RTK “Shift” Problem: Why Vertical Errors (Altitude) Tend to Be Large and How to Deal With It

Table of Contents

• Introduction (the convenience and spread of RTK surveying)

• Why are vertical errors more likely with RTK?

• Specific impacts of altitude errors (field examples)

• Common countermeasures and their limits

• Practical solutions: low-cost, high-precision alternatives (e.g., LRTK)

• Features and benefits of LRTK

• Conclusion: choices that work in field practice

• FAQ

• Simple surveying with LRTK

Introduction (the convenience and spread of RTK surveying)

In recent years, RTK surveying has been rapidly spreading in civil engineering survey sites. RTK (Real Time Kinematic) is a real-time high-precision positioning technique using GNSS (satellite positioning); by using two GNSS receivers, a base station and a rover, it achieves centimeter-level position accuracy. Conventional standalone GPS positioning produced meter-level errors, which was insufficient for construction accuracy and as-built control, but with RTK positioning it became possible to correct positioning errors in real time and obtain coordinates with accuracy within a few centimeters. This has streamlined many surveying tasks such as setting batter boards and checking as-built conditions, becoming an important technology supporting digitalization of worksites like smart construction and ICT earthworks.

However, convenient RTK surveying also has challenges. Field surveyors and construction managers sometimes report that “the heights measured by RTK didn’t match and the ground elevation was off.” In other words, RTK can exhibit cases where the vertical (altitude) measurements deviate more than expected. This article explains why vertical errors in RTK surveying tend to be large, organizes their impacts and common countermeasures, and introduces LRTK as a low-cost, high-precision alternative that is attracting attention, along with its features and benefits.

Why are vertical errors more likely with RTK?

RTK positioning tends to show larger vertical (height) errors compared with horizontal positions. The main reasons are as follows.

• Satellite geometry effects: GNSS positioning calculates position by measuring distances to satellites overhead. Because satellites are not below the horizon, the geometric conditions for vertical positioning are unfavorable. Compared with the horizontal dilution of precision (HDOP), the vertical dilution of precision (VDOP) is generally larger (worse). Simply put, height can only be measured from the sky, so the measurement conditions for the vertical direction are worse than for the planar directions measured from all around, making errors more likely.

• Atmospheric layer effects: GNSS signals experience delays and refraction when passing through the ionosphere and troposphere. RTK cancels out these error factors by differential positioning with a base station, but when the rover and base are far apart, residuals remain because atmospheric conditions differ between the two locations. Tropospheric delay in particular can introduce a bias in the vertical component, so with long-range RTK height errors tend to become significant.

• Multipath (reflection) errors: When satellite signals reflect off the ground or buildings before reaching the receiver, interference between direct and delayed reflected waves causes positioning errors. Reflections from the ground or walls can introduce phase shifts in range measurements and destabilize height measurements. Cases have been reported where vertical values fluctuate greatly due to multipath in urban canyons or when surveying near slopes.

• Equipment setup and handling errors: Human factors in RTK surveying also affect height errors. For example, if the rover antenna mounted on a pole (staff) is tilted, an error occurs in the vertical direction corresponding to the tilt angle. The pole must be kept plumb using a bubble level, but in the field perfect verticality is sometimes hard to achieve. Even a slight tilt can manifest as a deviation of several centimeters, so caution is needed. Similarly, mistakes in entering the antenna height (distance from the ground to the antenna reference point) or poor calibration of measuring instruments can shift height results.

Because these factors can overlap, RTK vertical accuracy tends to degrade relatively more easily. In practice, even under normal conditions RTK vertical accuracy is worse than horizontal, and the Geospatial Information Authority of Japan’s public surveying specifications sets RTK allowable errors as “horizontal within 15 mm (0.59 in) ・ vertical within 50 mm (1.97 in),” which is a looser standard for height. This implies that height deviations of a few centimeters are considered acceptable within RTK practice. Next, let’s look at how those few centimeters can affect field operations with concrete examples.

Specific impacts of altitude errors (field examples)

If RTK positioning produces altitude errors on the order of a few centimeters to several tens of centimeters, what problems can arise in civil engineering and construction sites? Here are some cases.

• Batter boards / reference elevation errors: If batter boards (height reference markings) are set using RTK and there is a 5 cm (2.0 in) height error, the finished elevation of structures can be off. For example, if the pavement final elevation was constructed based on RTK-set batter boards and ended up slightly higher than design, drainage problems could occur. Even a difference of a few centimeters can affect slope and drainage planning, so height offsets directly impact site quality.

