keyboard. However, AutoCAD offers you several tools for drawing this box much more easily. Two of these tools are the relative Cartesian and relative polar coordinate systems.
When you’re drawing lines, these coordinate systems use a set of new points
based on the last point designated rather than on the 0,0 point of the drawing
area. They’re called relative systems because the coordinates used are relative to the last point specified. If the first point of a line is located at the coordinate 4,6 and you want the line to extend 8 units to the right, the coordinate that is relative to the first point is 8,0 (8 units in the positive X direction and 0 units in the positive Y direction), whereas the actual—or absolute—coordinate of the second point is 12,6.
The relative Cartesian coordinate system uses relative x- and y-coordinates in
the manner shown, and the relative polar coordinate system relies on a distance and an angle relative to the last point specified. You’ll probably favor one system over the other, but you need to know both systems because you’ll sometimes find that, given the information you have at hand, one will work better than the other. A limitation of this nature is illustrated in Chapter 4, “Developing
Drawing Strategies: Part 1.”
To designate the usage of relative coordinates, you’ll need to prefix each
coordinate you enter with an at symbol (@). In the previous example, you would enter the relative Cartesian coordinates as @8,0. The @ lets AutoCAD know that the numbers following it represent coordinates that are relative to the last point designated.
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