Quick Answer
How do you set control points with a total station?
Set control points by running a traverse from known existing control: occupy a known point, sight a second known point as backsight, measure angles and distances to new traverse points, and continue until the traverse closes back onto known control. Adjust for closure error, assign coordinates to each traverse point, and monument the points permanently. Verify the network by checking measured distances between control points against computed distances.
How to Set Control Points with a Total Station
Applies to: Topcon GT series, Trimble S series, Leica TS/MS series, Sokkia SRX series; all construction and survey control network work
Control points are the anchor network for all construction stakeout and survey work on a project. Without accurate, stable control, every subsequent layout operation — building corners, pipe grades, finish elevations — compounds the control error. Setting good control points with a total station takes more care than routine stakeout but creates a foundation the project crew will rely on for the life of the project. This guide covers the full workflow from planning through monumenting and verification.
Step 1: Plan the Control Network
Before setting up the instrument, plan the control network on paper or in the field controller. Determine how many control points the project needs and where they should be located: visible to multiple working areas, on stable ground away from construction activity, intervisible with at least one other control point (for backsight verification), and on points that won't be disturbed by utilities, grading, or traffic.
On small sites (under 200m), three to four control points are typically sufficient. On large grading or road projects, space control points every 200-400m along the work corridor. Include at least two points at each end of the project that can serve as anchor points for extending the network.
Step 2: Establish Connection to Existing Control
New control points must be tied to the project's existing reference control — typically NSRS monuments, GPS-established project control set by the engineer, or client-furnished control benchmarks. Set up the total station on a known control point and verify orientation by backsighting a second known point. Confirm the instrument setup residuals are under 5mm before beginning the traverse.
Step 3: Run the Traverse to New Points
Occupy the starting known control point. Backsight the second known point and zero the horizontal circle (or record the backsight direction). Measure the angle and distance to the first new control point location. Record the raw angle (in both direct and reverse face to eliminate systematic error), and the slope distance. Reduce to horizontal distance and compute the horizontal angle from the mean of direct and reverse readings.
Move the instrument to the new point, set up, and repeat: backsight the previous point, measure to the next new point. Continue until the traverse returns to or connects with another known control point. This creates a closed traverse — the closure error can be computed and adjusted.
Measure each distance twice (or use multiple observations). For construction control, the angular accuracy of the total station (typically 2-5 arc seconds for 5" instruments) limits the achievable accuracy. Use a minimum of two face observations for each angle to detect blunders and reduce systematic error.
Step 4: Compute Traverse Coordinates and Closure
In the field controller or office software, compute the coordinates of each traverse point from the measured angles and distances. The traverse closure error is the difference between the computed final position and the known end-point coordinates. Express closure as a ratio: 1:5,000 (1mm error per 5 meters of traverse) is a minimum for construction control. 1:10,000 or better is preferred for primary control.
Apply a compass (Bowditch) adjustment or least-squares adjustment to distribute the closure error proportionally through the traverse points. Most data collector software (Trimble Access, Topcon Magnet Field) includes a traverse adjustment routine.
Step 5: Monument and Mark the Control Points
Drive rebar, set concrete nails in pavement, or cast concrete monuments at each adjusted control point location. Mark the monument with a nail or punch mark at the precise control point. Number or letter each control point clearly and record the point IDs in the field notes. Drive a lathe or PK nail marking the control point ID adjacent to each monument.
Photograph each monument in place and record a witness sketch showing distances to nearby permanent features (fences, corners, manholes). Witness sketches allow re-finding the monument if the marking is obscured.
Step 6: Verify the Network
After computing and adjusting the traverse, verify the control network by measuring between non-adjacent control points and comparing measured distances to computed distances. The difference should be within the expected accuracy of the traverse. Also verify by setting up on one new control point, backsighting another, and shooting a third new control point as a check — the residual confirms the network is internally consistent.
Frequently Asked Questions
What accuracy is required for construction control points?
Construction control typically requires 1:5,000 linear closure or better. Primary horizontal control for building or structural layout should achieve 1:10,000 or better. Vertical control (benchmarks) should close within 5mm over a 1km loop for most construction applications.
What is a traverse and why is it used to set control?
A traverse is a series of connected survey lines where each line's length and direction are measured. Traverses are used to extend control from known points to new locations because they allow a closure check — the accumulated error can be detected and distributed. A traverse that doesn't close reveals measurement errors before they affect the construction layout.
How do I know if my traverse closure is acceptable?
Compute the closure ratio: total traverse distance / closure error distance. A closure of 1:5,000 means 1mm of error per 5m of traverse length. For a 500m traverse with 100mm closure, the ratio is 1:5,000. Check project specifications for the required closure ratio. If closure is worse than 1:3,000, re-measure suspicious legs before adjusting.
Can I use GPS to set construction control points instead of a total station?
Yes — RTK GPS is efficient for setting primary control on large sites. GPS-established control achieves +/-10-20mm with RTK Fixed on a properly set up base. For very high-accuracy structural control (+/-5mm), a total station traverse adjusted from GPS control points is the preferred approach: GPS for primary control, total station for secondary layout.
Store control point coordinates, traverse records, adjustment reports, and monument sketches with Gradelog. Your control network, organized and accessible. Free to start at gradelog.com.
