How to run fast and error-free underground utility projects with GNSS

Underground utilities are a part of a construction project you can’t easily revisit once they’re covered. 

A sewer pipe with the wrong invert elevation or slope can mean reverse flow or a failed inspection. If the as-built record doesn’t match the actual install, a future excavation can strike the pipe. The fix in any of those cases is the same: dig it up.

Modern GNSS gear changes the math. With GNSS receivers the project team can stake the alignment, verify invert elevations as the pipe goes in, and capture the as-built before backfill, without a survey crew on standby at every step.

Three GNSS moments matter in a utility and wet utility install: layout, grade verification, and as-built. Each one calls for a different approach.

This article walks through each one—part of a series on how construction subcontractors win with GNSS.

Key takeaways

• GNSS helps at three points on a utility job: laying out the pipe alignment, checking grade and inverts, and capturing the as-built before backfill.
• Put offset stakes 1.5–3 m off the centerline. The excavator leaves them alone, and the pipe crew can still measure from them once the trench is open.
• Keep the pipe laser for grade on each section and use GNSS to check inverts at manholes and along the run, where errors larger than a few centimeters show up.
• Record the as-built while the trench is still open, with a survey code on every invert, rim, lateral, and fitting. Laterals need the most care, since tape-and-sketch as-builts usually give them only an approximate offset.
• Points sync to Emlid Flow 360 as the crew logs them. If an invert is out of spec or a feature is missing, the office sees it before anyone backfills.

Why is wet utility layout and grade hard to get right?

Traditional offset staking on a utility job goes like this: a surveyor lays out stakes for manholes, pipe centerlines, and offsets, then leaves.

The crew works from those stakes for the day. If a stake gets bumped, knocked over, or buried by spoil, work pauses until the surveyor is back. On a multi-week run, that’s a lot of waiting.

Then there’s grade. Gravity sewers and storm drains rely on a continuous slope from upstream to downstream, so every section needs to sit at the right invert elevation. The pipe also has to tie into existing manholes and laterals at the right depth.

Minimum slopes are shallow. For an 8-inch sewer, the smallest size allowed for a public gravity sewer, the Recommended Standards for Wastewater Facilities (Ten States Standards, 2014 edition) recommend a minimum slope of 0.40 ft per 100 ft. That’s 4 mm per meter. The same standards require a uniform slope between manholes. Set one invert wrong, and the next section either has the wrong slope or doesn’t connect cleanly.

With conventional tools, checking each pipe section’s invert takes time, so it gets spot-checked. The crew installs a few pipe sections, and the surveyor verifies the grade at the connection points. In between, the pipe laser sets every section. The Ten States Standards also name a laser beam or lamping as the way to check straight alignment on sewers 24 inches or smaller.

The real risks are in how the laser is set up and how the checks are recorded:

• The laser is set at the wrong starting elevation, so every section after it sits at the wrong elevation.
• The benchmark used to set the laser is off, with the same result.
• The laser gets bumped mid-run, and the sections after that follow a different line.
• Grade checks happen but never get written down, so there’s no record of where the pipe actually sits.

If one of these errors gets through, it may only show up once the trench is buried. Iowa DOT’s construction specifications, for example, call for video inspection of all new sanitary and storm sewers after backfill and compaction, before paving, unless the contract says otherwise.

Water is run through the pipe first so low spots show up on camera, and any low spot outside the depth tolerances is reported to the engineer. By then, the sag sits under compacted backfill, and fixing it usually means digging it back up.

The software and products

The workflows in this article are built around:

• Reach RX2: pocket-sized RTK rover that pairs to your phone or tablet over Bluetooth. Perfect for fast mobile GIS and mapping tasks where internet connectivity is available.
• Reach RS4 (and Reach RS4 Pro): professional pole-mounted receivers that connect to Emlid Flow for survey workflows in the field. Works over both NTRIP (internet) and radio, making it ideal when you don’t have cellular coverage on site.
• Emlid Flow: the field app–it runs on the surveyor’s phone or tablet and handles stakeout, point collection, and as-built capture.
• Emlid Flow 360: the cloud workspace for the office side–importing design files, setting up coordinate systems, and managing teams.

How do you stake a pipe alignment with GNSS?

When working with GNSS for utility and wet utility, layout comes first. The job is to mark the design on the ground: manhole centers, pipe centerlines, and offset references. The marks have to be accurate enough that the excavator and pipe crew can work from them for the next stretch of trench.

