3.3 Survey Stakes, Cut/Fill Notation, Differential Leveling (HI Method) & Percent Slope

Why Survey Stakes Exist

Stationing and offsets describe a location on paper, but someone has to translate that plan coordinate into a physical mark the contractor can build to. That is the purpose of a survey stake: a wood or steel marker driven into the ground at, or near, a plan-called point, carrying enough written information for an equipment operator or inspector to find grade without re-surveying the point themselves.

Hub-and-Tack Stakes: The Point of Reference

The most precise stake on a highway project is the hub-and-tack stake: a wooden hub driven flush with the ground, with a small nail or tack in its top marking the exact survey point to the nearest hundredth of a foot. Because a hub is driven flush with the ground, it is easily destroyed by traffic, equipment, or rain, so it is almost always paired with a nearby guard stake (sometimes called a witness stake), driven at an angle and left standing above grade, with the station, offset, and cut/fill information written on it in flagging marker or lumber crayon so the point can still be located and its data read even after the hub itself is gone.

Offset Stakes: Protecting the Point From Construction Traffic

Where a hub would be run over or destroyed by grading or paving equipment before the contractor can use it, the survey crew instead sets an offset stake — a stake placed a fixed, known distance (commonly 5, 10, or 25 ft) away from the true design point, out of the path of construction. The stake is marked with both the offset distance and the cut or fill needed at the true point, not at the stake itself, so the equipment operator or inspector must apply the offset distance first and only then apply the grade information.

Applying an Offset Stake in the Field

A stake at the edge of a proposed lane reads: "10' Rt Offset — C 1.4." The equipment operator needs to know two things before touching the grade: where the true design point is, and how far below the existing ground surface the finished grade sits at that point.

1. From the stake, measure 10 ft toward the centerline (or as marked) to locate the true design point.
2. At that true point, "C 1.4" tells the crew to cut, or excavate, 1.4 ft of existing material to reach proposed grade.

Because the offset stake itself sits outside the work area, its own ground elevation is irrelevant to the cut/fill figure — only the true point's cut/fill matters. Reading the offset distance and the cut/fill together, rather than in isolation, is one of the most common sources of grading error on a highway job, and catching that kind of misread is a routine part of the inspector's fieldwork.

Reading Cut and Fill Notation

Cut and fill describe the relationship between the existing ground elevation and the proposed, or design, grade elevation at a given point:

• Cut — the existing ground is higher than proposed grade; material must be excavated, or removed, to reach grade
• Fill — the existing ground is lower than proposed grade; material must be placed, or added, to reach grade

A grade stake is typically marked with a letter and a number, such as:

The inspector should never assume a stake's cut/fill figure is self-explanatory to the equipment operator; confirming that grading crews are reading the correct stake, at the correct offset, for the correct feature, is a routine and important field check.

Grade Stakes for Slopes, Ditches & Pipe

Beyond the roadway surface itself, grade stakes are used to control side slopes, ditch inverts, and pipe grades — anywhere the finished surface must follow a specific plane or slope rather than a single point elevation. Slope stakes, typically set at the top and bottom of a cut or fill slope, carry the cut/fill at the stake plus the design slope ratio, such as 2:1, so the grading crew can shape the full slope face rather than just the toe or top. Ditch and pipe grade stakes carry the cut/fill needed to establish the correct invert, the flow-line elevation, at each point — a concept tied directly to the percent-slope calculations covered next.

Stake Markings and Local Conventions

Many survey crews use colored flagging or lumber crayon to distinguish stake types at a glance — for example, one color for hubs and guard stakes, another for offset stakes, and another for slope stakes. These color codes are not standardized nationally; they are set by the surveying firm or agency on each project. Rather than assume a color means the same thing job to job, an inspector new to a project should confirm the local key with the survey crew before relying on stake color alone, and should always read the written station, offset, and cut/fill data on the stake itself as the authoritative source.

Benchmarks: The Foundation of Every Elevation

Every elevation on a highway project traces back to a benchmark (BM) — a fixed, stable point of known, established elevation, referenced to a vertical datum such as NAVD 88. Benchmarks are set by the project surveyor at durable locations, such as a bridge abutment, a manhole rim, or a permanent monument, specifically so they will not move or be disturbed during construction. Every leveling operation on the job — checking a subgrade elevation, verifying a pipe invert, confirming a curb grade — starts from a benchmark and works outward.

