RTK horizontal accuracy vs vertical accuracy: Key points contractors should know

Table of Contents

• Introduction (spread and benefits of RTK surveying)

• Differences between RTK horizontal accuracy and vertical accuracy

• Why is RTK vertical accuracy worse than horizontal?

• Impacts of vertical errors on the site

• Common countermeasures to ensure vertical accuracy

• Practical solution: utilizing LRTK

• Features and benefits of LRTK

• Conclusion (how to balance accuracy and efficiency on site)

• FAQ

• Simple surveying with LRTK

Introduction (spread and benefits of RTK surveying)

In recent years, RTK surveying has been rapidly spreading on construction and civil engineering sites. RTK (Real Time Kinematic) is a real-time high-precision positioning technology using GNSS satellite positioning; by combining two GNSS receivers—a base station and a rover—it can achieve centimeter-level positional accuracy. Standalone GPS positioning used to produce errors of several meters, which was insufficient to meet construction accuracy and as-built control requirements. With the advent of RTK positioning, positioning errors can be corrected in real time, enabling coordinates to be obtained with high accuracy within a few centimeters. This has greatly streamlined many surveying tasks such as setting batter boards and checking as-built conditions, making RTK an important technology that supports on-site digitalization like smart construction and ICT earthworks.

However, RTK surveying also has weaknesses. On-site technicians sometimes report that "the height measured by RTK did not match and ground elevation was off." In other words, RTK can exhibit greater deviations in height (vertical) than in the horizontal direction. This article focuses on the differences between horizontal and vertical accuracy in RTK surveying, explains why vertical errors tend to be larger, their impacts on the site, and general countermeasures to ensure vertical accuracy. We also introduce LRTK, a new low-cost, high-precision technology attracting attention in practice, and describe its features and benefits.

Differences between RTK horizontal accuracy and vertical accuracy

RTK positioning achieves dramatically higher accuracy compared to traditional GPS positioning, but accuracy characteristics differ slightly between horizontal and vertical (height) directions. Generally, RTK planar position (horizontal) errors under good conditions are around ±1–2 cm (±0.4–0.8 in), while vertical (height) errors are about ±3 cm (±1.2 in). These figures vary depending on equipment specifications and measurement environment, but it is important to note that the vertical direction tends to have slightly greater uncertainty than the horizontal.

In fact, the public surveying regulations established by the Geospatial Information Authority of Japan specify allowable RTK surveying errors as "within 15 mm (0.59 in) horizontally and within 50 mm (1.97 in) vertically." The looser (larger) vertical tolerance than horizontal shows that RTK vertical accuracy tends to be worse than horizontal. Conversely, this means that height deviations of a few centimeters are considered within the expected range for RTK surveying, and users need to handle positioning results with this characteristic in mind.

Why is RTK vertical accuracy worse than horizontal?

RTK positioning tends to show lower accuracy in the vertical (height) direction than in the horizontal direction. The main causes are as follows.

• Satellite geometry issues: GNSS positioning calculates position from distances to satellites in the sky, but satellites do not exist below the horizon, so vertical positioning is geometrically disadvantaged. Horizontal position can be measured from satellite signals all around the horizon (360 degrees), whereas vertical measurement must rely on signals from above. As a result, the vertical dilution of precision (VDOP) — an index of vertical position accuracy degradation — tends to be larger than the horizontal HDOP. Simply put, height measured only from the sky is more prone to error than horizontal position which can be measured from all directions.

• Atmospheric effects: Satellite signals are delayed and refracted passing through the ionosphere and troposphere. RTK cancels much of the atmospheric error via relative positioning with a base station, but residual errors remain when the atmospheric conditions differ between the base and rover locations as distance increases. In particular, tropospheric delay residuals tend to introduce bias in the vertical component, so RTK surveys with long baselines (far from the base station) often show larger vertical errors.

