Paving
Grade control: stringline, laser, and machine control for grading and paving
How grade control sets the design elevations, slopes, and ride, from stringline and blue tops to laser, 2D, and 3D GPS and total-station machine control, and how to check it behind the machine.
Direct answer
Grade control is the system of setting and holding the design elevations, slopes, and smoothness while you grade and pave, using stakes, stringline, lasers, or GPS and total-station machine control. It decides drainage, layer thickness and yield, and ride. The project survey control, agency grade and smoothness spec, and equipment govern the tolerance.
Key takeaways
- Grade control sets and holds design elevations, slopes, and smoothness, deciding drainage, layer thickness and yield, and ride.
- GNSS RTK machine control typically holds horizontal near 10 mm and vertical near 15 to 20 mm, not tight enough for fine grade or the wearing surface.
- Robotic total station (UTS) gives millimeter-class position, commonly 3 to 5 mm vertical, the accuracy the paver, fine-grade trimmer, and mill need.
- Cross-slope around 2 percent is common on roads and lots, but the plan and agency spec set the number; flat or reversed slope ponds water.
- Always check grade and cross-slope behind the machine with a rod and smart level; the systematic error that reads consistent costs the most.
What grade control is and why it decides the job
Grade control is the work of building each surface to the design elevations and slopes, and holding the smoothness, while you grade and pave. It is how you put the dirt, the base, and the mat exactly where the plan says, in the vertical and the horizontal, so water runs where it should and the section ends up the thickness it was paid to be. Every method in this guide, from a string on two hubs to a satellite-steered screed, is doing the same job: keep the cutting edge or the screed on the design surface.
Three things ride on getting it right, and all three cost money when you miss. Drainage comes from the slopes and the cross-slope. Flatten them and water ponds, freezes, and tears the pavement apart at the low spot. Thickness and yield come from the gap between the surface you built and the surface above it. Leave the base high and the mat comes in thin, so you bust the structure or you eat tons of asphalt filling a low base. Ride comes from how smooth you held the grade as you went, and on a paving job ride is a line item with a bonus or a penalty attached.
Grade control is not one tool. It is a decision about which reference the machine follows and how tight that reference has to be for the surface you are building. The subgrade tolerates more slop than the wearing course. Pick the method to the layer.
Why does grade control decide whether the job pays?
Grade control decides the money on three fronts: drainage, yield, and ride. Hold the grade and the pavement sheds water, the tonnage lands on the estimate, and the smoothness earns instead of costing. Miss it and you pay on all three.
Yield is the one that quietly eats a paving job. The mat is priced by the ton against a plan thickness, and that thickness is the distance between the base you built and the finished surface. Leave the base an average of a quarter inch high across a parking lot and you have stolen a quarter inch of asphalt off every square yard, which is real tonnage the truck never delivers and the structure never gets. Leave it low and you pour the difference back in asphalt at asphalt prices to bring the surface up. Either way the base grade just rewrote your material cost. The base and subgrade guide covers building that platform; this guide is about hitting its elevation.
Drainage is the failure you see later. Cross-slope and longitudinal grade move water off the surface. Flat spots and reverse falls hold it, and standing water is what starts the cracking and the potholes that follow. Ride is the failure you get paid or docked for the day the profiler runs. None of the three forgives a grade that was never checked.
Elevation, slope, cut and fill, and the tolerance
Grade control runs on two ideas: elevation, how high a point sits, and slope, how the elevation changes across distance. Everything else is bookkeeping on top of those two.
Elevation is referenced to a benchmark, a fixed point of known height that the whole site is tied to. The surveyor sets site control, the primary benchmarks and the secondary points the crew works from, and every cut and fill on the job is measured from that datum. Lose the benchmark or work off the wrong one and every grade on the site is off by the same amount, which is the worst kind of error because it looks consistent.
Slope shows up two ways. Longitudinal grade is the fall along the direction of travel, the slope of the road or the lot from one end to the other. Cross-slope, the crown or the tilt across the width, is what sheds water sideways off the surface. A common figure on roads and lots is around 2 percent, but the plan and the agency spec set it. Cut and fill is just the difference between the existing ground and the design grade at each point: cut where the ground is high, fill where it is low.
Tolerance is how far off the design you are allowed to be, and it tightens as you go up the section. A rough-grade tolerance might be a tenth of a foot. Fine grade under pavement is held far tighter, and the wearing surface tighter still. The project earthwork and paving specs carry the actual numbers, so read them before you set the first hub.
