How ASCE 38 quality levels work with RTK GNSS for utility mapping

Underground utility mapping helps design and construction teams understand what is below ground before excavation begins. Depending on the project, that means reviewing existing records, surveying visible utility features, running geophysical equipment over the site, or physically exposing the utility.

ASCE 38-22 gives design and construction teams a standard way to describe how confident they can be in that information. It defines four Quality Levels, from QL-D to QL-A, based on how each utility location was found and verified. The level tells anyone using the map what evidence backs it up.

This guide walks through how the four ASCE 38 Quality Levels map to a typical utility investigation, and where Emlid Reach receivers—Reach RS4 Pro in particular—fit into that process.

If you’re documenting a newly installed or relocated utility instead, that’s a different workflow. Our utility and wet utility workflow guide covers stakeout through as-built, and our as-built survey with RTK GNSS guide covers documenting it before backfill. This article is about mapping utilities that are already in the ground.

Key takeaways

• ASCE 38 defines four Quality Levels (QL-D to QL-A) that describe how utility information was obtained and verified—not how accurately it was surveyed.
• A GNSS receiver doesn’t detect buried utilities or assign a Quality Level. It surveys the evidence: a visible manhole, a geophysical designation mark, or an exposed utility.
• Reach RS4 Pro delivers 7 mm + 1 ppm horizontal and 14 mm + 1 ppm vertical RTK accuracy, enough to position utility features at every Quality Level.
• Emlid Flow and Emlid Flow 360 let you attach the Quality Level, utility type, material, and photos directly to each surveyed point, so that context travels with the data into CAD or GIS.
• Reach also works alongside your existing GIS data collection app, so you don’t need to change your workflow to get high-accuracy positioning.

What is ASCE 38?

ASCE 38 is a standard from the American Society of Civil Engineers for investigating and documenting existing underground utilities. It’s closely tied to Subsurface Utility Engineering (SUE) and uses four Quality Levels to show how utility information was established, from existing records all the way to physical exposure.

It’s worth separating this from an as-built survey. ASCE 38 investigates utilities that are already buried. As-built surveying documents utilities that were just installed or relocated, ideally before they’re covered. ASCE 75 covers how to record and exchange that as-built data.

With that distinction in mind, here’s how the four ASCE 38 Quality Levels work, and where RTK GNSS comes in.

What are ASCE 38 Quality Levels?

ASCE 38 defines four Quality Levels to describe how information about an existing utility was obtained and verified. They give everyone on a project a shared way to talk about the source and reliability of a utility location.

The levels run from QL-D, where information comes mainly from existing records, to QL-A, where the utility is physically exposed and directly measured.

Quality level	How utility information is obtained	What it establishes
QL-D	Existing records and other available information	Utility information based primarily on existing sources
QL-C	Visible utility features are surveyed and correlated with existing information	Relationship between records and observable surface features
QL-B	Appropriate surface geophysical methods are used to designate the utility	Horizontal position of the detected utility
QL-A	The utility is physically exposed and directly measured	Directly observed position and characteristics at the exposure

Moving from QL-D toward QL-A means the utility location rests on increasingly direct field evidence, from existing records to physical exposure. The Quality Level describes how that information was obtained and verified, not how precisely its coordinates were measured.

That distinction matters for RTK GNSS. A GNSS receiver doesn’t detect an underground utility or decide its Quality Level. It surveys the features that establish the utility information: a visible manhole at QL-C, a designation mark at QL-B, an exposed utility at QL-A. The detection and verification method sets the Quality Level. GNSS just gives you the position in your project coordinate system.

How do you map utilities at each Quality Level?

Each Quality Level is a different stage, using a different method, to investigate an existing utility.

QL-D: Start with existing information

QL-D starts before anyone goes to the field. Gather what’s already known about the utility:

• Utility owner records.
• As-built drawings.
• GIS data.
• Previous survey information.
• Other project records.

This tells you what utilities are likely there and roughly where. QL-D information hasn’t been verified in the field, so treat it as a starting point, not a confirmed position.

QL-C: Survey visible utility features

Next, compare that existing information against what you can actually see on site. Survey features such as:

• Manholes.
• Valve boxes;
• Utility pedestals.
• Risers.
• Hydrants.
• Other visible utility structures.

Correlating these surveyed features with the records sharpens your picture of the utility network. RTK GNSS puts these visible features into your project’s coordinate system.

QL-B: Designate buried utilities

At QL-B, the investigation moves past records and visible features. A qualified utility locator uses electromagnetic locating equipment, ground penetrating radar (GPR), or another appropriate geophysical method to detect and mark buried utilities. Those marks then get surveyed into the project’s coordinate system according to the applicable requirements.

This is really two separate tasks:

1. Locate the utility using geophysical equipment.
2. Survey the resulting designation into the project’s coordinate system.

RTK GNSS handles the second task, not the first. Surveying a painted or flagged mark doesn’t make it QL-B on its own. The Quality Level depends on how the utility was investigated, designated, surveyed, and documented against the applicable requirements.

QL-A: Survey an exposed utility

QL-A means physically exposing the utility, usually through vacuum excavation or another appropriate method. Once it’s exposed, you can observe and measure it directly, recording things like:

• Horizontal position.
• Elevation or depth.
• Utility type.
• Material.
• Size.
• Condition, where required.

RTK GNSS ties that exposed utility’s position to the project coordinate system, and photos add useful supporting documentation of the exposure.

QL-A gives you direct evidence at that specific exposure point. It doesn’t mean the entire utility run has been exposed.

Where does RTK GNSS fit into the workflow?