• Multipath errors: When satellite signals reflect off buildings or the ground before reaching the receiver, interference between direct and reflected waves causes positioning errors. Reflections from the ground or walls can shift the phase of the satellite signal and destabilize vertical measurements. In urban areas surrounded by tall buildings or near cut slopes, multipath effects can cause large fluctuations in vertical values.

• Errors in equipment setup and operation: The way surveying equipment is used can also cause vertical errors. For example, if the rover pole (staff) is not perfectly vertical, its inclination will introduce height errors. Poles should be kept vertical using a bubble level, but slight tilts occur on site. Even a few degrees of tilt can produce height deviations of several centimeters, so care is needed. Similarly, mistakes in inputting antenna height (height from ground to antenna reference point) or poor equipment calibration can shift vertical positioning results.

These factors make RTK vertical accuracy relatively more susceptible to degradation. As noted earlier, vertical deviations of a few centimeters can be within expected ranges for RTK, so it is important to perform corrections and checks as needed when using RTK results.

Impacts of vertical errors on the site

If RTK positioning produces vertical errors on the order of a few centimeters, what concrete problems can this lead to on civil engineering and construction sites? Here are representative cases.

• Errors in batter boards and reference elevations: If batter boards (height reference marks) are set using RTK and there is, for example, a 5 cm error, the finished elevation of structures may be off. For instance, setting pavement elevation based on RTK batter boards resulted in pavement being built slightly higher than designed, causing drainage problems. Even a few centimeters can affect slopes and drainage plans, so vertical deviations directly impact on-site quality.

• Impacts on as-built control and quantity calculations: When checking as-built conditions or calculating earthwork volumes using RTK-equipped drones, vertical errors can accumulate and distort results. For example, a drone RTK survey of a large fill area measured heights on average a few centimeters higher than actual, leading to an overestimation of fill volume. The discrepancy was later detected through validation leveling, causing rework in quantity calculations. Vertical accuracy instability can reduce the reliability of acceptance judgments and quantity settlements, so caution is required.

• Effects on tasks requiring strict height control: Tasks requiring millimeter-level height control—such as setting bridge bearing heights or verifying foundation elevations—require caution when using RTK. If installation heights are set directly by RTK, even small positioning errors can cause component installation heights to be off, necessitating later adjustments. Where millimeter precision is required, RTK errors of a few centimeters are insufficient. Therefore, in critical height control tasks practitioners often use RTK for efficient rough measurements and then verify and correct with precise leveling.

As shown above, RTK vertical deviations can cause rework and quality issues depending on the situation. In processes requiring ensured drainage gradients or accurate installation heights, avoid taking RTK results at face value and perform careful verification. Next, we will look at common on-site measures to address such vertical errors and their limitations.

Common countermeasures to ensure vertical accuracy

To minimize vertical errors in RTK surveying, the following countermeasures are commonly implemented on site. Below we summarize each point along with its effectiveness and limitations.

• Optimize satellite reception environment: Place receivers in open locations with as little obstruction overhead as possible. Surrounding tall buildings or trees can block or reflect satellite signals, so it is ideal to site both base and rover in locations with good visibility. Low-elevation satellites that negatively affect vertical accuracy are often obstructed, so securing open sky improves positioning stability. However, in urban or forested areas there are limits to environmental improvement, and ideal conditions cannot always be achieved.

• Keep distance to the base station short: If you set up your own base station, placing it near the measurement area increases commonality of error sources between stations and improves correction accuracy. Generally, positioning accuracy improves with shorter baselines, and long distances over 10 km (6.2 mi) can lengthen time to obtain a fixed solution and increase errors. When using network RTK (e.g., VRS), configuring virtual base stations based on nearby control points helps reduce vertical residuals. Nevertheless, site constraints or the need to survey wide areas may make it impossible to keep the base station close.