What is a stringline in grade control?
A stringline is a taut wire or cord strung between offset stakes at the exact design grade and alignment, set off to the side of the work so a paver, trimmer, or grader can follow it without running it over. It is the original automated grade control, and on slip-form concrete paving and curb-and-gutter it is still the method the work is built around.
Here is how it works. The survey crew sets hubs along the run, commonly every 25 or 50 ft, offset a known distance from the edge of the pavement. Each hub gets the cut and the offset marked. The crew sets the stringline brackets to the design elevation off those hubs, then strings the line and tensions it hard, because a sagging line is a sagging grade. The machine carries sensing wands that ride the line: one wand against the bottom of the string controls elevation, a second against the side controls steering. The machine chases the string, and the string is the design surface.
The stringline is proven and it is honest, in that you can see the reference with your eyes and check it with a level. It is also labor and time: stakes to set, line to string and tension, and a line that is easy to bump with a truck mirror or knock out of grade with a careless boot. That vulnerability is exactly what 3D machine control was built to remove.
Stakes, cut sheets, and the survey layout
Before any machine follows anything, the survey crew puts the grade on the ground. The work product is stakes and a cut sheet, and the grade crew lives off both.
Blue tops are the finish-grade reference. A blue top is a hub driven so the top of the stake sits exactly at finished grade, marked with blue paint or a blue whisker so the operator can see it and blade right to it. You set them in a grid across the area, and the grade is correct when the dozer or motor grader has cut or filled the surface flush with every blue top. Offset stakes carry the same grade information but stand back from the work, out of the machine's path, with the cut or fill and the offset distance written on them. A stake reading 4 ft offset, 1.5 ft cut tells the operator the design surface is 4 ft over and 1.5 ft down from the bottom of the stake.
The cut sheet is the table behind the stakes. It lists each station, the existing elevation, the design elevation, and the cut or fill between them, so the grade checker can verify any point without re-shooting the whole site. On a stake-based job the surveyor is back constantly, because stakes get knocked out, buried, and graded over, and a job that runs out of grade stakes stops. That recurring cost is the case 3D control makes against itself.
Laser grade control and where it stops
Laser grade control puts a rotating laser on a tripod that sweeps a level or single-sloped plane of light across the site, and a receiver on the machine's mast or on a hand-held grade rod reads where the surface is relative to that plane. Move the receiver up or down the mast to dial in the elevation, and the machine blade or box holds whatever the laser plane says. For flat work and constant single slopes, it is fast, cheap, and accurate enough for a lot of grading.
The strength is also the limit: a laser defines one plane. A standard rotating laser holds a flat grade or a single fixed slope, and that is all it can hold. The moment the design has a crown, a varying cross-slope, a transition, or any compound surface, a single laser plane cannot describe it, and you are back to stakes or to 3D. Dual-slope lasers handle two slopes at once, which covers simple crowned pads, but a true warped or curving surface is past what a plane can do.
Typical laser-receiver work holds vertical to roughly a tenth down to a couple hundredths of a foot depending on range and the gear, which is plenty for rough and intermediate grading and not enough for the wearing surface. Confirm the accuracy against the manufacturer's spec for the laser and receiver you actually have, and check the laser's calibration before you trust it on tight work.
Sonic sensors and the averaging ski
A sonic sensor reads grade without touching anything, bouncing sound off the surface below it and timing the echo, so it can follow an existing surface, a curb, or a stringline from a few inches up. On a paver it is the smoothness tool, and the way it is mounted matters more than the sensor itself.
Mount a single sonic sensor and the screed copies whatever is under that one point, bumps and all. Mount several sensors along a long beam, an averaging ski, and the system averages the readings over the length of the beam, so a short bump or dip under one foot gets diluted instead of telegraphed into the mat. The longer the ski, the more it filters. Common big-ski setups run on the order of 30 to 40 ft, long enough to ride over the medium-wavelength roughness that wrecks an IRI number while still following the real grade. Reference the existing surface with the ski and the screed lays a mat smoother than the surface it paves over, which is the whole point.
Two field habits make or break it. Keep the ski clean and the feet clear, because a stone or a clump under a ski foot lifts the beam and prints a high spot you will pay for on the profiler. And give the paver a steady head of material and a constant speed, because a sonic ski cannot fix a screed that is surging on a starved or flooded auger.
