A route or corridor survey supports a project that is long and narrow: a road, a pipeline, a transmission line, a railway, a canal, a fibre run. The defining characteristic is that the project is organised along a centerline rather than around a site, and almost every convention in this branch of surveying follows from that one fact.
Because the work is linear, errors behave differently. On a compact site a small scale error is invisible. Along thirty miles of alignment the same error becomes a large absolute displacement at the far end, and the far end is usually where the project has to meet something that already exists.
Stationing: the coordinate system of a corridor
Position along a corridor is expressed as a station, the distance measured along the centerline from a chosen origin. In foot units it is written with a plus separating hundreds of feet from the remainder, so station 12+45.67 is 1245.67 ft from the origin. Metric practice uses kilometers or a similar convention. Offset perpendicular to the centerline completes the position, so a point is described as, for example, station 12+45.67, 22.5 ft right.
Two features of stationing catch people out. First, station is measured along the alignment, which means along arcs through curves, not along the tangents. Second, when an alignment is revised in the middle of a project, rather than renumbering everything downstream the designer inserts a station equation: the stationing jumps at a specified point. Anyone computing across a station equation without applying it will be systematically wrong from that point on.
Horizontal alignment geometry
An alignment is a chain of tangents connected by curves. Circular curves are defined by radius and total deflection angle, and the relationships used to lay them out are covered in the construction staking article. For higher-speed roads and for rail, simple circular curves are usually insufficient because a vehicle cannot transition instantly from straight running to a constant radius.
The transition is provided by a spiral, a curve whose radius decreases uniformly with distance along it, so lateral acceleration builds gradually rather than as a step. The spiral also gives the length over which superelevation, the cross-slope of the pavement, is rotated from normal crown to full banking. On rail the equivalent is the transition into cant, and the geometry is unforgiving because the constraint is the vehicle's suspension rather than the driver's comfort.
Vertical alignment and the profile
The vertical alignment is a profile of grades connected by parabolic vertical curves. A parabola is used rather than a circular arc because it produces a constant rate of change of grade, which is what governs both sight distance and ride quality.
Take an incoming grade of +2.50 percent meeting an outgoing grade of −1.75 percent, with a curve length of 400.00 ft and the beginning of the vertical curve at elevation 512.40.
A = g₂ − g₁ = −1.75 − (+2.50) = −4.25 percentelevation at x = 512.40 + (g₁/100)x + (A / (200L))x²x at the high point = −g₁L / A = −2.50 × 400.00 / (−4.25) = 235.29 ftThe ratio K = L/A, here 400.00/4.25 = 94.12 ft per percent, is the parameter designers use directly, because sight distance requirements are expressed as minimum K values. The rate of change of grade is A divided by the number of stations, or −1.0625 percent per station.
Right-of-way is where the survey becomes a boundary survey
The centerline defines the engineering. The right-of-way defines what the project owner may legally occupy, and establishing it is boundary work with all the evidence problems that implies. Existing right-of-way must be retraced from records that are frequently old, sometimes described only by a width either side of a centerline that has since been reconstructed, and often held by deeds, dedications and prescriptive use in various combinations.
- Retrace the existing right-of-way and the boundaries of every parcel the corridor crosses, since each is a potential acquisition.
- Locate occupation: fences, driveways, buildings and cultivation, which frequently disagree with the record and drive negotiations.
- Prepare acquisition exhibits for each affected parcel, with a description of the take and of any temporary easement, in a form the acquiring agency and its counsel can use.
- Compute areas of the take and of the remainder, since compensation usually depends on both.
- Set and reference monuments that will survive construction, and file the survey where the jurisdiction provides for it.
- Track utility easements crossing the corridor, because relocations are often the critical path of the whole project.
Practical field considerations
| Constraint | Consequence for the survey |
|---|---|
| Length of the project | Scale and datum errors accumulate; tie to published control at intervals rather than only at the ends |
| Grid versus ground coordinates | Over a long corridor the difference is substantial and may need to be handled in zones with defined breaks |
| Access across many ownerships | Notification and permission logistics often exceed the field time; plan them first |
| Live traffic or live rail | Flagging, protection and work windows dominate productivity; some observations are only possible at night |
| Overhead conductors | Restrict where equipment can be raised and require clearance measurements that are themselves a specialised task |
| Environmental and cultural constraints | May prohibit clearing sight lines, forcing satellite or aerial methods where a total station would be simpler |
For long corridors, define the coordinate treatment in writing at the start. A project that begins on a low-distortion ground system and later receives design files on state plane grid will produce a discrepancy that grows steadily along the alignment and is discovered at the worst possible moment, at the tie-in.