• Effects on as-built control and volume calculations: When checking as-built surfaces or calculating earthwork volumes using RTK-equipped drones, accumulated vertical errors can distort results. For instance, in a case where a large reclaimed area was surveyed by drone RTK for earthwork volume calculation, heights were measured on average a few centimeters (a few inches) higher, causing an overestimation of fill quantity. The discrepancy was later discovered by verification leveling, resulting in rework. Vertical accuracy instability can undermine the reliability of acceptance decisions and quantity settlements.

• Effects on foundation work and structure installation: Attention is needed when using RTK for tasks requiring strict height control, such as setting bearing elevations for bridges or foundation elevations for buildings. If components are installed based on RTK-measured elevations with large positioning errors, installed member elevations may not match and adjustments may be needed later. Where millimeter-level precision is required, RTK’s centimeter-level errors are insufficient. Therefore, in critical height-control tasks a common approach is to use RTK to get quick approximate elevations and then verify and correct them with conventional leveling as a finishing step.

As shown above, RTK-induced height offsets can cause rework and quality issues in some cases. Especially for tasks that require accurate drainage gradients or alignment of structural elevations, one should not accept RTK vertical results uncritically and must perform careful verification. Next, we review common countermeasures used in the field and their limitations.

Common countermeasures and their limits

To minimize vertical errors in RTK surveying, several countermeasures are taken in the field. The main measures and their effectiveness/limits are outlined below.

• Optimize satellite reception environment: Place receivers where sky visibility is as open as possible. High buildings or trees around can block or reflect satellite signals, so it is ideal to secure clear lines of sight for both base and rover. Low-elevation satellites, which negatively affect vertical accuracy, are often blocked by obstacles; ensuring open sky can improve stability. However, in urban areas or forests there are limits to environmental improvement, so creating ideal conditions is often difficult.

• Keep the distance to the base station as short as possible: If you operate your own base station, placing it near the survey area increases the similarity of atmospheric errors and improves correction accuracy. In general, the shorter the baseline between base and rover, the higher the accuracy; with long baselines exceeding 10 km, it can take longer to obtain a fixed solution and errors tend to increase. When using network RTK (e.g., VRS), generating correction information using a nearby permanent station as a virtual base helps reduce vertical residuals. Still, it is not always possible to place a base station nearby due to site constraints or when surveying a large area at once.

• Choose times with a favorable number and geometry of satellites: Positioning accuracy varies with satellite geometry, so selecting time windows with many visible satellites and good geometry helps. Today, multi-GNSS receivers that handle GPS, GLONASS, Galileo, BDS, and QZSS increase the number of satellites available. Visible satellite counts and DOP values can be predicted with dedicated apps, so surveying when vertical DOP is low (good satellite geometry) can slightly stabilize height accuracy. However, construction schedules may not allow flexible timing, and weather or ionospheric conditions also affect accuracy independent of time of day.

• Perform precise equipment setup and correct input: For both base and rover, carefully level instruments and measure antenna heights when installing. When using a pole, confirm plumb with a bubble level and eliminate pole tilt for each observation point. For tripod-based base stations, firmly secure legs to prevent wind-induced motion. Avoid moving equipment during observation, and measure antenna heights accurately with a tape measure and input the correct values into the receiver and software. Rigorously managing these basic procedures prevents human-caused vertical offsets. Nonetheless, even with such care, errors of a few millimeters to about 1 cm (a few hundredths to about 0.4 in) are unavoidable, and accumulating such small errors can amount to several centimeters overall.

• Combine multiple observations and verification surveys: Do not accept RTK results at face value; for important height data take multiple observations at different times and average them or verify with another method. Measure each point several times at different times and check for large scatter. Also, verify key reference elevations with conventional leveling (spirit leveling) to detect RTK offsets. Leveling is labor-intensive but provides millimeter-level vertical accuracy, making it an effective backup for RTK heights. However, covering everything with leveling every time is inefficient and undermines the immediacy and efficiency that make RTK attractive.

These measures can secure RTK vertical accuracy to some extent, but completely eliminating errors is difficult. Particularly for factors like atmospheric errors and satellite geometry that cannot be fully controlled by human measures, it is worthwhile to consider using the latest technologies. The next section describes a practical low-cost, high-precision alternative—LRTK—that addresses RTK altitude error issues.

Practical solutions: low-cost, high-precision alternatives (e.g., LRTK)

One approach to tackling RTK height offsets is to utilize the latest high-precision GNSS technologies and devices. A representative example that has emerged in recent years is LRTK. LRTK is a new positioning solution that makes existing RTK technology easier to use and provides low-cost, high-precision positioning as an alternative to expensive conventional surveying equipment.

A key feature of LRTK is that it enables centimeter-level positioning (centimeter-level accuracy (half-inch accuracy)) by using a compact high-precision GNSS receiver that pairs with a smartphone, allowing anyone to perform centimeter-class positioning easily. Conventional RTK equipment required a full set of surveying receivers, controllers, and radios, which was costly, but LRTK can achieve RTK-equivalent accuracy by attaching a device to a smartphone and using a dedicated app. This makes high-precision positioning more accessible to site technicians who are not surveying specialists.

