Modeling from Above

So far in this book, you've been working with X- and Y-coordinates only. X- and Y-coordinates by themselves define a two-dimensional plane known (logically enough) as the X,Y plane.

In this and the following two chapters, you supply a third value, a Z-coordinate, in order to locate geometry above or below the X,Y plane. You use the same methods to specify Cartesian (X,Y,Z) coordinates as the ones I set out in Chapter 7 — you just add a comma and another number. The following techniques can be used to specify points in 3D:

• Coordinate and direct distance entry
• Point filters
• Object snaps
• 3D object snaps
• Object-snap tracking

Using 3D coordinate input

I introduce you to 2D coordinate entry in Chapter 7. To recap, the input formats are:

• Absolute Cartesian coordinates: Expressed as X,Y — the distances along the X- and Y-axes from the origin (0,0).
• Relative Cartesian coordinates: Expressed as @X,Y — the distances parallel to the X- and Y-axes from last point.
• Relative polar coordinates: Expressed as @d<a — the distance and angle in the X,Y plane from the last point.

While you can use these 2D coordinate input methods in 3D, you usually have to give AutoCAD just a little more information when you want to work in three dimensions. You can use absolute or relative Cartesian coordinates by simply adding a Z-coordinate to the end. In addition, there are two 3D-only coordinate formats, both based on 2D polar coordinates. You can enter 3D coordinates using the following methods:

• Absolute Cartesian coordinates: Expressed as X,Y,Z. Working with absolute coordinates in 3D should look familiar because the input is like working in 2D, except you add a Z-coordinate at the end.
• Relative Cartesian coordinates: Expressed as @X,Y,Z. Relative coordinates in 3D work just like they do in 2D, except you add the Z-coordinate.
• Cylindrical coordinates: Expressed as @d<a,z. Cylindrical coordinates are similar to relative polar coordinates (d is the distance and a the angle from the last point in the X,Y plane) with the addition of a Cartesian Z-coordinate to locate the point above or below the X,Y plane.
• Spherical coordinates: Expressed as @d<a1<a2. Spherical coordinates are also based on relative polar coordinates, but instead of a Z-coordinate to specify a location above or below the X,Y plane, you specify an angle from the X,Y plane.

Using point filters

Coordinate entry as described in the previous section is fine if you already know the coordinate values or distances in 3D space where you want to locate your points. Often, however, you may need to derive 3D points from existing geometry without knowing those exact values. Using point filters, you can locate new points based on existing points. You can use point filters to construct 2D and 3D coordinates.

When AutoCAD prompts you to specify a point, you can either enter point filter values at the command line, or bring up the right-click menu and choose a filter option from the Point Filters submenu. After selecting a point filter option, specify a point to filter out part of the coordinate value and then enter the requested value. For example, use the .XY filter to enter your own Z-coordinate value after specifying a point on the current working plane.

Object snaps and object snap tracking

Object snaps allow you to accurately specify points on existing objects in a 3D model. You can use object snaps such as endpoints, midpoints, and center on 3D objects, but AutoCAD also provides a set of object snaps specific to 3D modeling.

3D object snaps allow you to specify points on 3D objects that regular object snaps won't recognize — for example, the center of a face, or a vertex. You enable 3D object snaps by clicking the 3D Object Snap button on the status bar. Right-click the 3D Object Snap button and choose the running 3D object-snap modes you want to use.

Object snap tracking allows you to calculate points that are not on an object by using object snap points on existing objects. When working in 3D, you can not only track points on the current X,Y plane but also along the Z-axis. While tracking points along the Z-axis, AutoCAD provides feedback in the form of a tooltip that lets you know you are moving along the Z-axis in the positive or negative direction.