AutoCAD 2002 Bible

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You can use relative coordinates in the same way by including the change in coordinates. For example, to draw the line from (3,2,1) to (6,4,3), shown in Figure 21-4, you can start with the absolute coordinate (3,2,1) and then specify @3,2,2 because that’s the difference between (6,4,3) and (3,2,1).

Cylindrical and spherical coordinates


Just as polar coordinates are often more useful than Cartesian coordinates in 2D, cylindrical and spherical coordinates can be more useful in 3D. Here’s how they work.

Figure 21-4: A rectangle and triangle viewed from plan view and southeast view


Cylindrical coordinates have the format (@)distance<angle,distance. Here’s how it works:


♦    The first distance is the number of units in the XY plane from the origin (for absolute coordinates) or your last point (for relative coordinates).


♦    The angle is the number of degrees from the X axis in the XY plane.


♦    The second distance is the number of units along the Z axis.


Cylindrical coordinates can be absolute or relative. Add the @ for relative coordinates. When you draw a line using cylindrical coordinates, neither distance you specify is the length of the line. In essence, you are defining the lengths of two sides of a triangle to draw the hypotenuse. Figure 21-5 shows an example of a line drawn with cylindrical coordinates. The line was drawn from 0,0,0 to @5<30,3, which results in a line 5.8310 units long. (The @ wasn’t necessary because the line was drawn from 0,0,0.)


The two sides of the triangle are 5 and 3 units long. To calculate the length of the hypotenuse, use the Pythagorean theorem, which says that a2 + b2 = c2 where a and are the two sides of the triangle and c is the hypotenuse. Therefore, the hypotenuse is the square root of 25+9 or 34, which is 5.8310. Of course, you can use the DIST or LIST command to check it out.

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