Moreover, LRTK includes advanced functions that help improve vertical reliability. For example, whereas traditional setups required a base station or a network RTK subscription, some LRTK receivers can receive the centimeter-class augmentation service (CLAS) broadcast by Japan’s QZSS. This allows receivers to obtain correction information directly from satellites even in mountainous areas without internet coverage, maintaining high-precision positioning including altitude. Multi-GNSS and multi-frequency-capable LRTK receivers can use more satellites for positioning, providing stable fixed solutions and faster initialization, which in turn helps reduce vertical accuracy degradation and solution instability.

Traditional RTK operations involved complex setup and expertise as barriers. LRTK improves usability for field operation, enabling easy workflows. The next section examines specific LRTK functions and benefits in detail.

Features and benefits of LRTK

LRTK is designed as a “more user-friendly RTK” system and offers many useful features for field practitioners. The main characteristics and benefits are summarized below.

• Compact, lightweight, and highly portable: LRTK receiver devices are extremely compact and lightweight compared with traditional fixed GNSS receivers. Some models attach to a smartphone, greatly reducing the burden of carrying equipment to the site. A single worker can carry a device that fits in a pocket and perform high-precision positioning, making it convenient even for quick surveys.

• Ease of use through smartphone integration: Pair a smartphone with a dedicated app via Bluetooth and start RTK positioning from the app’s interface. Traditionally you needed a dedicated controller or field terminal, but with LRTK your familiar smartphone serves as the operating terminal. Starting positioning, saving data, and recording coordinates can be done intuitively in the app, making it easy for first-time users. Smartphone connectivity also allows configuring network RTK (corrections via Ntrip) for all-in-one operation.

• Multi-GNSS and multi-frequency support: LRTK receivers use modern GNSS chips that support GPS, GLONASS, Galileo, BeiDou, QZSS, etc. By receiving multiple frequency bands such as L1/L2, they can compute fixed solutions quickly and stably. With more and higher-quality satellites available, positioning becomes less unstable due to satellite shortages even in urban areas, and vertical solution precision is improved. Multi-frequency use also helps correct ionospheric delays better, reducing height component errors.

• Tilt compensation: Some LRTK models include tilt sensors that allow accurate coordinate calculation of the pole tip even when the pole is not perfectly vertical. For example, when you must tilt the pole to avoid obstacles, the sensor compensates for pole tilt in real time and corrects the positioning result including height. This enables height measurements even in places where standing the pole was previously difficult, increasing field flexibility and reducing the risk of missed or erroneous measurements.

• Data integration and combined use: Leveraging smartphone linkage, LRTK can immediately link recorded coordinates with photos and notes on site. For example, storing high-precision geotagged photos in the app lets you later identify exact locations in the office while reviewing images. Collected data can be uploaded to the cloud for sharing, enabling remote offices to monitor surveying results in real time. Tasks that previously required separate equipment or software, such as photogrammetry or AR-based visualization, can be handled within LRTK, expanding on-site applications.

• Low-cost and easy to introduce: LRTK offers high functionality at a lower initial cost than traditional surveying GNSS equipment. You can leverage existing base station networks or satellite augmentation information rather than installing large base station infrastructures, reducing upfront investment. Subscription-based usage plans are available, allowing flexible operation for only the needed periods. This makes centimeter-class positioning accessible to small and medium-sized companies and sites that previously could not afford high-precision equipment.

LRTK can be a practical solution to the RTK vertical error problem. It is not a magic tool that dramatically improves altitude accuracy on its own, but technical measures reduce error factors and help achieve stable on-site results. Its ease of use and cost advantages also make it a tool for overall improvement and sophistication of field surveying operations.

Conclusion: choices that work in field practice

We have reviewed the causes and countermeasures for the RTK “height shift” problem. By nature, satellite positioning is less favorable in the vertical direction than horizontal, but by understanding factors such as satellite geometry, atmospheric errors, and human error and addressing them appropriately, it is possible to keep errors within practically acceptable ranges. In the field, practitioners secure vertical reliability through measures such as improving satellite reception environment, careful equipment handling, and combined use of leveling.

At the same time, traditional countermeasures have limits and impose burdens. This is where new high-precision positioning solutions like LRTK come into play. LRTK maintains RTK-level accuracy while simplifying equipment and lowering costs, and it offers various functions to mitigate vertical error risks. For site practitioners, technologies that allow efficient surveying while paying attention to height accuracy are a major asset.

