Highway alignment staking requires placing stakes at each specified station along the roadway centerline, including key geometric points: PC (Point of Curve), PT (Point of Tangency), PI (Point of Intersection), and spiral transitions. Use a total station to stake each point from control, or run a traverse along the alignment. Check horizontal closure before staking begins — an alignment error propagates to every stake placed from it.
Station Equations
A station equation is an adjustment to the stationing at a specific point on a project — usually needed when alignments are combined (e.g., a bypass connects to an existing highway at a different station), or when an alignment revision changes the length of a portion of the project. A station equation appears on the plans as: “STA 42+15 = STA 43+00 AHD” meaning the stationing jumps from 42+15 to 43+00 at that point (a 0+85 increase). Every stake past the equation must use the adjusted station, not the original.
Missing a station equation is the kind of mistake that puts a structure in the wrong place by up to a hundred feet. Always check the plans for station equations before starting layout.
Horizontal Curve Geometry
For each horizontal curve on the alignment, the plans provide:
- PC (Point of Curvature): Station where the tangent transitions to the curve
- PI (Point of Intersection): Station where the two tangents intersect
- PT (Point of Tangency): Station where the curve returns to tangent
- Radius (R): Horizontal curve radius in feet
- Delta (Δ): Central angle (deflection angle) of the curve
- Arc length: Length of the curve from PC to PT
Staking the Alignment
On most DOT projects, the engineer provides a coordinate file with the XY coordinates of every 25-ft or 50-ft station, PC, and PT. Load this file into the total station data collector and stake each point using the layout/stakeout function. If coordinates are not provided, calculate them from the tangent bearing, PI location, and curve geometry, then stake from control using the total station.
On each stake, mark: station (e.g., STA 12+50), offset (CL, 25 LT, etc.), and cut or fill to design grade if the stake also serves as a grade stake.
Closure Check
Before staking a long alignment, verify the coordinate geometry by computing the distance and bearing from the first PC to the last PT and comparing to the plan. If the computed geometry does not match, there is a calculation error or a coordinate system mismatch. Find it before staking.
See also: Offset Stakes for Road and Utility Layout and Total Station Setup and Backsight Procedure.