Construction Stakeout with Coordinate Discrepancy
A surveyor is staking out the corners of a building foundation. The design coordinates for corner A are (N 5000.000, E 3000.000). The surveyor sets up the total station on a control point with coordinates (N 5000.000, E 3000.000) and backsights another control point with coordinates (N 5100.000, E 3000.000). The instrument height is 1.500 m, and the prism height is 1.500 m. The surveyor computes the azimuth and distance to corner A from the setup. The computed azimuth is 90°00'00" and the distance is 100.000 m. The surveyor turns the angle and measures the distance, placing a stake. Later, a second surveyor checks the stake using a different control point and finds the stake is 0.030 m off in the easting direction. The first surveyor's instrument was calibrated recently, and the prism constant is set correctly. What could cause the discrepancy? How should the surveyor resolve it?
1. Identify possible sources of the 0.030 m easting discrepancy.
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Possible sources: (1) Instrument centering error: if the instrument was not set exactly over the control point, a 0.030 m error is possible. (2) Backsight error: if the backsight was not precisely aimed or the control point coordinates are incorrect. (3) Prism constant: although stated as correct, a mistake in applying it could cause a constant offset. (4) Atmospheric correction: if not applied, temperature and pressure could cause a scale error; for 100 m, a 10 ppm error is 0.001 m, so not likely 0.030 m. (5) Instrument and prism height: if not measured accurately, affects vertical only, not horizontal. (6) Angular error: a 1' error at 100 m gives 0.029 m, so a small angular error could cause this. (7) Coordinate error in the control point: the second surveyor used a different control point, so if the control network has a discrepancy, the check could differ. (8) Stake placement: the stake may have moved or was not set precisely. (9) Total station horizontal collimation error: if not calibrated, could cause angular error. (10) The first surveyor's setup: if the control point coordinates are the same as the design coordinates for corner A, that is suspicious: the surveyor set up on a point with the same coordinates as the design point? That would mean the control point is exactly at the design location, which is unlikely. Actually, the setup coordinates (N 5000, E 3000) are the same as the design coordinates for corner A. That means the surveyor is set up at the design location of corner A? That would be a mistake: the surveyor should be setting up on a control point, not on the point to be staked. If the control point has the same coordinates as the design point, then the surveyor is staking from the design point to the design point, which makes no sense. This is a red flag: the surveyor may have used the wrong coordinates for the control point. The control point coordinates should be different from the design coordinates. This could be the source of the discrepancy: the surveyor used incorrect control coordinates. The second surveyor used a different control point, which may be correct, revealing the error.
2. Recommend steps to resolve the discrepancy and ensure correct stakeout.
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The surveyor should first verify the control point coordinates. The setup point has coordinates (N 5000, E 3000), which are identical to the design coordinates for corner A. This is highly suspicious: it is unlikely that a control point coincides exactly with the design location of the corner to be staked. The surveyor should check the control point list and the design plans. Possibly, the surveyor mistakenly used the design coordinates as the control point coordinates. The correct control point coordinates should be obtained from the project control network. The surveyor should re-set up on the correct control point, backsight the correct reference, and re-stake the corner. Additionally, the surveyor should check instrument centering, backsight accuracy, and prism constant. A second independent check using a different control point is good practice. If the discrepancy is confirmed, the stake should be moved. The surveyor should also document the error and take corrective action.