10 Checks to Improve Buried Pipe AR Accuracy｜Pitfalls of Reference Points, Coordinate Systems, and Elevations

Introduction

Buried underground conduits such as water supply pipes, sewer pipes, gas pipes, power cables, and fiber‑optic communication cables are important lifelines that support social infrastructure. However, because they are literally buried underground and out of sight, a major challenge for their maintenance and at construction sites is "not knowing what is buried where." If buried utilities are accidentally damaged, it can directly lead to serious accidents such as water leaks, gas leaks, and power outages. In fact, damage to pipelines during excavation is reported in various locations every year, and many of these incidents are caused by "mistakes in identifying burial locations" due to deficiencies in drawings and records.

In recent years, a technology that combines high-precision positioning using GNSS (RTK) and AR (augmented reality) to visualize buried underground pipes on-site as if seeing through the ground has been attracting attention as a solution to this problem. By equipping smartphones and tablets with compact GNSS receivers and ensuring centimeter-level positioning accuracy with RTK, it is becoming possible to overlay the network of pipes laid beneath the ground in AR displays in the exact correct positions. Because it enables intuitive on-site understanding of the locations of buried objects without relying on experience or guesswork, it is expected to dramatically improve the safety and efficiency of infrastructure inspections and excavation work.

However, no matter how high the positioning accuracy of RTK-GNSS is, neglecting accuracy management in the field can cause fatal shifts in the positions displayed in AR. For example, if a survey control point is set incorrectly, even if positioning accuracy of a few centimeters (a few cm (a few in)) is achieved, the coordinates themselves could indicate a location offset by tens of centimeters to several meters (tens of cm to several m (e.g., 30 cm (11.8 in) to 1 m (3.3 ft))). Also, if discrepancies in coordinate systems or differences in vertical (elevation) datums are overlooked, large errors can occur in the underground depth direction even when horizontal positions match. To display buried pipes accurately in AR, appropriate datum alignment and accuracy control are required throughout the entire process from GNSS positioning to drawing data.

This article explains 10 checkpoints to improve the accuracy of AR for buried pipes, aimed at practitioners at construction consulting firms, surveying companies, municipalities, and infrastructure management companies who are considering using AR for buried pipes. It comprehensively covers commonly overlooked pitfalls in accuracy management and countermeasures—such as methods for installing control points, unifying coordinate systems, correcting elevation data, and cautions for RTK positioning. By checking these points in advance, you can realize the "visualization" of underground infrastructure more safely and reliably.

Check 1: Appropriate selection and placement of reference points

First, secure the reference point (control point). When using RTK-GNSS, the position of the rover (mobile station) is determined by relative positioning to the base station (control point). Therefore, if the control point is set incorrectly, the coordinates for the entire survey will be shifted and the accuracy of AR display will be fundamentally compromised. As a countermeasure, it is important to use, as the reference, a known point that can be trusted as much as possible. For example, adopt on-site control points whose national coordinates are provided, such as the Geospatial Information Authority of Japan's electronic reference points or existing public-survey control points (fourth-order triangulation points or higher). Even when no known points are available, using a network RTK service provides virtual reference stations based on the nationwide electronic reference point network, allowing positioning consistent with national coordinates without newly installing a physical base station.

If you must set up your own reference point on site, choose a stable location with good visibility and firmly mount it using a tripod or pole. Select an open area free of tall buildings or trees nearby, and secure the antenna so it does not move. Also, measure the reference point’s height (antenna height) accurately and enter it into the receiver. Be careful, because mistakes when entering the antenna height can lead to vertical errors. Furthermore, if possible, verify the coordinates of the installed reference point by other methods. For example, observe a nearby known point or a point with published public coordinates together and check for discrepancies with the reference point coordinates—this lets you detect setup errors in advance. Properly securing the reference point lays a solid foundation for RTK positioning and greatly contributes to improved accuracy thereafter.

Check 2: Verify differences in geodetic datums

Next is the unification of geodetic datums. In 2002, Japan’s surveying coordinates were changed from the former “Old Japanese Geodetic Datum (Tokyo Datum)” to the World Geodetic System, and currently the “Japan Geodetic Datum 2011 (JGD2011)” is used in principle. However, drawings and ledger data for long-buried infrastructure such as water and gas pipes still sometimes use coordinates in the old datum. There can be a displacement on the order of several hundred meters (several hundred ft) between the old datum and the World Geodetic System depending on the region; around Tokyo there is a persistent difference of approximately +300 m (984.3 ft) in the east–west direction and about +150 m (492.1 ft) in the north–south direction. Therefore, if a design drawing was created based on the old datum, a direct comparison with the latest GNSS positioning results (World Geodetic System) can yield coordinates for the same point that differ by several hundred meters (several hundred ft).

