Alignments and Profiles
Alignments
An alignment is a horizontal control line built from fixed, floating, and free sub-entities. The distinction matters: fixed entities are fully constrained by geometry, floating entities attach to one fixed neighbor, and free entities attach to two neighbors and solve between them. Questions often present a scenario where a curve must remain tangent to two lines while one line moves - that requires a free curve, not a fixed one. Design criteria and check sets validate speed-dependent parameters such as minimum radius and superelevation attainment.
Profiles
A profile is a vertical representation tied to an alignment. Surface profiles are sampled dynamically from a surface; layout profiles are drawn from tangents and curves; superimposed profiles overlay a second surface or design for comparison. Profile views control vertical exaggeration, which is simply the ratio of vertical to horizontal scale and is the source of many label-placement complaints.
The Classic Traps
First, profile views do not contain profiles - they display them, so deleting a profile view leaves the profile intact. Second, editing a surface profile requires the profile to be re-sampled or updated; it is dynamic only while the link is maintained. Third, vertical curves are defined by K-values, and a common test asks you to compute the length from a K-value and algebraic grade difference.
Worked Example
Grades of +3.0% and −1.5% meet at a crest with K = 40. The algebraic difference is 4.5%, so the vertical curve length is 40 × 4.5 = 180 units. Half that length on each side of the PVI gives the PVC and PVT stations.