• Choose times with many satellites and good geometry: GNSS satellite geometry changes over time affecting positioning accuracy, so choosing times when a greater number of satellites with good geometry are available helps. Multi-GNSS receivers supporting GPS, GLONASS, Galileo, QZSS (Michibiki), and BDS are now common, increasing the number of usable satellites. Visible satellite counts and DOP values can be predicted in advance with dedicated apps, so scheduling surveys when vertical DOP is low (good satellite geometry) can somewhat stabilize vertical accuracy. Still, construction schedules may not always permit ideal timing, and weather or ionospheric conditions also influence accuracy.

• Proper equipment setup and accurate input: During base and rover setup, perform careful leveling and accurate measurement of antenna height. When using a rover pole, use the bubble level to ensure verticality at each point and prevent pole tilting. When using a tripod for the base, secure the legs firmly to prevent movement from wind. Avoid moving equipment during observations, which can introduce errors. Measure antenna height accurately with a tape measure and input the correct value into the receiver or software. Strict handling procedures help prevent human-induced vertical errors, though some millimeter-to-1 cm level errors remain unavoidable and can accumulate into several centimeters.

• Combine multiple observations and verification: Do not overtrust single RTK measurements; for important height data take multiple measurements and average them, or verify with alternate methods. Measure a point several times at different times to check for large variability. Also verify key reference heights with traditional leveling to detect RTK offsets. Although leveling is labor-intensive, it is orders of magnitude more precise vertically than RTK and serves as an effective backup. However, performing leveling for every point is inefficient and undermines RTK’s real-time benefits, so a flexible approach combining RTK and leveling is practical.

These measures can secure some level of vertical accuracy for RTK surveying, but completely eliminating error sources is difficult. In particular, factors such as satellite geometry and atmospheric errors cannot be fully controlled by human measures, making it worthwhile to consider the latest technologies as well. Next, we introduce LRTK, a new positioning solution gaining attention as a practical remedy to RTK vertical accuracy issues.

Practical solution: utilizing LRTK

One approach to address RTK vertical deviations is to utilize the latest high-precision GNSS technologies and devices. A representative example is the recently introduced LRTK system. LRTK is a positioning solution that makes existing RTK technology more user-friendly; it aims to deliver low-cost yet high-precision positioning on site as an alternative to traditional expensive and complex surveying instruments.

A key feature of LRTK is that it enables centimeter-level positioning easily by pairing a compact high-precision GNSS receiver with a smartphone. Traditional RTK equipment required a dedicated receiver, controller, radio equipment, and more—costly and requiring specialized knowledge. With LRTK, attaching a small receiver to a smartphone and operating a dedicated app delivers RTK-equivalent accuracy, making high-precision positioning accessible even to personnel who are not surveying specialists.

LRTK also includes advanced functions that help improve vertical reliability. For example, while conventional setups required a private base station or a network RTK subscription, some LRTK receivers can directly receive centimeter-level augmentation services (CLAS) provided by Japan’s QZSS "Michibiki." This allows receiving correction information from satellites even in mountainous areas without internet, maintaining high-precision positioning including altitude. Multi-GNSS and multi-frequency support increases the number of usable satellites, stabilizing initialization and maintenance of fixed solutions, which can suppress vertical accuracy degradation and solution instability.

Traditional RTK systems required specialized setup and operation knowledge, but LRTK is designed for on-site usability and is much easier to handle. Below we look at LRTK’s specific features and benefits.

Features and benefits of LRTK

LRTK was developed with the concept of "easier RTK," and has various features that can transform on-site surveying. Main benefits include:

• Compact and lightweight: LRTK GNSS receiver devices are designed to be very compact and lightweight. Some models attach to a smartphone and fit in a pocket while delivering high-precision positioning. A single worker can easily carry the device, making it convenient for quick surveys and improving mobility.

• Ease through smartphone integration: Pair a smartphone with a dedicated app via Bluetooth, and start RTK positioning through on-screen operations. Previously, dedicated controllers or field terminals were necessary, but with LRTK your familiar smartphone becomes the controller. Starting/stopping positioning, saving data, and recording coordinates are intuitive in the app, making it easy for first-time users. Smartphones’ communication functions can also be used to connect to network RTK via NTRIP, enabling all-in-one operation.