To avoid this pitfall, it is essential to check the geodetic datum of the design data in advance and convert it when necessary. Check the drawing legend and notes for entries such as "Coordinate system: ○○ (JGD2011)" or "Old Japanese geodetic datum used." If the coordinate system used in the design drawings is the old datum, you need to convert the coordinates to the World Geodetic System using the transformation parameters or software published by the Geospatial Information Authority of Japan. Some recent surveying software and GNSS receivers include automatic old-to-new datum conversion functions. The most reliable method is to survey a single known point on site and use the difference between that known point's World Geodetic System coordinates and the coordinates on the drawing as a correction value. In any case, leaving a datum mix-up unaddressed can cause meter-level errors, so be sure to coordinate so that both the design side and the positioning side use the same reference coordinate system. If you standardize on the World Geodetic System (JGD2011), you can safely compare and overlay RTK-GNSS measurements with the drawing data.

Check 3: Unification of Coordinate Systems (Planar Coordinates)

Pay attention to differences in horizontal coordinate systems. Even if the reference frame (Earth coordinates) of the positioning results matches, if the planar coordinate system differs from the design drawings, the coordinate values for the same point will not agree. For example, if the design drawings adopt the national public plane rectangular coordinate system, Zone ○ (JGD2011), but the GNSS receiver outputs are left in latitude/longitude or are converted to a plane rectangular coordinate system with a different zone number, positioning discrepancies of tens of meters or more (tens of ft or more) will occur. Also, some sites use their own local coordinate systems. This includes coordinates that set an arbitrary position within the site to (0,0) or coordinate systems that define an arbitrary direction as the X-axis—in other words, design drawings created using a “site-local reference.” In such cases, the coordinate values differ greatly from those expressed in the public coordinate system, including in digit places, so applying GNSS positioning values as-is will not match at all.

As a solution, it is necessary to transform (localize) positioning data to match the design coordinate system. First, check the legend of the design drawings to determine which coordinate system is being used (for example, "Plane Rectangular Coordinate System — system ○", "Local coordinates set at reference point ○○", etc.). If it is the plane rectangular coordinate system, set the system number correctly on the GNSS side and confirm you are not performing calculations in a different system. For a local coordinate system, the process of converting GNSS-acquired positioning values into that local system (localization) is indispensable.

Specifically, measure several known points whose coordinate values are identified on the site drawings, and from those results calculate the two-dimensional planar transformation parameters (translation amounts and rotation angle, and scale if necessary). With measurements of at least two known points you can correct translation and rotation of the planar positions, and with three or more points you can precisely fit including differences in scale. For example, if the drawing's coordinate axes are rotated 10° from true north, adjusting with only a single point will cause errors that increase with distance; at a distance of 1 km (3280.8 ft) this results in a lateral offset of approximately 170 m (557.7 ft).

By localizing with multiple points and matching GNSS coordinates to the design drawing's coordinate system, you can align the coordinates that serve as the basis for AR display.

Check 4: Aligning Height Reference (Elevation)

Unifying the reference for height (elevation) is also indispensable. The height information obtained from RTK-GNSS is usually a value called "ellipsoidal height", which indicates height above the global reference ellipsoid. On the other hand, the heights used in design drawings and infrastructure ledgers are "elevation (orthometric height)", based on mean sea level. Naturally there is a consistent difference between the two, and if this geoid separation is not corrected, large discrepancies will occur in the vertical direction. Around Japan, ellipsoidal heights are about 30–40 m (about 98.4–131.2 ft) larger than orthometric heights (because the geoid lies below the ellipsoid), and for example, if the design orthometric height of a known point is 50.000 m (164.042 ft) but RTK positioning yields an ellipsoidal height of 84.321 m (276.604 ft), that difference of about 34.3 m (112.6 ft) is the regional geoid height (a positive geoid separation). If GNSS height values are used as-is without applying this correction, the displayed depth of buried pipes will be off by tens of meters (tens of feet), and their appearance in AR will be completely inaccurate.

