AutoCAD 2002 Bible

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Most 2D commands, however, accept 3D coordinates (that is, a coordinate that includes a Z value) only on the first point. After that you must omit the Z coordinate because AutoCAD requires that it be the same as that of the first point. For example, if you draw a rectangle, you can specify its first corner as 2,3,8 but the second corner must be specified without the Z value, as in 6,7. The Z value for the opposite corner is automatically 8.


The LINE command is an exception. It is a true 3D command, so you can specify X,


Y, and Z values at all points.


Figure 21-3: The three axes in 3D drawing


Cross-    Working with the User Coordinate System (UCS) is essential in 3D work. If you are


Reference^ not famj|jar wjth the User Coordinate System, review the discussion in the section «User Coordinate Systems» in Chapter 8.

Absolute and relative Cartesian coordinates in 3D


You don’t use absolute coordinates more in 3D than you do in 2D — maybe less. But understanding absolute coordinates is important to understanding the Cartesian coordinate system that AutoCAD uses to define every point in your drawing. Figure 21-4 shows a wireframe model of a square and a triangle drawn with absolute coordinates, viewed from above (plan view) and from the southeast view (above, to the right, and in front). The square is drawn in 2D — which means that the Z coordinates are all zero—to be a reference point for visualizing the 3D points of the triangle.

Скачать в pdf «AutoCAD 2002 Bible»