What is the difference between 2D and 3D machine control?
The difference is what the machine knows about where it is. A 2D system controls elevation and slope relative to a local reference, a laser plane, a stringline, a sonic-tracked surface, or the machine's own slope sensor. It holds a grade and a cross-slope, but it does not know where the machine sits on the site. You tell it the slope and it holds it.
A 3D system knows the machine's exact position in three dimensions, x, y, and z, and steers the cutting edge or screed to a digital design surface at that position. It does this with GNSS, a robotic total station, or both, feeding position to a controller that compares the blade to the model and drives the hydraulics to match. The operator is not setting a slope. The model already carries the slope, the elevation, and the alignment at every point, including crowns, transitions, and warped surfaces a single plane cannot hold.
Pick by the surface and the tolerance. 2D is right for constant grades and slopes, and it is cheaper, simpler, and survey-light. Typical 2D laser work holds vertical on the order of a tenth down to a couple hundredths of a foot, fine for mass grading and a flat pad. 3D earns its cost on complex surfaces, tight tolerances, and cutting out the stake crew, but it is only as good as the model and the site calibration behind it. Verify the accuracy class against the manufacturer's spec for your exact system.
How accurate is GPS machine control?
GPS, or more correctly GNSS, machine control with RTK corrections typically holds horizontal position to roughly 10 mm and vertical to roughly 15 to 20 mm under good sky, per the manufacturer's spec. That is excellent for dozers and graders moving dirt to a model, and it is the workhorse of modern earthwork. It is not tight enough for fine grade or the paving wearing surface, and that gap is the single most important thing to understand about it.
The reason is in the numbers. Vertical is always the weak axis on satellite positioning, and fine grading and paving commonly want vertical control on the order of a few millimeters, inside what GNSS alone can promise. So GNSS runs the rough and intermediate work, and where the spec gets tight you augment it: a rotating laser feeding the vertical while GNSS handles horizontal, or a robotic total station taking over position entirely.
GNSS also has places it simply does not work. It needs sky. Under heavy tree cover, against a tall building or a high wall, in a deep cut, or in a tunnel, the signal drops or multipaths off the surfaces and the position goes bad, sometimes without an obvious warning. A base station on a known control point and a rover on the machine is the standard setup, and if the base sits on the wrong point or the site calibration is off, every position is confidently wrong. Treat a GNSS grade as verified only after you have checked it against site control with a rod.
Total station and UTS for fine grade and the mat
Where GNSS cannot hold the vertical, a robotic total station does. A universal total station, the UTS, is a robotic instrument that locks onto a prism or target on the machine and tracks its exact position many times a second, feeding the controller a position far tighter than satellites can. Manufacturers put millimeter-class systems in the range of roughly 3 to 5 mm vertical, the accuracy fine grading and the paving surface actually need. Confirm the class against the spec for the instrument you are running.
This is why the paver and the fine-grade trimmer on a tight job run off a total station, not off GPS. The screed laying the wearing course, the trimmer cutting the base to a millimeter tolerance, the mill profiling an existing road, these want UTS accuracy. The cost is line of sight and range. The instrument has to see the target, so on a long pull you leapfrog instruments or hand the machine off from one to the next, and an obstruction that breaks the line breaks the control until it reacquires.
The total station also wins exactly where GNSS loses. In a cut, under cover, against a structure, or anywhere the sky is blocked, line-of-sight UTS keeps working. The trade-off you accept is setup and the discipline of keeping the instrument over known control and the line clear.
The digital model and site calibration
Every 3D system follows a digital design surface, a model that says what elevation the finished grade should be at every x and y on the site. It goes by a few names, a DTM or digital terrain model, a TIN surface, the design surface, but it is the same thing: the plan turned into a continuous three-dimensional surface the machine can be driven to. The model is the job. A machine following a bad model builds the bad model perfectly.
The model gets built from the engineer's design, and the data prep is real work. Surfaces have to be clean, closed, and continuous, with breaklines right at the crowns and the curbs and the grade breaks, or the machine interpolates across a feature that should have been an edge. A gap or a flipped triangle in the surface becomes a hole or a hump in the dirt. A lot of 3D problems are born here, in the office, before a machine ever moves.
