Table of Contents

• Introduction

• Challenges of Height Accuracy in RTK

• Height Correction Using Geoid Models

• Accurate Measurement and Setting of Antenna Height

• RTK Positioning Settings Checkpoints

• Other Points for High-Precision Positioning

• Summary: Simple Surveying with LRTK

• FAQ

Introduction

RTK positioning (real-time kinematic) is a widely used technique for obtaining centimeter-level high-precision position information in surveying and construction sites. In particular, height (elevation) measurements are often important for construction management and infrastructure maintenance, but ensuring their accuracy is generally more difficult than for horizontal measurements. If height errors amount to tens of centimeters, they can hinder the control of water slopes and verification of design elevations, so securing vertical accuracy is an important issue on site. In this context, RTK vertical accuracy tends to be worse due to satellite geometry and radio propagation characteristics, which often result in larger positioning errors in the vertical direction.

This article focuses on three points—“geoid,” “antenna height,” and “settings check”—as ways to improve RTK vertical accuracy, and explains practical know-how useful on site.

Challenges of Height Accuracy in RTK

In general, GNSS positioning tends to have poorer vertical accuracy than horizontal accuracy. One reason is that satellites cannot be observed from below the horizon, which makes the geometry unfavorable in the vertical direction. As a result, RTK positioning also tends to have larger height errors than horizontal errors; height accuracy is often said to be about 1.5 times that of horizontal accuracy. In addition, the measurement environment affects vertical accuracy. In places with limited sky view or surrounded by buildings and trees, signal reflections (multipath) and loss of satellites can make height measurements unstable.

Another factor influencing vertical accuracy is the handling of reference values. When discussing heights on Earth, it is necessary to distinguish between “elevation” (height above mean sea level) and the “ellipsoid height” directly obtained from GNSS. If the geoid model described later is not used, heights obtained by RTK remain as ellipsoid heights and can differ from actual elevations by tens of meters. With these issues in mind, let us look at the points to watch on site to obtain highly accurate height information.

Height Correction Using Geoid Models

The first point to improve RTK vertical accuracy is to appropriately use a geoid model. The geoid is a surface of equal gravitational potential (a hypothetical sea surface that corresponds to mean sea level) and serves as the reference surface for actual elevation. The height directly computed by a GNSS receiver is the height from the reference ellipsoid used in satellite positioning (such as WGS84), i.e., the ellipsoid height, whereas the elevation we commonly use is measured from the geoid surface (orthometric height).

In Japan, the difference between ellipsoid height and elevation (the geoid height) varies by region by about 30-40 m (98.4-131.2 ft); for example, around Tokyo it is about 30 m (98.4 ft), while in mountainous areas it can exceed 40 m (131.2 ft). Therefore, to obtain accurate elevations with RTK positioning, you must correct for this geoid height. Specifically, enable the geoid model (geoid height conversion) in your positioning software or device settings, and subtract the geoid height from the ellipsoid height to calculate the elevation. For example, if GNSS positioning yields an ellipsoid height of 80.00 m (262.47 ft) at a point near Tokyo, subtracting the region’s geoid height (about 35 m (114.8 ft)) gives an elevation of approximately 45.00 m (147.64 ft). By correctly applying geoid correction in this way, RTK positioning can provide directly usable elevation values on site. Using the latest geoid models provided by the Geospatial Information Authority of Japan (for example, geoid models compatible with JGD2011 or the new Geoid2024 introduced in 2025) makes it possible to obtain elevations consistent with public standards. In fact, the Geospatial Information Authority of Japan (the national geodetic agency) revised nationwide elevation results in April 2025 based on the latest geoid model, making the use of geoid correction increasingly important for obtaining accurate heights from GNSS surveys.

Applying a geoid model fills the gap between sea-level height and RTK results and can bring height accuracy into a few centimeters. Conversely, without geoid correction, no matter how precisely RTK positioning is performed, the resulting height will be far removed from mean sea level. Surveyors and construction site personnel who need elevations on site should always check that geoid height conversion is enabled in their equipment settings and acquire height data on the correct reference surface.

Accurate Measurement and Setting of Antenna Height

The second point to improve vertical accuracy is to accurately set the antenna height. In RTK surveying, the GNSS antenna is mounted on a tripod or pole for measurements, and you must enter the height difference between the antenna’s phase center (the point used for positioning signals) and the ground measurement point. This is called antenna height or instrument height. If the antenna height input is incorrect, the resulting vertical coordinate will be offset by that amount.