The Reach RX2 rover gets RTK corrections over the internet, through the phone it’s paired with. There are two ways to do that.

The first is Emlid Corrections, our RTK correction service. It uses Virtual Reference Station (VRS) technology to create a virtual base at your location, so the rover reaches 1–2 cm accuracy without a physical base on site. All it needs is a mobile data connection on the phone. Emlid Corrections is available in the United States, the United Kingdom, Europe, Australia, and New Zealand. You can check your site on the coverage map.

The second is an NTRIP service or a Continuously Operating Reference Station (CORS) network. This also runs over the internet, so it needs cell coverage and a reference station close enough to deliver the accuracy the job requires.

On sites without cell coverage, set up your own base: a base receiver on known control, broadcasting corrections over LoRa radio. For this setup, use a rover with a built-in LoRa radio, like the Reach RS4 or RS4 Pro.

Whichever correction source you use, the stakeout workflow in Emlid Flow stays the same.

Related reading: RTK corrections delivery method explained: how to choose the right setup?

Working from design files

The design reaches the field engineer in whatever format the office prepared. For utility and civil construction projects, this is often a CAD drawing exported as DXF containing pipe alignments, stationing, manhole locations, rim and invert elevations, and lateral connection points. 

For utility alignments or as-built data coming from GIS platforms, the project may instead be exported as KML or CSV.

The project manager or survey manager imports these files into Emlid Flow 360, sets up the coordinate system, and adds the survey code library, imports additional data like surfaces in the office before the field crew arrives on site. 

The project syncs to Emlid Flow, so the field engineer arrives on site with the latest design data already loaded.

Then, the surveyor opens the stakeout view, selects the needed object, and walks toward it. The app displays real-time distance and direction guidance, along with audible feedback when the rover reaches the required tolerance.

Each stakeout point carries its design metadata automatically. This may include descriptions such as manhole center, pipe end, or offset stake at a specific station:

• In DXF workflows, Emlid Flow reads this information from layer names or block attributes. 
• In CSV, KML, and SHP workflows, the descriptions are imported directly from the description column: no manual entry in the field.

Staking manholes and pipe ends

A stake on the centerline doesn’t survive the excavator: the first bucket through the trench takes it out, and the pipe crew loses its reference at the worst possible moment.

That’s why offset stakes go in first. They sit 1.5 to 3 meters off the centerline—far enough to one side that the dig leaves them alone, close enough that the pipe crew can measure straight off them once the trench is open.

In Emlid Flow, the workflow is the same for every feature: a stake on the design point first, then a second stake at the offset—perpendicular to the alignment, with the offset distance saved in the description. Manholes get both. 

So do pipe ends, wherever a segment terminates at a fitting, a lateral, or a manhole.

On long runs, the surveyor adds offset stakes at regular intervals along the alignment. The spacing depends on soil conditions and what the contractor prefers to work from.

Can GNSS check pipe grade and inverts?

Once the trench is open and the pipe goes in, the work shifts from layout to grade control. The pipe laser handles the in-trench reference: it shoots a line at the design slope, and the crew sets each pipe section to it. On gravity pipe, the laser owns grade.

GNSS adds a check on top of the laser, and that check works at some scales but not others.

At structures, the tolerance is tighter than RTK. Iowa DOT’s storm sewer specification requires line and grade at any structure to stay within 1 % of the pipe’s inside diameter or 1/4 inch, whichever is larger. On an 8-inch pipe, that’s 1/4 inch, about 6 mm. The RTK vertical precision of the Reach RS4 Pro is 14 mm + 1 ppm, about 0.05 ft on a short baseline. Holding pipe to that tolerance at a manhole is the laser’s job.

Between joints, the slope is too small to measure. At 0.40 %, the Ten States Standards minimum for an 8-inch sewer, a 20 ft length of pipe falls 0.08 ft (24 mm). That’s close to the receiver’s vertical precision, so GNSS can’t reliably confirm the slope of a single length.

Over a full run, the trend is easy to see. A 400 ft run falls 1.6 ft (about 0.5 m) at the same slope, and 400 ft is the Ten States Standards’ maximum manhole spacing for sewers 15 inches or smaller. Against a 1.6 ft drop, 14 mm is small. Checking inverts along the run against the design shows whether the pipe is following the design grade. It also catches a laser set at the wrong elevation or bumped mid-run, as long as the error is larger than a few centimeters.

For grade checks, Emlid Flow 360 loads the LandXML design surface alongside the alignment. Once it’s in the project, every captured point shows its cut or fill in centimeters.

