To set a job site benchmark: locate the surveyor's project control benchmark, set up a builder's level midway between the source and your TBM location, backsight to compute HI (HI = Benchmark Elevation + Backsight), then foresight to compute the TBM elevation (TBM = HI minus Foresight). Close the loop by shooting back to the source benchmark to verify accuracy within 0.01 ft.
Every grade shot on a job site traces back to the project benchmark — the known elevation reference established by the surveyor. A correctly set temporary benchmark (TBM) gives your crew a reliable, convenient elevation control point that survives the daily site conditions. An incorrectly set TBM propagates elevation errors into every grade shot taken from it.
Topcon AT-B4, Spectra LL300N, or Leica NA724. A builder's level uses a telescope with a crosshair and a bubble vial to achieve a horizontal line of sight. Auto-levels use a compensator for faster setup than manual leveling. For short transfers (under 100 ft), a rotary laser with receiver is also adequate.
Use a fiberglass rod graduated in decimal feet for direct compatibility with engineering plan elevations. Check the rod foot for wear — a damaged foot biases every reading by a fixed amount. A standard 12 or 16 ft rod covers the range needed for most TBM transfers.
Aluminum or fiberglass, set on firm stable ground midway between source and TBM. The balanced-sight principle reduces the effect of any systematic error in the instrument. Tighten all leg clamps before leveling.
Concrete nail or rebar for physical TBM. Flagging and a witness stake for marking. Paint for a ring around the TBM location. A weatherproof tag or paint marker for writing the elevation on the witness stake. The TBM must be stable, findable, and clearly labeled with the elevation value.
Find the project control benchmark on the plan set. The benchmark is typically shown on the cover sheet or site plan with a description (monument type, location) and an elevation value with datum reference (e.g., NAVD88 or project arbitrary datum).
Physically locate the monument in the field and confirm it matches the plan description. Do not use a benchmark you cannot positively identify. If the project has multiple benchmarks, confirm which one you are using and that its elevation matches the plan value.
Select a location for your TBM that is:
For large projects, set multiple TBMs around the perimeter so any one can be verified against another if disturbance is suspected.
Place the level tripod approximately halfway between the survey benchmark and your TBM location. Set the tripod on firm ground, spread the legs wide, and level the instrument using the circular bubble and fine-adjustment screws.
Balanced sight distances are important because a small systematic error in the level (collimation error) produces equal errors in both the backsight and foresight, canceling out in the final elevation. Unbalanced distances amplify the collimation error.
Hold the rod plumb on the survey benchmark. Read the rod precisely through the level telescope. Record the backsight (BS) reading.
Move the rod to the TBM location (the nail or stake top). Read the rod through the telescope. Record the foresight (FS) reading.
Write the computed TBM elevation on the witness stake with a paint marker. Include the date and the source benchmark identifier (e.g., "BM-1 per plans, NAVD88").
Without moving the instrument, turn around and take a new foresight from your TBM back to the survey benchmark. Alternatively, set up the instrument again and run the loop in reverse — TBM backsight to benchmark foresight.
The computed elevation of the survey benchmark in the return loop should match its known elevation within 0.01 ft. A larger misclosure indicates a reading error in one of the shots.
Log the TBM elevation, closure error, date, and benchmark source in Gradelog. This documents that the TBM was properly established and verified at the start of the project.
Use the Gradelog elevation calculator to work through backsight/foresight calculations and verify TBM elevations in the field.
Field Documentation
Use Gradelog to log and verify your grade shots digitally — free to start. Record TBM elevations, closure checks, and benchmark verification with timestamps.
A TBM is an elevation reference point established on or near the site from a permanent survey control benchmark. TBMs are set in locations convenient for daily grade control. They are called temporary because they may be disturbed during construction, unlike permanent survey monuments set by a licensed surveyor.
A properly set TBM should be accurate to within 0.01 ft of the source benchmark. A closure error greater than 0.02 ft when running a level loop back to the source indicates a reading error. For finish grade and sewer invert work, TBM accuracy should be verified to 0.01 ft before use.
Common TBM materials: concrete nail in existing pavement, rebar driven into compacted ground and topped with a stake, a hub stake with a tack nail, or a chisel mark in an existing structure. The TBM must be stable, protected from traffic and disturbance, and clearly marked with the elevation value.
Yes, a self-leveling rotary laser with a grade rod and receiver can transfer elevations and set a TBM. The procedure is identical: backsight to compute HI, foresight to compute TBM elevation. Accuracy is comparable to a builder's level for distances under 200 feet. For longer transfers, a precision automatic level provides higher accuracy.