Then the model has to be tied to the ground. Localization, or site calibration, is the step that ties the model's coordinate system to the physical control points the surveyor set on the site, so the model's grade lands on the real benchmarks. A bad localization shifts or tilts the whole model relative to the site, and like a wrong benchmark it puts every grade off by an amount that looks consistent and reads correct on the screen. Check the calibration against known control before you cut, and re-check it if the base moves.
Grade control on the mill
A cold mill cuts the existing pavement to a depth and a profile, and how it is referenced decides whether you get a flat, true surface to pave over or a milled-in copy of the old road's problems. The drum cuts whatever the side plates and the grade control tell it to.
The simplest reference is depth off the existing surface: a sonic or contact sensor rides the un-milled lane or a ski, and the machine holds a constant cut below it. That is fast and fine when the existing surface is already true, but it carries the old grade's dips and bumps straight down into the milled surface, because you are referencing the very surface you are trying to fix. Run the mill to a stringline and you cut to an independent grade instead. Run it to 3D, with a total station or GNSS driving the drum to a design surface, and the mill ignores the old surface and cuts the road back to a true profile, taking more where the road is high and less where it is low.
That last mode, profile milling to a model, is how a rough old road gets made smooth in one pass, and it is the case for 3D on a mill: the smoothness you cut is the smoothness you pave over. The overlay and resurfacing decisions that sit on top of this are their own topic. Here the point is that the mill's reference, not just the paver's, sets the ride.
The paver, the floating screed, and the sensors that hold it
The paver lays the mat, and the part that sets the grade and the smoothness is the screed, which floats. The screed is towed from arms pivoted on the tractor, and it rides on the fresh mat at an angle of attack set by the tow-point height, so it seeks its own level instead of being held rigidly. Change the tow-point height and the screed slowly climbs or settles to a new thickness. That floating action naturally damps small irregularities in the base, and it is also why a paver hates speed changes and starved augers. Anything that disturbs the head of material in front of the screed moves the screed and shows in the mat.
Automatic screed control takes the tow point off the operator's hands and gives it to sensors. One side of the screed runs grade, off a sonic ski, a stringline, or a 3D position, and the other side runs slope, off a slope sensor that holds the set cross-slope to a fine resolution. The two together put the screed at the right elevation and the right tilt without the operator chasing it. A joint-matching shoe or sensor references the adjacent lane so the new mat meets the cold joint at the right height.
Hold the chain in your head: stable material, steady speed, clean reference, sensors doing the fine work. Break any link and the screed tells on it. The compaction that locks the mat in behind the screed is its own subject. The screed sets where the surface is, the rollers set how dense it gets.
What smoothness spec do I have to hit?
On most paving today the smoothness spec is an IRI number, the International Roughness Index, measured with an inertial profiler after the mat is down and cooled. The agency sets a target and a pay schedule around it, and the result is real money. Beat the number and you earn a smoothness bonus, miss it and you take a disincentive or have to grind and correct. Many agencies have moved from the older profilograph and its profile index to IRI, and IRI is commonly reported over fixed segments, often a few hundred feet, so a single bad stretch can sink a segment's pay. The exact target, segment length, and pay adjustment are in the project's smoothness spec, which you read before you pave, not after.
IRI catches the roughness a driver feels, the medium and longer wavelengths, which is exactly what a long averaging ski or a 3D-controlled screed is built to smooth out. The biggest IRI killers are the things that move the screed: stop-and-go paving, surging material, a cold joint that steps, and bumps copied up from the base. Hold the head of material, keep the paver moving at a constant speed, reference a long ski or a model rather than the rough surface under you, and fix the base before you pave instead of paving over it.
The cheapest smoothness is built into the base. You cannot reliably profile-correct a bad base out of the surface course, and grinding the bonus back off the top is the most expensive way to learn that.
Cross-slope and crown for drainage
Cross-slope is the tilt across the width of the surface that runs water off to the side, and holding it is a drainage requirement before it is a smoothness one. A road is usually crowned, falling both ways from the centerline. A parking lot or a single lane often runs one constant cross-slope to a curb or a swale. A figure around 2 percent is common on roads and lots so water clears without the slope being felt by traffic, but the plan and the agency spec set the number, and superelevated curves and ADA-governed walks have their own.
On the machine, cross-slope is the job of the slope sensor. It holds the set tilt to a fine resolution while the grade side of the screed or blade controls elevation, so the surface keeps its fall even as the elevation rises and drops along the run. In 2D that slope is a number you dial in. In 3D the model carries the cross-slope at every station, including the transitions through curves, which a single dialed slope cannot do.
