Chainage is the distance measured along a defined alignment from an agreed starting point. Civil engineers use it to identify locations along roads, railways, tunnels, pipelines, channels and other linear infrastructure.
For example, Ch 1+250 means a point 1,250 metres along the alignment from chainage 0+000. Chainage is measured along the alignment, including its curves, rather than as the shortest straight-line distance between two coordinates.
What Does Chainage Mean?
A project alignment is assigned an origin or starting chainage. Distance increases in the nominated direction along the alignment.
Common notation includes:
| Chainage | Meaning |
|---|---|
| 0+000 | 0 m from the origin |
| 0+250 | 250 m from the origin |
| 1+000 | 1,000 m from the origin |
| 1+250 | 1,250 m from the origin |
| 12+345 | 12,345 m from the origin |
Some drawings and reports may show the same locations as CH 1250, Ch 1,250 m or another project-specific format. The drawing legend and project conventions should be checked before interpreting the notation.
How Is Chainage Calculated?
For a simple alignment:
\[ \text{Chainage at a point} = \text{Starting chainage} + \text{Distance measured along the alignment} \]If a road alignment begins at 0+000 and a test pit is located 1,250 m along the alignment, the test pit chainage is:
\[ 0+000 + 1{,}250\,\text{m} = 1+250 \]Survey and design software normally calculates chainage along the horizontal alignment. On a curved road, the chainage follows the curve and is not the direct distance between the start and end coordinates.
Chainage and Offset
Chainage identifies a position along the alignment. An offset identifies the perpendicular distance from the alignment to the actual feature.
A location may be recorded as:
- Chainage: 1+250
- Offset: 5.0 m left
- Reduced level: 12.45 m AHD
Together, chainage and offset provide a practical location reference for boreholes, test pits, monitoring points, structures and construction activities.
The meaning of left and right depends on the direction of increasing chainage. Reports and drawings should make this direction clear.
How Chainage Is Used in Geotechnical Engineering
Chainage is useful when ground conditions and engineering works need to be related to a linear project.
| Application | Use of chainage |
|---|---|
| Boreholes and test pits | Records investigation locations along the alignment |
| Geological long sections | Plots soil, rock and groundwater conditions against distance |
| Material boundaries | Identifies where soil or rock units change |
| Earthworks | Defines cut, fill and treatment zones |
| Pavement investigation | Locates test points, defects and pavement changes |
| Design recommendations | Applies recommendations to defined chainage ranges |
| Construction records | Links inspections, tests and site observations to locations |
Example Borehole Schedule
| Borehole ID | Chainage | Offset | RL (m AHD) | Depth (m) |
|---|---|---|---|---|
| BH01 | 0+250 | 5.0 m L | 12.45 | 15.0 |
| BH02 | 0+500 | Centreline | 11.82 | 18.5 |
| BH03 | 1+050 | 3.0 m R | 10.96 | 12.0 |
This format allows the investigation data to be compared directly with survey drawings, alignment models and geological long sections.
Chainage Equations and Alignment Revisions
A chainage equation may be used when an alignment is revised or when two chainage systems meet. It records that one location has different chainage values under the adjoining or revised systems.
Because a changed alignment can alter the relationship between chainage and physical location, reports should identify the alignment name, revision and chainage convention used. Coordinates should also be included where they are required for unambiguous set-out or future verification.
Common Chainage Mistakes
Common errors include:
- Measuring a straight line instead of following the alignment
- Omitting the alignment name where a project has multiple alignments
- Using left or right offsets without stating the direction of increasing chainage
- Mixing chainage formats in the same schedule
- Using chainages from an outdated alignment revision
- Recording chainage without an offset for features away from the control line
Good Practice for Reports and Field Records
For reliable location records:
- Identify the alignment or control line.
- State the alignment revision where relevant.
- Use a consistent chainage format.
- Record offsets and reduced levels where needed.
- Include coordinates when chainage alone is insufficient.
- Check field records against the current survey or design information.