Ultimately, the choice that works in the field balances accuracy and efficiency. Where millimeter-level accuracy is required, optical leveling remains indispensable, but for many general civil surveying tasks, RTK or LRTK with centimeter-level accuracy (half-inch accuracy) is sufficient. The important point is to understand RTK’s vertical error characteristics, perform necessary checks and corrections as needed, and proactively adopt new tools to optimize field operations. For example, a practical approach is to use LRTK for rapid data collection over wide areas and verify key elevations with leveling as required.

Vertical errors in RTK surveying are certainly a concern, but solutions are steadily improving. Use the latest technologies wisely to carry out safe and efficient field surveying.

FAQ

Q1. Why does RTK tend to have larger errors in height than in horizontal position? A. This is due to geometric constraints of satellite positioning and atmospheric error effects. Because satellites only exist overhead, the satellite geometry is inherently weak for height (the vertical dilution of precision becomes larger). In addition, differential positioning with a base station cannot perfectly cancel tropospheric delays and other altitude-influencing errors, so RTK tends to have worse vertical accuracy than horizontal.

Q2. How accurate is RTK vertically in concrete terms? A. Under favorable conditions, errors can be within a few centimeters (a few inches). For example, when close to a base station and with few obstructions, height may stabilize around 3–4 cm (1.2–1.6 in). However, when environment degrades, errors can grow and in urban or forested areas deviations exceeding 10 cm (about 4.0 in) have been reported. The national public surveying standard sets RTK allowable vertical error at 50 mm (1.97 in), but this is only a guideline; on-site practice should allow margins.

Q3. What are the key points to reduce height shifts in RTK surveying? A. First, improve satellite reception conditions as much as possible: ensure open sky and avoid reflective surfaces to reduce multipath. Keep the base–rover distance short, choose times with favorable satellite geometry, set up equipment carefully and input antenna heights correctly. For important heights, take multiple measurements and average them or verify with leveling to eliminate incidental errors. Even with these measures, some deviation can still occur, so plan operations with that possibility in mind.

Q4. Should leveling be used instead of RTK when high vertical accuracy is required? A. It depends. For benchmark surveys and precision works requiring millimeter-level heights, conventional leveling is still superior in reliability. Leveling takes more effort but achieves accuracy orders of magnitude better than RTK in vertical. On the other hand, RTK is sufficient for many general earthwork and as-built checks. A recommended strategy is to use RTK for efficient surveying and double-check critical points with leveling, combining the strengths of both methods.

Q5. What is LRTK and how does it differ from conventional RTK? A. LRTK is a new high-precision positioning system developed to make RTK easier to implement. By using a small GNSS receiver that interfaces with a smartphone, LRTK achieves centimeter-class positioning at lower cost and with simpler operation than conventional RTK gear. Its principle is the same—relative positioning using correction information—but LRTK supports multi-GNSS and multi-frequency, includes convenient field functions such as tilt compensation, and can use satellite augmentation services like CLAS as well as network corrections. In short, LRTK preserves RTK accuracy while simplifying hardware and operation.

Q6. Will LRTK completely solve RTK’s vertical error problems? A. Because LRTK is still GNSS-based, it cannot escape fundamental constraints such as satellite geometry and atmospheric effects. However, LRTK incorporates modern technologies that mitigate RTK weaknesses: multi-frequency capability for faster and more accurate solutions, the ability to use CLAS and network RTK for broad correction coverage, and automatic pole-tilt compensation to reduce human error. While not eliminating errors entirely, LRTK reduces the risk of height deviations and is a significant practical aid on site.

Q7. Does introducing LRTK require specialized knowledge? A. Compared with conventional RTK systems, LRTK is much more user-friendly. Operation via a smartphone app makes it accessible without specialized equipment training. Basic setup and an understanding of the principles are necessary, but following manufacturer manuals and support allows non-surveyors to operate it. Still, knowledge of GNSS basics and error factors helps with troubleshooting and judging accuracy, so some learning is recommended to fully utilize LRTK in the field.

Simple surveying with LRTK

With a deeper understanding of RTK vertical errors and countermeasures, we introduce simple surveying using LRTK as a new high-precision field method. As noted, LRTK is a smartphone-linked RTK system that enables anyone to perform centimeter-class surveys without specialized surveying equipment. For example, small-scale site elevation checks or in-progress as-built inspections that previously required a surveying crew can be performed quickly by site technicians using LRTK. Because you can record photos and notes while measuring positions on a smartphone, LRTK integrates measuring and recording and enhances convenience.

LRTK-based simple surveying thus pursues ease of use while maintaining high precision. By adopting LRTK, sites struggling with altitude errors can achieve both improved efficiency and measurement accuracy. As positioning technology continues to evolve, incorporating these advances will help boost site productivity and ensure quality.