Flatten the cross-slope and you build a birdbath. Water that should sheet off in seconds sits on the surface, soaks the joints and any crack it finds, and on the next freeze it does the prying. The low spot that ponds is the spot that fails first, and it traces straight back to a cross-slope that went flat or reversed. Check cross-slope with a smart level behind the machine, not just on the readout.
Checking grade behind the machine
Trust the machine, then verify it, every method, every day. The grade checker is a position, not an afterthought, and the job is to confirm the surface the machine built actually matches the design, because every control system can be confidently wrong.
The tools are simple and they have not changed much. A level and a grade rod shoot elevations off the benchmark and compare them to the cut sheet or the model. A smart level or a digital level reads cross-slope directly on the finished surface. On a 3D job the rover doubles as a check tool: walk the finished grade with the rod and the controller shows the cut-fill to the model at each point, which is the fastest way to catch a localization that has drifted. The grade checker walks behind the machine and the paver, not at the end of the shift.
What you are hunting is the systematic error, not the random one. A single point a hundredth off is noise. The whole pad reading a consistent tenth high means the benchmark, the localization, or the laser is wrong, and you stop and find it before you build the whole site to it. Check against an independent reference too. Shoot a known control point with the GNSS rover now and then to confirm the base and the calibration have not moved. The crews that get burned are the ones who watched the screen and never put a rod on the ground.
Grade through the layers: subgrade to surface
Pavement is built in lifts, and grade control runs through all of them, because the error in any layer carries up into the next. Subgrade, then aggregate base, then asphalt binder, then the surface course, each gets built to its own grade, and the surface is only as true as the sum of what is under it.
The trick is that the tolerance tightens as you climb. The subgrade can be a little rough and still do its job. The base has to be truer because it sets the mat thickness. The surface has to be truest of all because that is the ride and the drainage you get paid on. So the smart move is to spend the grade-control budget early enough that you are not trying to fix a wandering base with the most expensive layer on the job. A high spot in the base does not just create a thin spot in the mat. It either prints through to the surface or forces you to grind it, and both cost more than holding the base grade would have.
This is also where yield lives. The asphalt thickness is the gap between the base grade and the surface grade, so loose base grade is loose tonnage in both directions. Building each lift to grade, and checking it before the next one covers it, is how you protect the structure the design called for and the tonnage the estimate assumed. The base and subgrade work below this line is its own guide. The point here is that their grade is the floor everything above inherits, and the thickness design that sets those layers is a separate guide worth reading alongside it.
Big sitework, pads, and flat-floor grading
On large flat sites, a distribution center, a data center pad, a big-box lot, grade control turns into a flatness problem at scale, and 3D is what makes it pay. A modern site does the mass excavation and the rough grade with GNSS dozers cutting to the model, then brings in fine-grade trimmers and motor graders on total station or millimeter-GPS to hit the pad tolerance GNSS alone cannot.
The reason is volume and tolerance together. Moving the bulk of the dirt to a model with GNSS is faster and needs far fewer stakes than the old way, which matters on a site measured in acres. But a building pad or a slab subgrade has a flatness spec the rough system cannot hold, so the finish pass steps up to the tight reference. The pad that reads flat to GNSS can still be out of tolerance for the floor that goes on it.
The other large-site reality is the model's size and the localization holding across the whole footprint. A calibration that is fine in one corner can carry a tilt that shows up a thousand feet away, so big sites get more control points and more frequent checks, not fewer. Verify the pad against site control with a rod across the whole area, not just where the instrument happens to sit, before anyone pours on it.
The case for 3D over stakes
The shift to 3D machine control is a productivity and accuracy argument, and on the right job it is not close. Stakes are slow to set, easy to lose, and a recurring survey cost every time the machine grades them over. A 3D model puts the grade in the machine once and keeps it there, so the dozer and grader work without waiting on the stake crew and without stopping when the stakes run out.
The accuracy case is just as real. A machine holding a model holds it consistently, pass after pass, where a stake-and-eyeball grade varies with the operator and the light. Fewer stakes, faster cycles, tighter and more uniform grade, and less rework when the surface checks out the first time. That is the modern shift, and it is why mass grading on a large site has largely moved to GNSS and fine grade to total station.