As a countermeasure, it is important to appropriately apply geoid corrections. The most reliable approach is to observe one known field benchmark or BM (benchmark) with RTK, calculate the difference between the obtained ellipsoidal height and the design elevation, and use that difference as the correction value. In the earlier example, subtracting 34.321 m (112.802 ft) from the GNSS-derived height will bring the other survey points into alignment with the design elevation system.

Also, many recent GNSS receivers and positioning software include geoid models (in Japan, GSIGEO2011, etc.) and can automatically convert to elevation values. In that case, you only need to select the region in advance, and the RTK positioning values will be output directly as elevations, which is convenient. Either method is acceptable, but make it a habit to check and correct vertical offsets as well as horizontal positions. In AR visualization of buried pipes where depth information is important, even slight elevation errors can affect safety. By aligning the height datum, you can achieve accurate visualization that includes the vertical positions of underground structures.

Check 5: Unit systems and scale check

Differences in unit systems and scales must not be overlooked. Check that the coordinate values in drawings or CAD data are expressed in the same units as your positioning data. A typical example is when drawing data coordinates remain in millimeter units (mm (in)), so values that should be in meters are recorded as numbers 1,000 times larger. For example, a point that should be (120.00 m (393.70 ft) , 50.00 m (164.04 ft)) might be output in CAD as (12000 mm (472.44 in) , 5000 mm (196.85 in)); if you compare those to survey coordinates (meters (m (ft))) without realizing it, the coordinates will differ greatly. Also, with international data you need to be careful about confusing feet and meters (1 foot is 0.3048 m (1.0000 ft)). Because errors due to unit mismatches are extremely simple mistakes that can cause large discrepancies, it is important for both designers and surveyors to align the data units.

In addition, pay attention to the difference between distances on the coordinate plane and actual ground distances (scale errors). Japan’s plane rectangular coordinate systems are designed so that scale errors within each region are minimal, but small scale differences due to the projection still exist. Especially over wide areas, differences on the order of several tens of ppm can occur between distances on the coordinate plane and actual ground distances (for example, for a 1000 m (3280.8 ft) distance, on the order of a few centimeters (a few inches)). This level is not usually problematic for routine buried-pipe management work, but it cannot be ignored when very long sections require high accuracy. When comparing GNSS positioning values (Earth-referenced coordinates) with the survey reference used on site, it is advisable to consider scale corrections as necessary. However, for typical AR use in buried-pipe applications, as long as you do not neglect to check the unit system, the impact of scale errors is minor and there is no need for excessive concern.

Check 6: GNSS Equipment Configuration and Calibration

To fully realize the performance of high-precision positioning, it is also essential to correctly configure the GNSS receiver and positioning software. First, check the output coordinate system of the positioning data. When using network RTK services within Japan, results are often obtained in the JGD2011 plane rectangular coordinate system (regional system), but depending on the settings they may be output in global coordinates such as WGS84 latitude/longitude and ellipsoidal height. Be sure to check in the receiver or app settings that the output mode matches the coordinate system used for your project. Also, as noted above regarding height, it is convenient to enable the application of a geoid model if possible (if not applied, data will be output as ellipsoidal height and will require manual correction later).

Next, pay attention when entering information for base stations and known points. If you set up and operate your own base station, you must register that station’s precise coordinate values in the receiver. When entering data, take the utmost care not to select the wrong geodetic datum or coordinate units. If you make an error in even a single digit, all positioning results will be shifted by that amount. Even when using network RTK, you should confirm that operation follows the coordinate system specified by the service provider (often JGD2011).

Also, perform calibration of the sensors. Recent GNSS rovers are equipped with tilt compensation that automatically corrects for some pole tilt, but prior calibration is essential for it to function accurately. Follow the receiver’s instructions and perform internal IMU calibration before use by rotating horizontally and doing figure-eight motions. When using a smartphone or tablet, adjust the built-in electronic compass and gyroscope as needed. In environments with many magnetic sources nearby, the compass can become easily disturbed, which may affect the directional alignment of AR displays. Depending on the situation, restart the device or run the calibration mode to properly reset the sensors. In systems that link a GNSS receiver and a smartphone, also check the Bluetooth and serial connection settings and verify that correction information is being received correctly.

