Alignments provide the means to connect different line types together, end to end, provided they share common end points. In their construction they are very similar to polygons, except they do not have to close back to the starting point. All line types can be used in an alignment with the exception of circles and offsets.
Connecting lines together has the advantage of being able to compute distances and chainages along different line types.
There are two types of alignment, primary and secondary. A primary alignment has the ability to support Chainage Equations, Section Markers, Corridors of Interest, and Cross Sections. Secondary alignments have a primary alignment as a parent and therefore their chainages are controlled by their primary alignment. Secondary alignments do not support the same objects as primary alignments as support for these is provided through the parent primary alignment.
All alignments have a unique description, which is used as an identifier. They also each have a distance, which always starts at 0.000 and gives the length of the complete alignment. The distance can be either in grid, ground or, if working on a projection, ellipsoid.
Primary alignments are given a starting chainage, which can be any user-defined value. Secondary alignments are not given a chainage as this is generated from its primary alignment.
Alignments do not have attributes for line style, colour or group, as these are controlled by the individual lines that make up the alignment.
When an alignment is created it is created as a primary alignment. The only exception being when a parallel alignment is created, it is created as a secondary alignment to the primary alignment it was created from. Primary and secondary alignment relationships can always be changed through the Section Manager.
The principal reason for using secondary alignments is to maintain chainages with respect to a primary control alignment. This generally means setting up a single primary control alignment with several secondary alignments that all share a common chainage, at any given point, perpendicular to the primary alignment.
When setting up a secondary alignment, consideration needs to be given regarding the relationship to its primary alignment. For instance, if the primary and secondary alignments are approximately parallel to each other, but the secondary alignment is longer than the primary alignment, it will not be possible to compute chainages at all points along the secondary alignment. The active region of the secondary alignment will be restricted to the section where a perpendicular offset can be computed from the primary to the secondary alignment. Likewise, strange things may appear to happen if the secondary alignment doubles back upon itself, but its primary parent does not. This would cause the chainage to reverse direction at the point it starts to double back. Both these scenarios are possible, but the consequences may seem unpredictable, especially if long sections are taken out along these secondary alignments.
Chainages along a secondary alignment do not in any way reflect the distance along the secondary alignment.