Where 3D does not pay is the small or simple job. A short flat pad, a single-slope lot, a curb run, these are often faster and cheaper on a laser or a stringline than standing up a base station, building a model, and localizing a site. The honest answer is to match the method to the job: 3D where the surface is complex or the site is big, 2D and stringline where it is simple, and a rod in someone's hand to check either one.
Calibration and what goes wrong
Most grade-control failures are not the technology failing. They are the setup, and they share a nasty trait: the screen looks right while the dirt is wrong. Knowing the short list is how you catch them before you build the whole site to an error.
Uncalibrated or drifted sensors lead it. A laser out of calibration sweeps a plane that is not level, a slope sensor reading a bias holds the wrong cross-slope, a mast or prism height typed in wrong offsets every elevation by that amount. A bad model or a bad localization is the next tier: a flipped triangle or a missing breakline in the surface, or a site calibration set on the wrong control, tilts or shifts the whole job by an amount that reads as a clean, consistent grade. On stringline, a loose or bumped line is the classic, a sag you cannot see from the cab that prints straight into the pavement. And GNSS dropout, against a wall, under cover, in a cut, lets the position wander while the operator trusts a readout that has quietly gone bad.
The defense is the same for all of them. Calibrate the gear on a schedule, check the model and the localization against known control before you cut, walk the stringline and tension it, and put a rod on the finished surface independently of whatever the machine says. The error that looks consistent is the one that costs the most, because you build the whole job to it before anyone notices.
What to document
A grade that nobody can prove was checked is a grade you get to argue about later. The record is what answers the dispute when a low spot ponds or a segment's IRI comes in short and the question is whether the grade was ever right.
Capture the survey control and benchmark used, the design model or cut-sheet version and date, the method and equipment, the localization or calibration record for 3D, the as-built grade checks with their stations and cut-fill, the cross-slope readings, and the smoothness result against the spec. On a paving job, tie the as-built grade and the IRI back to the segment and the day, because the pay adjustment will. If you upsized the base cut or changed a grade in the field to fix a problem, write down why, because the next person reading the section will wonder.
| Field to record | Why it matters |
|---|---|
| Benchmark and site control used | Every grade on the job is referenced to it |
| Design model or cut-sheet version and date | Proves which surface was built to |
| Method and equipment | Stringline, laser, 2D, GNSS, or total station sets the accuracy |
| Localization or calibration record | A bad calibration offsets the whole site |
| As-built grade checks (station, cut-fill) | Shows the surface matched the design |
| Cross-slope readings | Ties drainage to the record |
| Smoothness result vs spec | Drives the IRI bonus or penalty |
Common mistakes
- Setting the machine off the wrong benchmark or a drifted localization, so the whole site is uniformly off grade.
- Leaving a stringline loose or letting it get bumped, and paving the sag into the surface.
- Running GNSS where it is not accurate enough for the surface, fine grade or the wearing course, instead of a laser or total station.
- Trusting the readout and never putting a rod on the finished grade behind the machine.
- Flattening or reversing the cross-slope, so water ponds and the low spot fails first.
- Building the base high or low and busting the mat thickness and the yield.
- Referencing the mill or paver off a rough existing surface and copying its roughness into the new mat.
- Building a 3D model with missing breaklines or a flipped surface, then grading the bad model perfectly.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
Grade and smoothness requirements come from the agency or owner spec, and for public work that usually means the state DOT standard specifications, built on AASHTO methods. AASHTO covers materials and the design and test framework, the Asphalt Institute publishes the paving and mix guidance the trade leans on, and ASTM carries the test methods, including the profiling and IRI procedures behind a smoothness pay schedule. The exact grade tolerance, cross-slope, segment length, and bonus-penalty numbers live in the project's own earthwork and paving specifications and the agency standard specs, so read those for the controlling values rather than any rule of thumb.
Survey control is its own discipline. The benchmarks and site control the whole job references should come from the project surveyor, and a 3D model's localization ties back to those same points. Confirm the datum and the control before you build to them.
Machine-control accuracy classes come from the manufacturer. The major systems, Trimble, Topcon, and Leica among them, publish accuracy specs for each configuration, GNSS RTK, total station, and millimeter-GPS, and slip-form and paving builders such as GOMACO and the paver and mill makers publish their own. Treat the accuracy numbers in this guide as typical figures to confirm against the spec for the exact equipment you are running. Specs and product capabilities change, so verify the adopted edition and the manufacturer's current data before you commit a tolerance on a submittal.