Check 7: Observation Environment Suitable for Positioning

The on-site positioning environment also has a major impact on accuracy. To fully leverage the performance of RTK-GNSS, it is fundamental to conduct observations in locations with as open a sky as possible. In urban areas where high-rise buildings stand close together or in places where tree branches and foliage are dense, signals from satellites can be blocked or reflected off building facades, causing positioning accuracy to deteriorate markedly (increased multipath errors and a reduced number of satellites acquired). When performing AR display of buried pipes, it is effective, if possible, to first complete RTK initialization (an instance) in a spot with good visibility and then move to the target location. Once a FIX solution has been obtained, high accuracy can sometimes be maintained for a short time even in locations with somewhat poor visibility. Also, when the sky cannot be seen at all, consider countermeasures such as moving a few meters (a few ft) to a spot where more satellites are visible to take measurements, observing at a different time of day (because satellite geometry changes), or supplementing with position alignment using a ground station or known points.

Pay attention to radio interference and communication conditions. Because GNSS receives very weak radio signals, it can be affected by nearby powerful radio equipment or high-voltage power lines. If accuracy is unusually poor only in a particular direction at the site, check whether there are sources of radio interference nearby. Also, when using network RTK, the mobile communication environment is important. In mountainous areas or underground spaces, communications can become unstable, and correction information may be interrupted, reducing accuracy. Measures such as preparing a pocket Wi-Fi or a relay antenna as needed, and surveying radio conditions in advance, can also be effective.

As for weather conditions, whether it is sunny or rainy does not have a major effect on GNSS positioning. However, it is somewhat affected by the ionosphere and troposphere in the atmosphere, so accuracy can become slightly unstable, particularly during daytime in summer. That said, it is usually within a few centimeters (a few inches), and rarely causes problems for buried-pipe AR applications. Reception obstruction caused by the site environment has a greater impact, so try to choose environmental conditions that make positioning as easy as possible. At some sites it may be difficult to achieve precise positioning using GNSS alone; in such cases, rather than relying solely on AR, it is important to combine conventional exploration methods (ground-penetrating radar surveys or trial excavation) to ensure safety.

Check 8: Monitoring RTK positioning accuracy

In GNSS surveying, it is important to constantly monitor the accuracy information obtained in real time. During RTK positioning, the receiver or the app screen displays the current solution (FIX/FLOAT, etc.) and error estimates (HDOP, estimated accuracy, etc.). To ensure the centimeter-level accuracy (cm level accuracy (half-inch accuracy)) required for buried-pipe AR, always maintain a FIX solution (integer-fixed solution). If it temporarily switches to a FLOAT solution (ambiguity not yet resolved) or a single solution, the positions during that time may include errors of tens of centimeters to several meters (tens of centimeters (several in) to several meters (several ft)), reducing the reliability of the AR display. In such cases, do not force the work to continue; stop and check the cause. It is important to wait until FIX is regained, move to a location with clear sky visibility, try restarting the equipment, or otherwise restore as stable a positioning state as possible before resuming work.

Also, monitoring the number of satellites and the geometric configuration (DOP values) is useful. In general, when the number of satellites acquired decreases or the geometry becomes biased, positional uncertainty (DOP values) worsens. For example, an ideal situation is having an HDOP value below 2 and acquiring more than 10 satellites, but if this rises above 5 or 6, a drop in accuracy is a concern. During observations, keep an eye on the satellite count and DOP indicators shown by the receiver, and improve the positioning conditions as necessary. If satellites are missing in a particular direction, changing the time of day may sometimes improve the situation.

Also, always perform a stability check of the positioning results. Even when a fixed solution is obtained, the position can still fluctuate slightly due to effects such as multipath. Therefore, when measuring an important point, it is advisable to remain still at the spot for several to around ten or so seconds and confirm that the positioning values have stabilized. Averaging over time can reduce temporary variations. It is also useful to re-measure the same point at a different time to see whether the results agree. If roughly the same coordinates are reproduced across multiple measurements, the reliability can be judged to be high; but if they differ significantly, some kind of error may have occurred. In that case, review the aforementioned control points, coordinate system, correction settings, etc., identify the cause, and then measure again.

