Learning Basic Commands to Get Started - AutoCAD 2011 and AutoCAD LT 2011: No Experience Required - page 48

using Relative Coordinates
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 earlier 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 the 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 direc-
tion), 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 will be illustrated in Chapter 4, “Developing
Drawing Strategies: Part 1.”
When the Dynamic Input tool is turned off, you'll need to prefix the coordi-
nate with an “at” symbol ( @ ). In the previous example, you would enter the rela-
tive Cartesian coordinates as @8,0 . The @ lets AutoCAD know that the numbers
following it represent coordinates that are relative to the last point designated.
When the Dynamic Input tool is turned on, relative coordinates are assumed,
and the @ symbol is not required.
Relative Cartesian Coordinates
The Cartesian system of coordinates uses a horizontal (x) component and a verti-
cal (y) component to locate a point relative to the 0,0 point. The relative Cartesian
system uses the same components to locate the point relative to the last point
picked, so it's a way of telling AutoCAD how far left or right and up or down to
extend a line or to move an object from the last point picked (see Figure 2.14). If
the direction is to the left, the x-coordinate will be negative. Similarly, if the direc-
tion is down, the y-coordinate will be negative. Use this system when you know the
horizontal and vertical distances from point 1 to point 2. To enter data using this
system, use this form: @x,y .