Principal Attributes of Linear Dimensions

The example illustrates the attributes of a linear Dimension that most affect its general appearance. (Default values are in brackets.)

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 takes 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 can be specified. Examples of setting this attribute are:

PLCL can be set at Dimension or Dimension Point level. PLCL set at Dimension level (which becomes the ‘default’ PLCL) applies 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.

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

(Refer to Principal Attributes of Linear Dimensions for further comments relating to OSHT.)

Other attributes are:

DOFF has a related attribute DPOS - the Dimension Line Position. DPOS is a fixed 2D paper coordinate through which the dimension line passes. DOFF and DPOS are mutually exclusive: setting one unsets the other. If a dimensioned element is moved the dimension line also moves when DOFF is used; if DPOS is used the dimension line still passes 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 unsets 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, for example:

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

The dimension direction can be set explicitly by a command, for example:

DIR E10N

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

DIR TRUE

The dimension direction is set as being that from the first to the second dimension point (TLIN is set to TRUE and DIR is unset).

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 the example 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, for example, 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 @

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