• Multi-GNSS and multi-frequency support: LRTK receivers use the latest GNSS chips and support not only GPS but GLONASS, Galileo, BDS (BeiDou), and QZSS (Michibiki). By receiving multiple frequency bands like L1/L2, fast and stable fixed solutions can be obtained. Increasing the number and quality of usable satellites reduces positioning instability due to satellite shortage in urban areas and raises vertical solution quality. Multi-frequency reception also improves ionospheric delay correction accuracy, reducing vertical component errors.

• Tilt compensation: Some LRTK models include tilt sensors that allow accurate calculation of the pole tip coordinate even when the pole is not perfectly vertical. For example, when avoiding obstacles forces the pole to be tilted, the sensor compensates the tilt in real time and automatically corrects positioning results including height. This enables coordinate acquisition in places where it was previously difficult to set a pole upright, reducing the risk of missed measurements and human-induced errors.

• Data utilization and integrated functions: Leveraging smartphone integration, LRTK allows you to immediately attach photos and notes to captured coordinates. Recording high-precision geotagged photos in the app makes it easy to identify exact locations later in the office. Measurement data can also be uploaded and shared to the cloud in real time so remote colleagues can instantly review results. Tasks that previously required separate equipment or software—such as photogrammetry or AR-based site visualization—can be handled on a single LRTK platform, broadening use cases.

• Reduced introduction costs: Despite these features, LRTK is generally less expensive than traditional surveying GNSS equipment. You can utilize existing control point networks and satellite augmentation information without setting up large fixed base stations, lowering initial investment. Subscription-based plans that avoid upfront hardware purchases are also available, enabling flexible, period-based usage. This lowers the barrier for adopting high-precision positioning, allowing small and mid-sized contractors or sites that previously could not afford such equipment to use RTK-equivalent surveying.

LRTK can be a practical solution to RTK vertical error problems. While it is not a magical device that instantly transforms vertical accuracy, its technological improvements reduce error sources and help obtain stable on-site results. Its usability and cost advantages also position LRTK as a promising tool to enhance and streamline surveying operations.

Conclusion (how to balance accuracy and efficiency on site)

So far we have examined the vertical accuracy issue in RTK surveying, its causes, and countermeasures. Due to GNSS principles, vertical accuracy is inherently disadvantaged relative to horizontal, but understanding factors like satellite geometry, atmospheric errors, and human mistakes—and addressing them appropriately—can bring accuracy within practically acceptable limits. On site, practitioners have managed vertical reliability through efforts to improve satellite reception, careful equipment handling, and, where necessary, combining RTK with leveling.

Nevertheless, traditional countermeasures have limits and can be cumbersome. This is where new high-precision positioning solutions like LRTK come in. LRTK maintains RTK accuracy while offering ease of use and lower cost, and incorporates functions to mitigate vertical error risks. For practitioners, tools that allow accurate and efficient surveying despite vertical accuracy concerns are powerful assets.

Ultimately, the key on site is balancing accuracy and efficiency. For tasks requiring millimeter-level precision, optical leveling remains indispensable. For many general civil surveying tasks, centimeter-level accuracy from RTK or LRTK is sufficient. The important thing is to perform checks and corrections when necessary, based on RTK’s vertical characteristics, and to adopt new tools to optimize on-site work. For example, using RTK/LRTK to quickly collect data over wide areas and double-checking key elevations with leveling can be a practical compromise that leverages the strengths of both methods.

RTK vertical errors are indeed a noteworthy issue, but remedies are steadily advancing. Use the latest technologies wisely to conduct safe, efficient, and accurate on-site surveying.

FAQ

Q1. Why is RTK vertical positioning accuracy often worse than horizontal? A. It is due to the geometric constraints of satellite positioning and atmospheric error influences. Satellites are only overhead, so vertical positioning is inherently disadvantaged (vertical DOP tends to be larger). Even with relative positioning to a base station, tropospheric delay and similar errors affecting height are not always fully canceled. These factors cause RTK to tend toward lower vertical accuracy than horizontal.