Units, terms, and conversions
Grade work mixes survey units, machine units, and spec units on the same job, so the same grade reads several ways across the drawings, the controller, and the field.
Elevation is in feet on most US plans, often to the hundredth of a foot, and a tenth of a foot is the rough-grade unit you hear most. Machine-control accuracy is usually quoted in millimeters, so a crew lives in both, roughly 0.01 ft to about 3 mm, 0.1 ft to about 30 mm. Slope is given as a percent, as a ratio like 4:1 (horizontal to vertical) on side slopes, or in degrees, and cross-slope is almost always a percent. Smoothness in IRI is reported in inches per mile in the US or meters per kilometer in metric. Keep the unit attached to the number, because a tenth of a foot and a tenth of a percent are different worlds and the controller does not care which one you meant.
- Benchmark
- A fixed point of known elevation that the whole site's grades are referenced to
- Blue top
- A grade hub set with its top at finished grade, marked blue for the operator to blade to
- Cut and fill
- The difference between existing ground and design grade: cut where high, fill where low
- Cross-slope
- The tilt across the width of the surface that sheds water, usually given as a percent
- DTM
- Digital terrain model, the three-dimensional design surface a 3D machine is driven to
- Localization
- Tying the model's coordinates to the physical site control, also called site calibration
- GNSS / RTK
- Satellite positioning with real-time corrections, the basis of GPS machine control
- UTS
- Universal total station, a robotic instrument giving millimeter-class machine position
- IRI
- International Roughness Index, the smoothness measure behind most ride pay schedules
FAQ
What is grade control in construction?
Grade control in construction is the system of building each surface to its design elevations and slopes and holding the smoothness, using stakes, stringline, lasers, or GPS and total-station machine control. It is how a grading or paving crew puts the dirt, base, and mat where the plan says so drainage, thickness, and ride come out right.
What is the difference between 2D and 3D machine control?
2D machine control holds a grade and cross-slope relative to a local reference, a laser, stringline, or slope sensor, but does not know where the machine is on the site. 3D control knows position in x, y, and z from GNSS or a total station and steers the machine to a digital design surface, including crowns and transitions.
What is a stringline?
A stringline is a taut wire set on offset stakes at the exact design grade and alignment, off to the side of the work. A slip-form paver, trimmer, or grader carries wands that ride the line, one sensing elevation and one sensing alignment, so the machine follows the string as its design surface and builds to it.
How accurate is GPS machine control?
GNSS machine control with RTK corrections typically holds horizontal to about 10 mm and vertical to about 15 to 20 mm under open sky, per the manufacturer's spec. That suits dozers and graders moving dirt to a model, but it is not tight enough for fine grade or the paving surface, which need a laser or total station.
How accurate does total station machine control get?
A robotic universal total station tracks a prism on the machine and gives millimeter-class position, commonly in the range of 3 to 5 mm vertical, per the manufacturer's spec. That is why the paver, the fine-grade trimmer, and the mill on tight work run off a total station rather than GNSS, which cannot promise that vertical accuracy.
What cross-slope do I need for drainage?
Cross-slope sheds water off the surface, and a figure around 2 percent is common on roads and lots so water clears without traffic feeling it. The plan and the agency spec set the actual number, and curves, ADA walks, and superelevation have their own. Flatten it below the spec and water ponds at the low spot, which fails first.
Why is my pavement smoothness or IRI failing?
IRI usually fails from the things that move the screed: stop-and-go paving, surging material, a stepped cold joint, and bumps copied up from a rough base. Hold a steady head of material and constant speed, reference a long averaging ski or a 3D model, and fix the base before paving rather than profiling it out of the surface.
Do I still need grade stakes with 3D machine control?
3D machine control cuts the stake count sharply, since the grade lives in the model instead of on hubs, which is most of its productivity case. You still need survey control points to localize the model and to check the finished grade with a rod. Fewer stakes, not none, and never skip the independent check.
What grade tolerance do I have to hold?
Grade tolerance tightens as you climb the section: rough grade might allow a tenth of a foot, fine grade under pavement far less, and the wearing surface tighter still. The project earthwork and paving specifications carry the controlling numbers, so read them before setting hubs. Treat any spec figure as the value to verify, not a rule of thumb.