RTK GNSS gives you accurate, georeferenced positions for the features your investigation turns up. Depending on the Quality Level, that can mean surveying:

• Visible utility features.
• Geophysical designation marks.
• Exposed utilities and test holes.
• Supporting site features.

GNSS doesn’t decide the Quality Level or detect utilities on its own—the completeness of the investigation and documentation does that. Your project coordinate system matters too: field measurements, utility information, CAD drawings, and GIS data all need to speak the same coordinate language, or the resulting map won’t line up with the rest of the project data.

Using Reach RS4 Pro for utility mapping

Reach RS4 Pro handles the surveying side of an ASCE 38 workflow. It doesn’t detect underground utilities or set their Quality Level—it records accurate coordinates for whatever evidence you’ve gathered, from visible features to designation marks to exposed utilities.

Hardware alone won’t get you to centimeter accuracy, though. You also need a source of RTK corrections.

That’s what the Emlid Corrections service is for. Instead of setting up your own base station, it uses a network of reference stations spaced roughly 40 km apart to build a virtual base station near you, giving you 1–2 cm accuracy. It’s built into compatible Reach receivers and ready to go as soon as you connect your device to Emlid Flow.

With RTK accuracy of 7 mm + 1 ppm horizontal and 14 mm + 1 ppm vertical, Reach RS4 Pro can:

• Survey visible utility features. Record manholes, valve boxes, risers, and other surface features for QL-C.
• Map utility designations. Once a locator marks a utility with EM, GPR, or another method, survey those marks for QL-B.
• Survey exposed utilities. Record a utility’s position once it’s been physically exposed for QL-A.
• Reach difficult points. Use IMU tilt compensation when you can’t hold the pole vertical, or Visual Capture when placing the pole directly on a point isn’t practical or safe.
• Keep field data organized. Collect positions, attributes, and photos in Emlid Flow, then sync with Emlid Flow 360 for office review and further use in CAD or GIS.

Reach RS4 Pro handles the positioning. The investigation method still determines the ASCE 38 Quality Level.

Recording utility data and its Quality Levels with Reach receivers

Once you know the applicable Quality Level, keep it attached to the utility feature you survey. That way, anyone using the data later can see both where the utility is and how confident they should be in that location.

Before heading to the field, set up your project in Emlid Flow 360 so your crew collects utility data consistently. Build a code library with survey codes for each utility feature type. Give each code its own color and attributes, so utilities are easy to tell apart on the map and the right information gets captured as you survey.

A typical utility record might include:

• Quality Level: QL-B
• Utility type: Water
• Material: PVC
• Diameter: 200 mm
• Detection method: EM
• Notes: Additional field observations
• Photos: Supporting visual documentation

For QL-A, you can also log what you observed at the exposure: depth, material, size, and condition.

To return to a known utility location, open the project in Emlid Flow and use Stakeout. Navigate to individual points, or stake out lines and segments with different guidance modes. For line features, intervals and offsets help you move along or relative to a utility alignment.

With Reach RS4 Pro, Visual Capture measures points that are hard or unsafe to reach with the survey pole directly—utilities sitting in open excavations, for example.

You can also attach photos to staked points in Emlid Flow, showing the condition of an exposed utility or its surroundings. Photos stay with the point data and sync to Emlid Flow 360.

Because Quality Levels and other attributes are recorded alongside each point, they stay connected to the surveyed features through the rest of the project. That makes it easier to tell utilities established at different Quality Levels apart later, whether you’re reviewing the project or handing the data off to CAD or GIS.

Together, Reach, Emlid Flow, and Emlid Flow 360 give you one connected workflow: position the utility, record its Quality Level and other attributes, add supporting photos, and keep it all organized for the final deliverable.

Requirements for preparing, reviewing, certifying, or sealing utility deliverables vary by project and jurisdiction. Follow the applicable ASCE standards, your contract requirements, and local professional regulations.

Using Reach with your existing GIS workflow

You don’t need to rebuild your field workflow to use Reach for utility mapping. Reach RS4 Pro feeds high-accuracy GNSS positioning into compatible third-party GIS mapping equipment and data collection apps, so your team keeps working with the maps, forms, and attributes it already uses.

If your GIS project already has a field for ASCE 38 Quality Level, utility type, material, diameter, detection method, and notes, keep collecting those the same way. Reach just supplies the position for each surveyed feature.

A typical workflow looks like this:

1. Prepare the GIS project. Set up utility layers, symbology, forms, and attributes, including a field for the ASCE 38 Quality Level.
2. Integrate Reach with your GIS software. Use the receiver as the high-accuracy GNSS position source for your data collection app.
3. Collect utility features. Survey visible features, designations, or exposed utilities according to your ASCE 38 workflow.
4. Record the supporting information. Complete the required attributes, and add photos where your GIS app supports it.
5. Sync with your GIS. The surveyed positions and attributes stay part of your existing GIS dataset and office workflow.

That means you can add Reach positioning to an established GIS workflow without switching platforms. Reach handles the positioning; the investigation method still determines the ASCE 38 Quality Level.

From utility detection to mapped data

A useful underground utility map tells the project team more than coordinates. It says what was found, how it was found, where it was measured, and how much evidence supports that. ASCE 38 Quality Levels give everyone a consistent way to communicate all of that.

Reach RS4 Pro fits into this process by accurately positioning whatever your investigation turns up. Paired with records research, utility-locating equipment, controlled excavation, and solid documentation, it turns field observations into georeferenced data your design and construction teams can actually use.

Looking to bring RTK GNSS into your utility mapping workflow?

Frequently asked questions