Corridors and Sections
Assemblies and Subassemblies
A corridor is the marriage of three inputs: a horizontal baseline (alignment), a vertical baseline (profile), and an assembly of subassemblies. Subassemblies carry their own logic - lane widths, side slopes, daylight targets - and many support conditional targeting. The corridor builds stations along the baseline at a specified frequency, applies the assembly at each station, and connects the resulting points into a triangulated surface.
Targets and Regions
Regions divide a corridor along station ranges so different assemblies or frequencies can apply. Targets tell subassemblies what to attach to: a surface for daylighting, a width or offset alignment for lane widening, or an elevation profile. The dominant exam trap is a corridor that fails to daylight because the daylight subassembly has no surface target assigned. A second trap is frequency: too coarse a frequency produces faceted, inaccurate surfaces on curves, and the fix is a tighter frequency or additional stations at curve points.
Sections and Quantities
Sample line groups cut cross sections from the corridor at chosen stations. Material lists and quantity takeoff then compute volumes between surfaces - typically comparing the corridor's datum or finished grade against the existing ground. Earthwork volumes depend on correct surface definitions on both sides, so a missing boundary on the existing ground surface inflates cut and fill.
Rule of Thumb
When a corridor looks wrong, diagnose in this order: baseline, profile, assembly, region, targets, frequency. Ninety percent of failures are missing targets or a region that does not span the intended stations.