Principal Attributes of Linear Dimensions

Those attributes of a linear Dimension that most affect its general appearance are shown in Figure 12:9 below. (Default values are shown in brackets.)

Figure 12:9. Key Attributes of a Linear Dimension

Note that OSHT, PLCL and DOFF are given initial default values at Layer level when the Layer is created. These values are cascaded down to Dimension level when those elements are created subsequently. Following this initial setting, an attribute set to ‘default’ at Dimension Point level will take its value from the same attribute of its owning Dimension element (if appropriate).

The Projection Line Clearance (PLCL) is the distance between the dimension point and the projection line at the dimension point end. Negative clearances may be specified. Examples of setting this attribute are:

PLCL may be set at Dimension or Dimension Point level. PLCL set at Dimension level (which becomes the ‘default’ PLCL) will apply to all subsequently created Dimension Points. PLCL set at Dimension Point level can only be reset from the same Dimension Point, not from Dimension level. When setting PLCL with the cursor, the resulting (paper) coordinate that does not lie in the same axis direction as the projection line is ignored. These comments also apply to the OSHT attribute (see below).

The Projection Line Overshoot (OSHT) is the distance by which the projection line ‘overshoots’ the end of the dimension line. Negative overshoots may not be specified. Examples of setting this attribute are:

(See PLCL description for further comments relating to OSHT.)

The other three attributes shown on Figure 12:9 above are:

DOFF has a related attribute DPOS - the Dimension Line Position. DPOS is a fixed 2D paper coordinate through which the dimension line will pass. DOFF and DPOS are mutually exclusive: setting one will unset the other. If a dimensioned element is moved the dimension line will also move when DOFF is used; if DPOS is used the dimension line will still pass through the same point.

A feature of these four attributes (DOFF, DPOS, PLDI, DIR) is that although they are attributes of the Linear Dimension they can also be set at Dimension Point Level without the need to navigate to the Dimension element.

DOFF and DPOS both control the position of the dimension line and so are mutually exclusive - setting one will unset the other. Examples of the relevant commands are:

When setting DPOS, the resulting (paper) coordinate that does not lie in the same axis direction as the required offset is ignored.

The projection line direction is set by commands such as:

Note: Projection line direction is treated as 90 degrees when it is nearly but not quite 90 degrees. This avoids an unexpected flip in the projection line text orientation.

The dimension direction can be set explicitly by a command such as:

DIR E10N

Another method is to set the ‘true length’ attribute (TLIN) using the command:

DIR TRUE

This will set the dimension direction as being that from the first to the second dimension point (TLIN is set to TRUE and DIR is unset). See Figures 12:10 and 12:11 below.

Figure 12:10. Definition of ‘True Length’

Figure 12:11. Effect of True Length Attribute Settings

The true length facility can be useful when it is not clear what explicit direction to set in order to achieve the desired result. The bottom left-hand dimension in Figure 12:11 has been created without regard for the Dimension direction, which turns out to be inappropriate. Setting TLIN TRUE produces the desired picture.

Points to note about the true length facility are:

• The true length is 2D, that means, the length is orthographic, not a slope length - ‘uppings’ are ignored.

• True length is meaningless for a Dimension with more than two points, unless the points are in line.

• Since a DPBA Dimension Point relies on the existence of a predefined dimension direction, such a point cannot be used as one of the first two points of a true length.

It is possible to set the dimension direction to that of a p-point, using the command:

DIR IDP @

(This command will set the DDNM attribute to the name or reference number of the primitive concerned, and the PPDI attribute to the appropriate p-point number; DIR will be unset and TLIN set to FALSE.)