Surveying & Layout Fundamentals

Before the first shovel of dirt moves, somebody has to say exactly where the building goes and how high it sits. Everyone who follows trusts those stakes. An error in layout does not stay small: 1 degree off at a corner puts you about 2 inches out over 10 feet, and nearly 2 feet out over 100 feet. Fixing that after the concrete is poured is expensive.

Layout work takes care and a habit of checking. Only a licensed land surveyor can establish or certify property lines, and on many jobs a surveyor sets the main control points. Layout crews and field engineers work from that control. Know which job is yours.

What You Will Be Able to Do

• Choose and set up the right instrument for the task.
• Read a grade rod in decimal feet and convert it to inches.
• Calculate height of instrument and elevations, including through turning points.
• Set batter boards and prove corners square.
• Work out cut and fill, mark stakes, and close a level loop.

Words You Will Use

Staking out a building means marking its exact location on the ground with stakes, based on the site plan.

Instruments

Builder's Level

A builder's level or automatic level gives a horizontal line of sight in every direction. It primarily measures elevation differences between points. A compensator inside an automatic level holds the line level once the instrument is roughly set up.

Setup:

1. Set the tripod on firm ground, with the legs spread and pushed in.
2. Fasten the instrument, and center the bubble using the leveling screws.
3. Rotate the telescope 90 degrees and recheck. Keep going until the bubble stays centered all the way around.
4. Focus the eyepiece on the crosshairs, then focus on the rod.

Parallax is when the crosshair appears to shift on the rod as you move your eye. Fix it by focusing the eyepiece and the objective until the crosshair stays put.

Lasers

A rotary laser spins a level beam around the site. Outdoors you cannot see the beam, so a laser detector (receiver) on the rod is essential. It beeps when it is on the beam. Line lasers project lines for interior work. Dot lasers transfer points up and down, replacing a plumb bob.

Check any level or laser regularly. Shoot two points from the middle of them, then repeat from close to one point. If the difference between points changes, the instrument is out of adjustment.

Total Station

A total station measures angles and distances at the same time and calculates positions and elevations. Its electronic distance meter (EDM) sends a laser to a prism reflector and measures the return. With a data collector, it can lay out points directly from plan coordinates. Check your control points first, because every point you lay out carries any error in them.

RTK GPS

Standard handheld GPS is accurate to several feet. RTK GPS gives centimeter-level accuracy using a base station over a known point plus a rover. It works well for mass grading and large sites, and it drives machine control on dozers and graders. It needs an open sky, and it struggles under trees and near tall buildings.

Grade Rods and Tapes

Rods are marked in feet, tenths and hundredths, not inches.

To convert, multiply the decimal by 12. For example, 3.87 feet is 3 feet plus 0.87 x 12 = 10.44 inches, or about 3 feet 10-7/16 inches.

Hold the rod plumb, using a rod level. A tilted rod always reads high, so the point looks lower than it is.

For a precise measurement with a steel tape over a long distance, pull about 10 to 15 pounds of tension and hold the tape level. On long runs and in hot or cold weather, temperature matters as well.

Reading the Site Plan

Before you go out, find:

• Property lines and pins.
• Setbacks. These vary by town, and a building over the line can be ordered moved.
• Building dimensions, tied to property lines or control points.
• Benchmark and FFE. For example, FFE 102.50 means the finished floor sits at elevation 102.50 feet.
• Utility locations and inverts. An invert is the inside bottom of a pipe.

If a pin is missing or disturbed, stop and call the surveyor.

Long stakes and hubs can hit buried lines too. Make sure utilities are marked before you drive them.

Elevations

Height of Instrument

The formula is simple:

• HI = Benchmark Elevation + Backsight Reading
• Target Elevation = HI - Foresight Reading

Worked example. A benchmark has an elevation of 100.00 feet.

1. Backsight on the benchmark reads 4.52. HI = 100.00 + 4.52 = 104.52.
2. Foresight on the target reads 3.87. Elevation = 104.52 - 3.87 = 100.65.

Moving the Instrument

When a site is too long or too steep for one setup, use a turning point. It must be solid, like a hub or a rock. Do not use something that can sink.

Worked example. Continuing from an HI of 104.52:

1. Foresight on the turning point reads 6.10. TP elevation = 104.52 - 6.10 = 98.42.
2. Move the level. The rod stays on the TP.
3. Backsight on the TP reads 5.25. New HI = 98.42 + 5.25 = 103.67.
4. Foresight on the target reads 2.87. Elevation = 103.67 - 2.87 = 100.80.

Closing the Loop

After your last point, level back to the benchmark. Your calculated elevation should match its known elevation. The difference is the closure error. For typical construction layout, 0.02 to 0.05 feet is acceptable. Anything more means you need to find the mistake or rerun it.

Balance your backsight and foresight distances. Equal distances cancel refraction, earth curvature and small instrument errors.

Cut and Fill

Cut and fill means removing earth from high areas (cut) and adding earth to low areas (fill) to reach design grade.

