Grid Versus Ground Control for Bridge Layout
An Iowa engineering firm is laying out a replacement bridge on a county road. The project datum is NAD83(2011), Iowa State Plane North zone, and the plans are on grid. The firm sets two GNSS-derived control points, CP1 and CP2, using static observations tied to an NGS monument; the processed solution reports a combined scale factor of 0.999941 at the site. A total station traverse from CP1 to CP2 measures a ground distance of 3,486.72 ft and a grid azimuth of 127°18'42". The contractor's GPS equipment, set to ground distances using a handheld scale factor, stakes the first abutment 0.21 ft from the plan position. The resident engineer asks whether the control is 'wrong' and whether to hold the staked point or the plans. The surveyor also finds that CP2's published coordinate differs from the processed value by 0.08 ft N and 0.05 ft E.
1. Compute the grid distance and explain why the contractor's stake is 0.21 ft from the plan position.
Read the worked answer
Grid distance = ground distance × combined factor = 3,486.72 ft × 0.999941 = 3,486.514 ft. The ground distance is therefore about 0.206 ft longer than the grid distance, which closely matches the 0.21-ft stake offset.
The plans are on State Plane grid, but a total station measures ground distance. The contractor apparently used a handheld scale factor that was not the project combined factor, or applied the factor in the wrong direction. The control is not necessarily wrong; the reduction from ground to grid was wrong. On a bridge, a 0.21-ft error is significant and must be corrected before abutment layout.
2. Should the field crew hold the staked point or the plans? Describe the correct procedure.
Read the worked answer
Hold the plans and the project grid coordinates. The staked point is not controlling when the stakeout reduction used the wrong scale factor.
Correct procedure:
- Convert plan grid distances to ground using ground = grid ÷ combined factor, or set the total station with the correct scale factor.
- Compute stakeout coordinates consistently in the same system as the plans.
- Re-stake the abutment from CP1 and CP2 and check the result by an independent tie.
- Record the combined factor, the source of the factor, and the software settings.
The resident engineer should not accept the contractor's point merely because it was staked first. The plans govern until a verified design change says otherwise.
3. CP2's published coordinate differs from the processed value by 0.08 ft N and 0.05 ft E. How should the surveyor evaluate and handle that difference?
Read the worked answer
The vector difference is √(0.08² + 0.05²) = 0.094 ft. Whether that is acceptable depends on the project tolerance and the accuracy required for bridge control. A difference under 0.1 ft may be acceptable for some earthwork, but bridge layout often requires tighter control.
The surveyor should not arbitrarily adjust CP2 to split the difference. Reoccupy CP2, check antenna height and centering, review the OPUS or static solution, check the monument for stability, and repeat an independent baseline. If CP2 is stable and the difference is within the network tolerance, use one coordinate system consistently and document the decision. If not, perform a least-squares adjustment of the control network and publish final coordinates. Mixing published and processed coordinates without adjustment is the real error.