Two Benchmarks, One Elevation, Four Stories of Steel
You are the layout surveyor for a four-story structural steel building. Control points CP-1 and CP-2 were set by a previous surveyor from the county NAD83(2011) network; their published distance is 480.215 ft (grid). Setting up on CP-1 with atmospheric correction applied, you observe 480.196 ft to CP-2. The combined scale factor at the site computes to 1.0000212. Column line A is to be staked at a 25.00 ft offset from the property line, with a required 10.00 ft setback. Separately, the structural drawings cite a benchmark elevation of 112.45 ft NAVD 88 for finished floor control, while the city's as-built record for the adjacent sewer in the same street gives 112.14 ft for the same manhole rim.
1. Is the existing control adequate to stake the building, and what does the scale factor do to your conclusion?
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Convert before you compare. The published 480.215 ft is a grid distance; your observation is a ground distance. Comparing them raw overstates the misclosure.
- Ground equivalent of the published value: 480.215 / 1.0000212 = 480.205 ft
- Your measured ground distance: 480.196 ft
- Difference: 0.009 ft, or about 1:53,000
Had you compared 480.215 to 480.196 directly you would have reported 0.019 ft (1:25,000) and half of that 'error' would have been the projection, not the observation.
Judgment: 1:53,000 relative between two control points is more than adequate for structural layout held to about 0.02 ft, and the two points define a usable baseline. The scale factor itself (2.1 ppm, or roughly 0.001 ft over this distance) is negligible for building dimensions - but it matters in principle: the building is built on the ground, so lay out on ground distances, while keeping coordinates in the project grid system with the factor documented. Never mix: a dimension scaled from a grid coordinate should not be used to set a steel embed.
Caveats: two points give you a bearing but no check on rotation or on the second point's own integrity. Before setting steel, close through one more control point or occupy CP-2 and re-shoot CP-1. Note in your field book what was held fixed, what factor was used, and the residual.
2. How do you resolve the 0.31 ft conflict between the structural benchmark and the city's as-built record?
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Do not average, and do not adjust a published benchmark to fit your loop. Work the possibilities in order:
- Different vertical datums? NAVD 88 versus NGVD 29 in California typically differs by roughly 2 to 3 ft, not 0.31 ft, so a datum mismatch alone doesn't explain it - but check anyway, because the sewer sheet may not say which datum it used.
- Different feature. As-built sewer records are notorious for referencing invert, crown, or top of casting instead of rim. A 0.31 ft difference is the right order of magnitude for a casting-versus-rim or a pipe-reference confusion.
- Blunder or transcription. Pull the actual datasheet for both, not the drawing callout.
- Settlement or construction change. A manhole rim reset during paving moves.
Resolution: run a closed level loop from the project's designated benchmark to the manhole rim and to a third published benchmark. If the project benchmark checks with the third mark, hold it fixed and treat the city figure as a record of a different feature or a different datum. If the loop shows your benchmark is the outlier, stop work and notify the owner and the structural engineer before any elevations are set - a 0.31 ft error carried into a finished floor is a costly, hard-to-reverse mistake.
Document the loop, the closures, the marks held, and the basis of the finished-floor elevation on the layout drawings.
3. The property line is established from record monuments. How do you set the 25.00 ft offset and protect the 10.00 ft setback?
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Measure the setback from the legal line, not from your control. The line is fixed by the record monuments as you have established them; your control merely transfers it. Compute the offset from the monument-based line, and record the tie from both ends so the offset can be re-established if a stake is destroyed.
Check what the setback actually applies to. The 10.00 ft side yard is measured to the closest building element, not to the column centerline. Footings, piers, and architectural projections typically extend beyond the structural line. If the drawing gives a 25.00 ft offset to column line A and the footing projects outward, the governing dimension is the face of the footing, and the effective clearance is reduced.
Tolerance and margin. Field layout is realistically good to about 0.02 to 0.05 ft off your control, plus any uncertainty in the record line. On a 10.00 ft setback that is comfortable if you are staking the true 25.00 ft, but if the design intent was 25.00 ft from the column line to the line and the footing pushes 0.75 ft out, the building is at 24.25 ft - still compliant with a 10.00 ft setback, but the number the contractor checks against will not match the plan.
Action: stake and clearly mark the offset, show the footing projection on the stakeout sheet, flag the difference to the contractor and the design team in writing, and keep the raw observations. Where a layout is within a few tenths of a regulatory limit, the surveyor's job is to make the margin explicit, not to decide it silently.