Q2. How accurate is RTK vertical positioning in practice? A. Under good conditions, errors are often within a few centimeters. For example, when close to a base station with few obstructions, vertical errors have been reported to stabilize around 3–4 cm (1.2–1.6 in). However, accuracy degrades in poor environments and there are cases in urban or forested areas where errors exceed 10 cm (3.9 in). National public surveying standards set an RTK allowable vertical error of 50 mm (1.97 in), but this is a guideline; on site it is wise to allow additional margin.

Q3. What are the practical points to reduce RTK vertical deviations? A. First, improve the satellite reception environment as much as possible—ensure sky visibility and avoid reflection sources like buildings or rock faces to reduce multipath. Keep the base station near the survey area, choose times with favorable satellite geometry, set up equipment accurately and input antenna heights correctly. For critical heights, take multiple observations and average them or verify with leveling to eliminate incidental errors. Since zero error is unattainable, be aware that deviations can occur and double-check important locations.

Q4. For tasks requiring high vertical accuracy, should leveling be used instead of RTK? A. It depends on the case. For benchmark surveys and precision installations requiring millimeter accuracy, leveling (optical leveling) remains superior in reliability. Leveling is time-consuming but far more precise vertically than RTK. On the other hand, RTK is adequate for many routine earthworks checks and quantity surveys. A recommended approach is to use RTK for efficient initial measurement and perform leveling double-checks at critical points. This combined method leverages both approaches while avoiding unnecessary rework.

Q5. What is LRTK and how does it differ from traditional RTK? A. LRTK is a newer high-precision positioning system designed to make RTK more accessible. By using a small GNSS receiver that connects to a smartphone, LRTK delivers centimeter-level positioning more affordably and simply than conventional RTK equipment. The underlying principle is the same relative positioning with correction information, but LRTK leverages multi-GNSS, multi-frequency reception, and field-friendly features such as tilt compensation. It can also use satellite-delivered augmentation like Japan’s CLAS, enabling high precision without a dedicated base station. In short, LRTK simplifies equipment and operations while maintaining RTK-like accuracy.

Q6. Will LRTK eliminate RTK vertical error problems? A. LRTK is still based on GNSS positioning, so it cannot completely escape fundamental constraints like satellite geometry and atmospheric effects. However, LRTK incorporates technological improvements that mitigate RTK weaknesses—for example, multi-frequency reception enabling faster, more accurate fixed solutions; CLAS and network RTK support for wide-area corrections; and automatic pole tilt compensation to reduce human error. While LRTK will not make errors vanish entirely, it reduces the risk of vertical deviations and is a practical aid on site.

Q7. Does adopting LRTK require specialist knowledge? A. Compared to traditional RTK systems, LRTK is far more user-friendly. Operation via a smartphone app means specialized surveying equipment training is less necessary. Basic setup and an understanding of core principles are still required, but following manufacturer manuals and support, non-surveyors can operate LRTK. Still, knowing GNSS fundamentals and error factors helps with troubleshooting and judging accuracy, so some knowledge acquisition is recommended for effective on-site use.

Simple surveying with LRTK

With a deeper understanding of RTK vertical errors and countermeasures, we introduce "simple surveying with LRTK" as a new high-precision method for on-site use. As described above, LRTK is a smartphone-integrated RTK system that enables anyone to perform centimeter-level surveying without specialized equipment. For example, small-scale elevation checks of engineered sites or on-site as-built inspections that previously required a survey team can be conducted quickly by site technicians using LRTK. Because photos and notes can be recorded simultaneously with positioning on a smartphone, the tasks of "measuring" and "recording" are integrated, increasing convenience.

Simple surveying with LRTK pursues ease while maintaining high precision. Even sites troubled by vertical errors can balance productivity and measurement accuracy by introducing LRTK. Continue to adopt advancing positioning technologies appropriately to improve site productivity and ensure quality.