Checking invert elevations in real time

Once a stretch of pipe is in place and bedded, the field engineer brings the rover to a manhole opening or pipe end, holds the pole on the invert (or as close as access allows), and captures a point.

There are two ways to compare that reading with the design.

If the design includes invert points, from the DXF or CSV the office imported, the field engineer stakes out the design point and taps Save as staked point. Emlid Flow saves the measured position with its deltas from the design. These pairs go into the stakeout report, a CSV that lists each design point next to its staked point with ΔE, ΔN, and ΔU, plus RMS and solution status for both. The stakeout report is available for projects in a local coordinate system.

If the project uses the LandXML design surface, Emlid Flow shows each captured point’s cut or fill in centimeters on screen. The stakeout report is built from design points and their staked points, so stake any invert you need in the report against a design invert point.

Either way, the field engineer adds a description, a feature code, and optionally a photo to the point, stored with the coordinates and timestamp in the project file. The office sees every point and its metadata in Emlid Flow 360, in a browser. Staked points show their deltas there too, so the office can review them before the report is exported.

If a reading is off from the design by more than a few centimeters, the field engineer flags the point the same way before backfill. The office picks it up through the same sync.

When something’s off

A failed grade check is much cheaper to fix while the trench is open than after backfill. 

The logged GNSS point is the documentation: a measured elevation, time-stamped, and exported to the office in the same coordinate system as the design.

The engineer notifies the foreman and the surveyor or project manager, and the conversation is concrete: “The invert at manhole 4 reads 4 cm above design..” 

From Emlid Flow, the data lands in Emlid Flow 360 within seconds—not at the end of the day. The decision to re-bed, adjust the next section’s slope, or escalate to the engineer of record happens while the crew is still on the pipe.

Related reading: Minimize construction site visits: how to support your field crew remotely

What changes on water mains and force mains?

Water mains and sewer force mains are pressure pipe. They don’t rely on gravity flow, so the slope problem from the previous stages matters less, and the force main tolerance is much wider.

Iowa DOT allows trenched force mains to vary from design line and grade by up to 3 inches, horizontally and vertically. That’s about 76 mm, more than five times the RTK vertical precision of the Reach RS4 Pro, so GNSS can check force main line and grade directly.

For water mains, Iowa DOT requires pipe, fittings, and valves to go in to the lines and grades in the contract documents. What water mains add is separation from sewers. By default, Iowa DOT keeps sewers at least 10 ft horizontally from water mains, with limited exceptions. Where a sanitary sewer crosses under a water main, the top of the sewer should sit at least 18 inches below the bottom of the water main.

On pressure pipe, GNSS fits in four places:

• Layout, with the same stakeout workflow as gravity pipe.
• Depth-of-cover checks against the owner’s specification.
• Separation checks where sewers and water mains cross.
• As-built capture before backfill.

For the as-built, capture the features that operators and future crews will need to find:

• Valves, hydrants, and fittings.
• Thrust blocks, which Iowa DOT requires on fittings 16 inches in diameter or smaller.
• Tracer wire, which Iowa DOT requires with all buried water main and on all force mains.

How do you capture a utility as-built before backfill?

The as-built is the most time-sensitive step in the whole job. On most public utility projects, it’s also non-negotiable—the jurisdiction requires a certified record before project closeout.

Once the pipe is bedded and verified, the crew is ready to backfill. The trench closes, the equipment moves to the next station, and whatever wasn’t captured in those few minutes is gone, buried under compacted material that nobody will dig up to verify.

Conventional practice handles this with tape measurements and field sketches: a crew member measures from the offset stakes, writes the offsets and elevations on a worksheet, and the office digitizes the result later.

It’s faster than calling a survey crew back, but the result is only as accurate as the tape and the sketch. Lateral connections are the usual weak spot, most get an approximate offset rather than a real measurement.

What to capture and how to code it

Before backfill, the crew sets up the Reach RS4 Pro and captures every feature that trades or maintenance teams will need to find later, using the survey codes library. The RS4 Pro’s IMU-based tilt compensation is particularly useful here. Around an open trench, holding the pole perfectly plumb isn’t always possible, and the receiver compensates automatically without slowing the crew down.

• Pipe centerline points collected at regular intervals along the run.
• Invert elevations at each manhole and at every change in direction or grade.
• Lateral connections, including the connection point at the main line, offset, and depth.
• Manhole rim elevations and invert elevations.
• Bends, fittings, valves, and cleanouts.