Check 9: Verification and Calibration at Known Points

After performing on-site positioning and AR display, verify the results using known control points or reference markers. This is an important step to independently confirm whether the positioning results and AR alignment are accurate. For example, if there are Geospatial Information Authority of Japan (GSI) benchmarks or leveling points near the site, measure them using RTK in the field. Compare the obtained coordinate and elevation values with the published values; if the differences are within a few centimeters (a few in), this supports that the entire system is functioning correctly. Conversely, if there are large discrepancies, there may be problems such as errors in the control point coordinates, mismatches in the coordinate system, or missing height corrections. Investigate the cause, and, if necessary, take measures such as applying offset corrections to the positioning data to adjust the entire system.

Even if there are no official reference points nearby, you can substitute with on-site markers whose positions are known from the design drawings. For example, if coordinates for infrastructure fixtures such as fire hydrants, manhole covers, or utility poles are included in the drawing data, you can locate those physical items and compare them. It is also effective to directly compare the positions of virtual objects displayed in AR with their corresponding real-world counterparts. Focus on distinctive features that are easy to match between the map and the site—such as road hatches, boundary markers, or the bases of poles—and check whether they appear to overlap in AR without any misalignment. If you discover an obvious offset of several tens of centimeters or more (several to dozens of inches or more), consider correcting the entire virtual model using in-app coordinate offset adjustment functions (many AR systems allow manual fine-tuning of model position). However, the need for correction implies there may be an issue somewhere in the original positioning or coordinate system settings, so be sure not to forget to address the root cause.

By making it a habit to check at known points like this, it becomes easier to notice configuration mistakes on site should they occur. Because AR visualization of buried pipes is directly tied to safety management, it is desirable not to omit the step of verifying accuracy against an objective standard as the final check.

Check 10: Verifying and Updating Data Accuracy

Finally, turn your attention to the accuracy of the management data itself. No matter how precisely positioning and alignment are performed, if the underlying digital data for buried pipes (drawings and ledgers) is inaccurate, the AR display results will not be correct. Unfortunately, it is also true that older infrastructure tends to have more discrepancies between drawings and actual field conditions. Past construction may have altered piping routes, or older surveys may not have been sufficiently accurate, resulting in discrepancies between the coordinates in the data and the actual positions. Therefore, when introducing AR on site, it is desirable to simultaneously verify the accuracy of existing data and perform updates as necessary.

Specifically, for important pipelines, you can preselect points and conduct on-site survey checks, and if any noticeable discrepancy appears during AR display, re-measure the position on the spot and feed the data back. Manhole covers, gate valve covers, and signs located above buried pipes—any landmarks that can be identified from the surface—should be actively measured in the field and compared with ledger data. If a large discrepancy is discovered, correct the coordinates recorded in the company database or GIS and update them so that the corrected, accurate data are used from the next time onward. By directly applying discoveries made in the field to subsequent work, data quality will improve incrementally, creating a virtuous cycle in which future AR applications can achieve "visualization" with higher accuracy.

Also, it is important to leave reliable survey records upon completion for newly buried pipelines. Even in cases where, traditionally, contractors would measure dimensions with a tape measure after burial and note them on drawings, going forward, if high-precision 3D records are captured using RTK-GNSS and 3D scanning, they will become valuable data for the future. By continuously pursuing such precision control efforts, the reliability of underground infrastructure information will increase, and inspections and construction support using AR will be able to be carried out more safely and reliably.

Conclusion

We explained ten points for achieving high accuracy in AR visualization of buried pipes. These may seem like extra work at first glance, but they are essential checks for safely and reliably "visualizing" underground infrastructure. Fortunately, in recent years advances in technology have made high-precision positioning in the field markedly easier. A representative example is new solutions like the high-precision GNSS receiver LRTK that can be attached to a smartphone. LRTK can be attached to mobile devices such as an iPhone to obtain centimeter-level (cm level accuracy (half-inch accuracy)) position information via RTK positioning, allowing easy and accurate alignment even without specialized surveying equipment. Because dedicated apps also support complex coordinate transformations and height corrections, personnel without surveying expertise should be able to achieve highly accurate AR displays.

The important thing is to rely on technology while ensuring that people understand and manage the fundamentals. By paying attention to the pitfalls of control points, coordinate systems, and elevations discussed in this article, and by effectively leveraging the latest tools, the visualization of buried pipelines can dramatically improve in both accuracy and reliability. Accurately understanding the lifelines lying underground will not only prevent excavation accidents but also improve efficiency in maintenance and planning tasks. Please proactively apply AR technology at your sites while addressing the key points of accuracy management, and achieve safe, smart infrastructure management.