The Height-of-Instrument (HI) Method

Differential leveling is the field procedure for transferring a known elevation from a benchmark to an unknown point using a level and a graduated rod. The most common computation method is the Height of Instrument (HI) method:

HI = Known Elevation + Backsight (BS) Unknown Elevation = HI − Foresight (FS)

A backsight (BS) is a rod reading taken on a point of known elevation, such as a benchmark or a turning point; a foresight (FS) is a rod reading taken on a point whose elevation is being determined. Because the instrument's line of sight is level, the backsight reading tells the crew how high the instrument itself sits above the known point, the Height of Instrument, and the foresight reading tells them how far below that same line of sight the unknown point sits.

Worked example. A crew sets up a level and takes a backsight reading of 5.62 ft on Benchmark "BM-4," which has an established elevation of 100.00 ft. They then take a foresight reading of 3.14 ft on Point A, whose elevation is unknown. What is the elevation of Point A?

1. Compute the Height of Instrument: HI = 100.00 + 5.62 = 105.62 ft
2. Compute the elevation of Point A: Elevation = HI − FS = 105.62 − 3.14 = 102.48 ft

If the crew needs to continue the level run beyond the instrument's range, Point A becomes a turning point (TP), a temporary, stable point rather than a permanent benchmark, that lets the crew move the instrument forward and start a new backsight/foresight pair from Point A's now-known elevation of 102.48 ft. A level run should always close back onto a known benchmark so the crew can check the total rise and fall against the expected elevation difference and catch any rod or reading error before it is built into the grade.

Percent Slope: Definition and Formula

Percent slope, also called percent grade, expresses the steepness of a line — a ditch invert, a pipe run, a roadway profile, a cross slope — as the ratio of vertical change, or rise, to horizontal distance, or run, multiplied by 100:

% Slope = (Rise ÷ Run) × 100

Worked example. A drainage ditch invert is at elevation 425.20 ft at Station 10+00 and elevation 423.70 ft at Station 11+50, which is 150 ft further along. What is the percent slope of the ditch?

1. Compute the rise, or drop in elevation: 425.20 − 423.70 = 1.50 ft
2. Identify the run: 150 ft
3. Divide and multiply by 100: (1.50 ÷ 150) × 100 = 1.0% slope

A positive result, as above, describes a downhill, or falling, grade in the direction of travel; if the elevation at the far station were higher than at the starting station, the result would describe an uphill, or rising, grade instead.

Cross Slopes, Ditch Grades & Pipe Inverts

Percent slope is not limited to the roadway profile. A cross slope — the transverse slope across a travel lane or shoulder, commonly around 2% on a standard travel lane for drainage — is checked with the same rise-over-run formula, just measured across the pavement width instead of along it. Ditches are graded to a minimum percent slope, often around 0.5%–1%, to keep water moving and prevent ponding or sediment buildup. Pipe inverts, the elevation of the inside bottom of a pipe, are checked the same way: the upstream and downstream invert elevations, divided by the pipe length between them, give the pipe's percent slope, which must meet the design grade for the pipe to drain properly. An inspector who finds a flat or reversed invert slope during a level check has caught a real drainage defect before it is buried.

Checking Your Work: Closing the Level Loop

Any leveling operation should be checked, not trusted on a single pass. The simplest check is to run the level back to its starting benchmark, or to a second known benchmark, and compare the closing elevation to the known value; the difference is the closure error. Agency specifications typically set a maximum allowable closure error tied to the length of the level run. An inspector who lets a leveling error carry into a pay-quantity or grade-acceptance decision has passed along a mistake that concrete, asphalt, or pipe will lock permanently in place.

Between the HI method and the percent-slope formula, an inspector has the two calculations needed to verify almost any elevation question that comes up in the field — whether it is confirming that a subgrade elevation matches the plan, that a ditch is draining in the right direction, or that a pipe invert meets its design slope before backfill covers it for good.