Points, Planes and Axes
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Points, Planes and Axes
ARC+ defines a point in space by using Cartesian coordinates. These coordinates are the values of the point measured along three axes at right angles to each other. The axes are designated x, y and z. The xy axes are usually assumed to be the horizontal plane and the z axis is the vertical plane.
A plane is defined by three points in space, i.e., three sets of three coordinates. Any plane parallel to two of the axes is known as a major or orthogonal plane (xy, xz and yz). A plane parallel to only one of the xyz axes is known as a diagonal plane, while a free plane is not parallel to any axis.
Origin
The origin is the reference location in space from which all measurements along the axes are made. The value of each axis at the origin is 0 and increases with the distance from the origin. The value may be positive or negative depending on the direction. The origin is represented on the screen by an asterisk.
IMPORTANT You can locate the origin (0 0 0) using the Snapping to a point (\snap) command, described in the next section.
It is important to remember that the working and viewing planes are not necessarily the same. The differences between the two are described in the following section.
Viewing Plane
To view any major plane (orthogonally) you must look parallel to the third axis. The default viewing mode in the working window is plan view.
In plan view you look at the xy plane by viewing it parallel to the vertical z axis.
When you switch the view to front elevation you are looking orthogonally at the xz plane from along the y axis.
If you rotate the view by 90 degrees, you are viewing the yz plane from along the x axis.
You can look at any plane along the third axis, in either direction, positive or negative. This allows you to see the front and rear elevations (xz) and the left and right elevations (yz).
The views parallel to one of the major axes (or perpendicular to one of the major planes) are called orthogonal views. A non-orthogonal view such as axonometric allows you to see more than one plane or two axes.
In addition to the standard plan, elevation and axonometric views, ARC+ enables you to manipulate the views in a variety of ways. Any viewing position can be displayed, at any distance and at any angle.
Active or Working Plane
In ARC+ the cursor moves on only one plane at a time. This is natural in orthogonal views since you cannot see the third axis. (The third axis is parallel to your viewing direction.) In axonometric, however, the cursor may appear to move in 3D space when in fact it remains on the currently active plane. This is because you cannot control the depth of the cursor along the viewing direction - perpendicular to the screen.
HINT The active plane is indicated on screen by the axis indicator at the top right of the screen. The active axes are of the same color, while the third (inactive) axis is displayed in a different color. You can also determine the active plane, if the axis indicator is not displayed, by observing which two coordinates in the status line change when you move your mouse. The third axis remains static.
When you change to one of the orthogonal views (plan or elevation) the system changes the active plane automatically parallel to the viewing plane. When switching to axonometric, the current active plane remains active.
NOTE In orthogonal projections, the active plane is usually the same as the viewing plane. However, you can specify a different active plane. In such cases the cursor appears to be active only along a line as the active plane is perpendicular to the viewing plane.
Activating the Third Axis
You can switch to a plane parallel to the current one but with a different value on the third axis by snapping to an existing entity on the required plane or by specifying a coordinate value on the inactive axis. The value of the third axis in the status line is updated. See the description of the snap (\snap) command below.
HINT In the control window, you can move the eye/target view on the inactive axis by holding the shift key while moving the mouse.