# Sections

Cross-sections are the core data in TunnelCAD. A cross-section represents a measured tunnel profile at a specific chainage (station) along the alignment. Together, the collection of cross-sections forms a complete picture of the as-built tunnel geometry, which can be compared against the design template for deviation analysis and volume computation.

## What is a cross-section?

Each cross-section captures the tunnel profile at a single point along the alignment and carries the following properties:

- **Station** -- the chainage (distance along the alignment) where the section is located
- **Name** -- a user-assigned identifier (e.g., "Sec 1" or "Sta 0+100")
- **Bearing** -- the surveying bearing of the alignment at that station
- **Template** -- the design template assigned to the section, used as the theoretical reference
- **Insertion Point** -- the world-coordinate position computed from the alignment engine
- **Elevation** -- the alignment elevation at the station
- **Grade** -- the longitudinal gradient (%) at the station
- **Superelevation** -- cross-fall applied at the station

## Line data

Each section contains multiple **line data** entries, one per line definition. Line definitions describe the types of lines measured or designed at each section -- for example, an "Excavation" line, a "Final Lining" line, or a "Shotcrete" line. Each line data entry is a polyline: an array of 3D points with optional bulge values to represent arcs.

```{note}
Line definitions are configured per tunnel. See {doc}`line-definitions` for details on creating and managing them.
```

## Viewing sections

Sections are displayed on two canvas tabs:

- **Sections tab** -- shows the current section in local coordinates, where X represents the horizontal offset from the alignment axis and Y represents elevation. This is the primary view for inspecting and editing individual profiles.
- **Alignment tab** -- shows all sections in world coordinates, positioned along the alignment path. This view provides the overall tunnel context.

## Navigating between sections

There are several ways to move between sections:

- **Ribbon navigation buttons** -- use the **Prev** and **Next** buttons in the ribbon Navigation pane to step through sections sequentially.
- **Section Details grid** -- double-click any row in the Section Details panel to jump directly to that section. See {doc}`section-details` for more information.

```{tip}
Use the Section Details grid for quick random access to any section, and the Prev/Next buttons when reviewing sections in order along the alignment.
```

## Chapter contents

```{toctree}
:maxdepth: 2

adding-sections
line-definitions
editing-sections
section-details
```