For example, if you set the rover antenna on a pole at a height of 2 m (6.6 ft) above the ground, you must register the antenna height as 2.00 m (6.56 ft) in the software; otherwise, the computed coordinates will be 2 m off the actual ground point. Similarly, when installing a base station, it is important to account for the antenna height correctly. When entering known coordinates for a base station, either add the antenna height to the elevation to set the antenna phase center’s height coordinate, or, if the software allows entering known point coordinates and antenna height separately, enter appropriate values accordingly.

It is desirable to measure antenna height carefully to the millimeter with a tape or a dedicated scale and to record it. Especially for precision surveys that require high vertical accuracy, do not neglect measuring and setting the antenna height for each installation. Although basic, performing this task reliably removes an unnecessary error source and helps maintain RTK vertical accuracy.

RTK Positioning Settings Checkpoints

Verifying various settings is essential to ensure the RTK system operates correctly and to maximize vertical accuracy. First, check that the coordinate system and geodetic datum you use are set correctly. In Japan, when public survey coordinates are required, systems such as JGD2011 or the latest JGD2022 are often used. If your controller or app allows you to select the coordinate system, set the correct one according to your purpose and, as mentioned earlier, turn on geoid height conversion. This allows the RTK receiver to automatically subtract the geoid height from the ellipsoid height it obtains so you can read elevation directly on site.

Next, check the communication between base and rover and the reference settings. When using network RTK (NTRIP services, etc.), verify that you are connected to the correct correction source and recheck the server address, port number, mountpoint, login information, and so on. Also, ensure that the correction data format (e.g., MSM4 or MSM7) is supported by your receiver. If you set up your own base station for surveying, always verify that the base station coordinate settings are appropriate. When placing a base station on a known point, calibrate it beforehand using public control points or benchmarks and set the precise elevation, latitude, and longitude. Any error in the base station coordinates will produce the same error in the rover’s computed coordinates. Vertical errors tend to be particularly noticeable, so if you place a base station at an arbitrary point away from control points, it is desirable, if possible, to verify accuracy at nearby known points and correct any height offset.

Additionally, confirm that positioning modes and filter settings on the RTK receiver or application are configured correctly. Set the system to record fixed solutions (FIX) only, and, if possible, enable a function to automatically exclude float (FLOAT) or single solutions. Configure the satellite elevation mask appropriately to exclude extremely low-elevation satellites (which are prone to noise and errors) while maintaining a balance that secures a sufficient number of satellites. Thoroughly checking these settings helps draw out the RTK system’s potential and prevent factors that degrade vertical accuracy in advance.

Other Points for High-Precision Positioning

Finally, here are other points to keep in mind to improve vertical accuracy. First, choose the measurement environment carefully. Selecting an open area and securing as wide a sky view as possible reduces vertical error sources. In canyons between high-rise buildings or within forests, satellite signal blockage and multipath reflections make obtaining a fixed solution difficult and degrade height accuracy. Conduct surveys in locations with good visibility and, if necessary, attach a ground plane (metal plate) to the antenna to suppress reflections from below.

Also pay attention to the baseline length between the base station and the rover. If the distance is too great, uncorrectable factors such as atmospheric changes increase and accuracy deteriorates. Generally, a distance within 10 km is desirable; beyond that, obtaining a fixed solution may take longer and vertical errors are more likely to increase. In network RTK, VRS (virtual reference station) methods set a virtual base station near the user to keep baseline lengths short.

Considering satellite geometry is also important. Use a GNSS planner to check satellite configurations in advance and choose times when satellite geometry is favorable and DOP values (dilution of precision) are low. In particular, a low PDOP value improves relative vertical accuracy. If your receiver can use multiple GNSS constellations (GPS, GLONASS, Galileo, QZSS, etc.), utilize multi-GNSS support to increase the number of satellites, which improves accuracy and stability in acquiring fixed solutions.

Observation time and averaging are effective measures to improve accuracy. A single observation is affected by instantaneous errors, but by continuously observing the same point for tens of seconds to minutes and adopting the averaged value, you can reduce random errors. Use any available on-site averaging functions. For example, averaging position data collected over a set time (e.g., 60 seconds) can greatly reduce the standard deviation compared to a single measurement.

Finally, make a habit of validating positioning results in real time. If there are points on site with known elevations (such as benchmarks or previously measured points), perform test RTK measurements there as well and compare the obtained elevations. If results are within a few centimeters, it is a good indication that the system is functioning properly. Detecting errors on site allows you to investigate causes and re-measure promptly.