The crew can attach a photo to any point—during point collection, linework, or stakeout. Photos go in from the app’s camera or the phone’s gallery, and save with the point data.

For as-built work, photos answer the questions that coordinates alone can’t. The office gets the coordinates, the code, the attributes, and an image of what’s actually in the trench. 

Two weeks later, a question about which way a cleanout was rotated, or whether the lateral coupling matched the spec, gets answered from the photo. No return trip.

Notes from our Community: Improve your as-built workflow with point photos attachment in Emlid Flow

Getting data to the office before backfill is done

Digital field capture often hits an end-of-day bottleneck: the field team exports the project, sends it to the office, and the office imports the data into spreadsheets or CAD. By the time anyone spots a discrepancy, the trench is buried.

Emlid Flow 360 closes that gap. Each captured point or line syncs in the background as the crew logs it, so the office sees the as-built data the moment the run is done. 

If something looks off—a missing feature, an out-of-spec invert, a coordinate that doesn’t tie into the existing manhole—the office can flag it before the foreman tells the operator to backfill.

For offices that don’t use Flow 360, the project exports as DXF for CAD or CSV for spreadsheets, with the design’s coordinate system and feature codes intact. Exports can also be partial—the upstream half of a pipe run, for example—so the field crew can share work mid-job without re-exporting the whole project.

Which utility checks can GNSS handle?

RTK GNSS gives centimeter-level precision at every stage, but some utility checks have tolerances tighter than RTK can measure. This table compares common checks with the RTK precision of the Reach RS4 Pro and shows which tool owns each one:

Check	Typical spec requirement	RTK fit (Reach RS4 Pro: H 7 mm + 1 ppm, V 14 mm + 1 ppm)	Tool that owns the check
Gravity pipe line and grade at structures	• Within 1% of inside diameter or 1/4 in (about 6 mm), whichever is larger • 1/4 in governs for pipe up to 25 in	• Vertical spec is wider than 1/4 in • Catches gross errors, not acceptance	• Pipe laser • Level and rod
Gravity pipe low spots	• Maximum low spot of 1/2 in for 8 in and 10 in pipe • Found by video inspection after backfill	• Can’t resolve	• Pipe laser during install • CCTV at acceptance
Gravity slope between manholes	• Uniform slope between manholes • Minimum 0.40 ft per 100 ft for 8 in sewer • Alignment checked by laser beam or lamping	• Can’t resolve slope joint to joint • Confirms the run-level trend over a full manhole run	• Pipe laser • GNSS for run-level checks
DOT drainage pipe hubs	• 0.01 ft horizontal and vertical • Robotic total station required	• Doesn’t meet the spec	• Total station
Centerline stakes	• 0.1 ft horizontal	• Fits	• GNSS
Force main line and grade	• Within 3 in of design line and grade	• Fits with margin	• GNSS
Sewer and water main separation	• 10 ft horizontal • 18 in vertical at crossings	• Fits	• GNSS
As-built capture before backfill	• Set by the owner or jurisdiction • Signed by a licensed surveyor or engineer where required	• Fits for position and attributes within the owner’s spec	• GNSS for field capture • Licensed surveyor or engineer to certify

Sources:

• Iowa DOT, Section 2503: Storm Sewers, Article 2503.03 F and Table 2503.03-1
• Iowa DOT, Section 2504: Sanitary Sewers, Articles 2504.03 J and L
• Iowa DOT, Section 2554: Water Mains, Valves, Fire Hydrants, and Appurtenances, Article 2554.03 A.7
• Great Lakes–Upper Mississippi River Board (GLUMRB), Recommended Standards for Wastewater Facilities (Ten States Standards), 2014 edition, ¶33.41, ¶33.44, ¶33.5, ¶38.31, and ¶38.32
• North Carolina Department of Transportation (NCDOT), Manual for Construction Layout, §3.3 and §8.4
• Emlid, Reach RS4 Pro specifications

The crew structure follows from the table:

• The licensed surveyor sets control, sets tight-tolerance hubs where the owner requires them, and certifies as-builts where required.
• The field engineer runs GNSS layout, gross-error checks on gravity pipe, pressure pipe checks, and as-built capture.

One person can cover both roles only when that person is licensed.

GNSS removes the bottlenecks that have historically forced teams to choose between speed and documentation, between waiting for the surveyor and skipping a check, and between sketching the as-built fast and getting it right.

Frequently Asked Questions