How P-lines Are Used For Generic Sections

P-lines defined in the Pointset of the Section Profile (as referenced by the SpecRef) follow the path defined by the GENSEC’s Spine. This means that, for a curved GENSEC, the p-line directions are not constant over their lengths. This must be taken into consideration when using them as datums referenced by attached items.

The command syntax used for referencing p-lines for SCTNs, as detailed in Positioning and Orientating Using P-lines, is extended when applied to GENSECS. The additional command options available are summarised here.

Positions Derived From P-lines (refer to Positioning by Using P-lines):

The distance value along the p-line may be preceded by the keyword DISTANCE. For example:

PPLINE TOS DISTANCE distance FROM END ...

The position along the p-line may be measured from the plane normal to the end of the GENSEC, from the cutback plane, or from the joint end preparation cutback. For example:

PPLINE TOS OF /BEAM1 NORMAL ...
PPLINE TOS OF /BEAM1 NOCUTBACK ...
PPLINE TOS OF /BEAM1 CUTBACK ...

It is possible to define a proportional and an absolute distance position at the same time. For example:

PPLINE TOS PROPORTION value DISTANCE distance ...

Positions may be measured along the actual p-line specified, or along the Spine and then projected onto the p-line. For example:

PPLINE TOS distance VIA SPINE FROM END ...
PPLINE TOS PROPORTION value VIA PLINE ...

The element specified by the OF keyword may be a POINSP or a CURVE. This redefines the element to be the GENSEC owning the POINSP or CURVE, rather than the current element or an element defined with the p-line choice. This sets:

• the direction along which DISTANCE and PROPORTION are to be measured;

• in the case of a CURVE, the POINSP to be used as the origin from which DISTANCE and PROPORTION are measured;

• the segment of the GENSEC to which PROPORTION is applied as:

• the named CURVE,

• the segment following the POINSP (from START), or

• the segment preceding the POINSP (from END).

For example:

PPLINE TOS PROPORTION 0.4 FROM END VIA SPINE OF /CURVE3

calculates a position 40% along /CURVE3 measured from its end.

If no POINSP or CURVE is specified, PROPORTION is calculated from the start of the GENSEC.

Directions and Offsets Derived From P-lines (refer to Orientating by Using P-lines):

Directions and offsets derived from p-lines can have a derived position inserted after the direction/offset option and before the OF option. This lets the user specify the position along the p-line at which the direction/offset is to be calculated. For example:

PPLINE TOS X DIRECTION PROP 0.4 VIA PLINE FROM END OF /CURVE3
PPLINE TOS OFFSET FROM PPLINE BOS DISTANCE 200 FROM END

If no position is given, the direction/offset is calculated using the start of the GENSEC or element specified by the OF option.

Picked Positions on P-lines

The following syntax lets the user derive a position on a p-line which is nearest to a probe line:

PPLINE pline_name NORMAL/CUTBACK NEAR direction THROUGH position [VIA SPINE/PLINE] BOUND/UNBOUND

The probe line starts at the THROUGH position and extends in the specified direction. The expression returns the nearest point on PLINE pline_name.

If BOUND is set, the point is either on the p-line or at the ends (where it intersects the NORMAL or CUTBACK end of the GENSEC). If UNBOUND is set, the point can be on the tangential extension of the p-line.

The point returned will be in or above the plane through the THROUGH position, normal to the probe line; that is, the point will not be behind the THROUGH position.

The VIA SPINE/PLINE option is used if the point is to be returned with its distance along the p-line from the start of the segment.

If pline_name is specified as ANY, all plines of the GENSEC will be probed and the closest p-line will be used.

Identifying P-line Elements

P-line expressions can be used in any positioning command (DISTANCE, PROPORTION, NEAR etc.) in the following way:

ID SEGMENT PPLINE ... positioning_command
ID ONPLINE PPLINE ... positioning_command

ID SEGMENT returns the identity of the POINSP at the start of the segment on which the specified position occurs. ID ONPLINE returns the identity of the p-line on which the specified position occurs. (The latter is only useful for the PPLINE ANY option, otherwise it simply returns the identity of the specified p-line.)

The following expressions use keywords analogous to the OFFSET option between the p-line specification and the subsequent positioning command:

PPLINE pline_name PLDIST positioning_command
PPLINE pline_name PLKEY positioning_command
PPLINE pline_name SEGMENT positioning_command
PPLINE pline_name ONPLINE positioning_command

PPLINE...SEGMENT and PPLINE...ONPLINE return references to the segment and p-line, respectively, on which the point is specified. PPLINE...PLDIST returns the distance along the segment on which the point lies, in the direction specified in the positioning_command (FROM START or FROM END). PPLINE...PLKEY returns the name (the PKEY) of the p-line on which the point lies.