To find the rod reading that matches grade:

Grade rod reading = HI - Desired Elevation

Compare the reading you actually get:

• A larger reading than needed means the ground is low, so you need fill.
• A smaller reading than needed means the ground is high, so you need cut.

Worked example. HI is 104.52 and the desired footing elevation is 99.50.

• Grade rod reading = 104.52 - 99.50 = 5.02.
• You read 5.35 in the trench.
• 5.35 - 5.02 = 0.33 feet too low. The crew dug about 4 inches too deep.

Footings must sit on undisturbed soil or properly compacted material, so tell the foreman. Do not just throw dirt back in.

Grade Stakes

Stakes tell equipment operators what to do. C 0.8 means cut 0.8 feet to reach grade. F 1.2 means fill 1.2 feet. Keep the stakes out of the way of machines, and flag them clearly.

Slopes

A 2 percent grade equals about 1/4 inch per foot, since 2 feet in 100 feet is 0.24 inch per foot.

Ground around a foundation. Under the International Residential Code (IRC), soil next to a house must fall at least 6 inches in the first 10 feet, which is 5 percent, or about 1/2 inch per foot. Paved surfaces next to the building must fall at least 2 percent, or about 1/4 inch per foot.

Drain pipes. Under the International Plumbing Code, typical minimum slopes are 1/4 inch per foot for small pipes up to 2-1/2 inches, and 1/8 inch per foot for 3- to 6-inch pipes, which includes 4-inch sewer lines. Plumbing codes differ, so check yours.

Worked example. A 4-inch sewer runs 120 feet at 1/8 inch per foot.

• Fall = 120 x 1/8 = 15 inches, or 1.25 feet.
• If the start invert is 96.50, the end invert is 96.50 - 1.25 = 95.25.

Laying Out a Building

Batter Boards

Batter boards are temporary frames that hold string lines marking the building's exact corners and elevation. They sit 4 to 6 feet away from the corners, so they survive the excavation undisturbed.

1. Find the corners from control points, and mark each with a stake.
2. Drive 2x4 stakes outside each corner, and nail horizontal boards across them, all set level at the same elevation.
3. Run strings over the boards. Where strings cross, the corner sits below. Mark the exact line with a saw kerf or nail so that it can be restrung.

Proving Square

The 3-4-5 method verifies that a corner is a true 90-degree angle. Measure 3 along one line, 4 along the other, and check for 5 between them. Bigger triangles are more accurate. Use multiples such as 6-8-10, 15-20-25, and 30-40-50, the largest commonly used on a long layout.

Then compare the diagonals. In any rectangle, they must match. Move strings until they do, and recheck the sides.

Drop a plumb bob from each string crossing to transfer the corner to the ground.

Protect your work. Flag batter boards, paint them, and tell the operators where they are.

Offsets and Trade Layout

Excavation destroys stakes, so set offset stakes a fixed distance from the building line, often 2 feet or more. For utilities, mark line and invert elevation at each stake, and use a laser in the trench to hold pipe slope. On slabs, snap wall lines, column grids and door openings from the plans.

Checks and Common Errors

Keep a field book or digital log with the date, instrument, rod person and readings. When someone questions a grade months later, it is your proof. An as-built survey does the same job at the end of a project, recording what was really built.

Staying Safe in the Field

• Wear high-visibility clothing, and stand where operators can see you. Do not turn your back on a working machine.
• Use traffic control near roads.
• Do not look into a laser beam. Most jobsite lasers are low-power classes, but they can still harm your eyes. Post warnings on shared sites.
• Keep rods away from overhead lines. Metal and wet fiberglass rods conduct electricity.
• Survey work means long days in the sun, so wear sunscreen and a hat, and drink water.

Check Your Understanding

1. A benchmark is at 250.00. Your backsight reads 3.45, and your foresight reads 5.80. What is the target elevation?

HI = 253.45. Target = 253.45 - 5.80 = 247.65.

2. With an HI of 253.45, you need a pad at 249.00. You read 5.00 on the ground. Cut or fill?

Grade reading = 253.45 - 249.00 = 4.45. You read 5.00, which is larger, so the ground is low by 0.55 feet. Fill.

3. Your level loop closes 0.12 feet off. What should you do?

That is too much for typical construction layout. Find the error, or rerun the loop.

4. Convert 0.42 feet to inches.

0.42 x 12 = 5.04 inches, or about 5 inches.

5. You mark 30 feet on one string and 40 feet on the other, and the diagonal reads 50 feet 1 inch. Is it square?

No. Move the strings until it reads exactly 50 feet, then recheck the diagonals of the full rectangle.

Key Takeaways

• HI equals benchmark elevation plus backsight. Target equals HI minus foresight.
• 0.01 foot is about 1/8 inch, and a 2 percent grade is about 1/4 inch per foot.
• A larger reading than needed means fill. A smaller reading means cut.
• Balance your sights and close your loops within 0.02 to 0.05 feet.
• Set batter boards 4 to 6 feet back, and prove square with large 3-4-5 multiples and diagonals.
• Check the benchmark before you trust anything else.