Summary: Simple Surveying with LRTK

We have discussed points to improve RTK vertical accuracy: using geoid models, strictly setting antenna height, optimizing various settings, and refining environment and observation methods. Putting these into practice enables you to obtain much more accurate elevation data than before. However, managing all of these perfectly on site is not easy.

This is where simple surveying with LRTK draws attention. LRTK is a system designed to make high-precision GNSS positioning easier, and with dedicated receivers and smartphone apps it is designed to achieve centimeter-level positioning without complex setup. For example, in an LRTK system, if you set the coordinate system and geoid correction in the app, corrected elevations are applied automatically. Antenna height management is also guided within the app, so one person can carry out measurements without mistakes. Advanced RTK settings that previously required specialist knowledge can be completed through intuitive operations with LRTK, greatly reducing the burden on site.

For surveyors and construction site supervisors seeking vertical accuracy, LRTK can be a reliable ally. Consider adopting simple surveying with LRTK to obtain high-precision elevation information without being troubled by complex procedures.

FAQ

Q: Which is superior for vertical accuracy, RTK surveying or leveling? A: Generally, leveling outperforms RTK surveying in vertical accuracy. Leveling performed by experienced technicians using a level instrument can measure height differences to millimeter order over short distances. However, leveling requires many personnel and time and is inefficient for measuring elevation differences over wide areas. RTK surveying, on the other hand, can quickly obtain elevation information over large areas. Its accuracy is on the order of a few centimeters, which is practically sufficient for many uses such as construction management and ground elevation mapping. Combining the two methods—e.g., measuring known benchmarks by leveling and using RTK to reference them—can balance efficiency and accuracy. For instance, when monitoring subsidence of a few millimeters, leveling is necessary, but for general earthworks and topographic surveys, RTK vertical accuracy is usually sufficient.

Q: How accurate is RTK vertical positioning? A: For typical RTK-GNSS equipment, if horizontal accuracy is on the order of a few centimeters, vertical accuracy is about 1.5 times that—i.e., on the order of a few centimeters to a few tens of centimeters. However, with favorable satellite geometry and environmental conditions, and with correct geoid correction and antenna height settings, vertical accuracy within a few centimeters is achievable. Using the latest equipment and services (e.g., LRTK), there are cases where vertical error has been reduced to about 3 cm (1.2 in).

Q: Is it mandatory to use a geoid model? A: If you need precise elevations (heights above mean sea level), geoid model correction is indispensable. Without geoid correction, positioning results remain as ellipsoid heights and are significantly higher than actual elevations. However, in cases where only relative height relationships within a limited site are needed or where you plan to correct values later, you may not need to apply geoid correction in the field. That said, if you produce public survey results or compare heights with other sites, it is more reliable to apply a geoid model from the start.

Q: Are there tips for measuring antenna height? A: When measuring antenna height, measure the vertical distance from the designated reference point on the antenna (many GNSS antennas have protrusions or marks for hooking a tape) to the installation point. For tripods, measure from the ground marker to the antenna underside and then add the offset from the antenna to the phase center. Commercial antennas list the offset to the phase center in catalogs, so include that value. For pole-mounted antennas, precisely measure the pole length and check with a bubble level that the pole is vertical. In all cases, millimeter-level errors affect vertical accuracy, so measure carefully.

Q: What should I do if RTK surveying does not produce good vertical accuracy (large errors)? A: First, check whether you have a fixed solution (FIX). Height accuracy cannot be guaranteed with a FLOAT solution. If FIX is not obtained, check satellite signal reception and the reception of correction information. If there are obstructions nearby, moving a short distance or mounting the antenna higher may help. Also check for misconfiguration of the base station (or NTRIP server) that might cause coordinate system or height datum offsets. Other measures include waiting for an improved satellite constellation, checking for antenna cable connection faults, and, if problems persist, resetting positioning (restarting the receiver or re-acquiring correction data) and attempting to acquire FIX again.

Q: What is LRTK? A: LRTK is a system (solution) for performing simple and high-precision GNSS surveying. It combines a dedicated high-precision GNSS receiver with a smartphone app to obtain centimeter-level position information in real time. LRTK greatly simplifies RTK setup and data processing that previously required specialized knowledge, making it easier not only for surveyors but also for construction supervisors and infrastructure inspection personnel to use. With LRTK, one person can efficiently perform field surveys and obtain the necessary elevation information with high precision.