How to Set Up On‑Site Calibration (Localization) with RTK

Table of Contents

• Introduction

• What is RTK

• What on‑site calibration (localization) is

• Procedure for on‑site calibration

• Network RTK and smartphone RTK

• Simple surveying with LRTK

• FAQ

Introduction

In recent years, the use of GNSS positioning technology has rapidly expanded in construction and civil engineering, and among these methods, RTK (Real Time Kinematic) has attracted attention as a technique that achieves centimeter‑level high‑precision positioning. While conventional GPS can produce errors of several meters, using RTK can reduce errors to the order of several centimeters. This improved accuracy dramatically enhances the efficiency and precision of surveying and construction management, and RTK‑GNSS is being used across a wide range of sites including infrastructure works, land surveying, drone measurements, and agriculture.

However, no matter how precisely RTK measures positions, if on‑site calibration (localization) is not performed correctly, the positioning results may have slight offsets relative to existing control points or the coordinate system used in design drawings. For example, when a public coordinate system defined by national or local governments (in Japan, the plane rectangular coordinate system based on JGD2011, etc.) or a locally defined coordinate system from past works is used on site, using RTK measurements (coordinates in latitude/longitude or ellipsoidal height) as‑is may not match existing drawings or control points. To solve this problem, it is essential to apply on‑site calibration, which corrects RTK positioning results based on known points (control points). Also called localization, this process aligns the position information obtained by RTK with the site’s coordinate system so that it is highly consistent with past survey results and design data.

This article explains from the basics, in a careful manner for beginners to intermediate practitioners in surveying and civil engineering, how to set up on‑site calibration (localization) with RTK. We first review the basics of RTK and the overview of on‑site calibration, then explain concrete procedures and key points step by step. We also touch on surveying methods using the increasingly widespread network RTK and smartphone RTK, and at the end introduce a simple surveying solution using the latest high‑precision positioning system LRTK. If you are considering introducing RTK surveying or improving accuracy, please refer to this guide.

What is RTK

RTK stands for Real Time Kinematic and is a technology that obtains high‑accuracy positions in real time by performing relative positioning using two or more GNSS receivers. One is designated as the base station and the other as the rover; using the differential data of satellite signals received simultaneously by both receivers, error components are canceled out. Because common errors from satellite orbit inaccuracies, receiver clocks, and atmospheric effects are eliminated by differential processing, errors that would be several meters in standalone positioning are reduced by RTK to the order of several centimeters horizontally and several to a dozen or so centimeters vertically.

This centimeter‑level accuracy allows RTK to achieve positioning precision comparable to total stations while providing the advantage of GNSS positioning even where line‑of‑sight cannot be maintained. For example, RTK‑GNSS is effective for large‑area topographic surveys with poor visibility and for as‑built (volume) management; since results are obtained in real time, measurement values can be checked immediately and reflected in construction on the spot. For these reasons, RTK is becoming an indispensable positioning technology in civil engineering and construction.

What on‑site calibration (localization) is

On‑site calibration (localization) is a coordinate correction method that aligns global coordinates obtained by RTK (latitude/longitude and ellipsoidal height) with the local coordinate system used on site. Survey instruments and software may call this function “site calibration,” but the meaning is the same. Specifically, known points (control points whose accurate coordinate values are already known) on site are observed with RTK, and the differences between the RTK‑measured coordinate values and the true coordinate values of those control points are calculated. Based on the differences measured at multiple control points, transformation parameters such as horizontal shifts (translations), rotation angles, and, if necessary, scale factors are computed.

By applying the calculated parameters to the RTK system, all subsequent points obtained by RTK will be output and recorded as coordinates in the site’s local coordinate system. Simply put, on‑site calibration is the process of “aligning GNSS‑obtained coordinates with the local coordinate system used on site.” For example, when using a public coordinate system in Japan, the RTK positions obtained in latitude/longitude can be transformed into the plane rectangular coordinate system, and heights can be corrected from ellipsoidal height to geoid/orthometric height, thereby producing coordinates that can be directly compared and integrated with conventional survey drawings and existing control points.

The reason this calibration is necessary is that many surveying and design tasks require consistency with existing drawings and control point coordinate systems. No matter how accurately RTK measures, coordinates left in a global geodetic system (latitude/longitude) may not match local XY coordinates used on site. In civil works particularly, it is important to eliminate discrepancies with control points provided by the client in the public coordinate system or local control points set in previous works. Performing on‑site calibration removes offsets between newly acquired point cloud data and existing data, eliminating the need for coordinate alignment in post‑processing.

Procedure for on‑site calibration

Now let’s look at the actual procedure for performing on‑site calibration (localization) using RTK. The general flow is as follows.

• Known point preparation: Prepare multiple control points on or near the site whose accurate coordinate values (X, Y, Z in a public coordinate system, etc.) are already known. It is desirable to secure at least one point, and preferably three or more control points. The more known points available, the more stable and reliable the transformation parameter estimates will be.

• RTK observation of known points: Use the RTK receiver (rover) to observe the coordinates of each known point. The positioning values obtained from GNSS are typically latitude/longitude and ellipsoidal height based on a global datum such as WGS84, but depending on the receiver or software settings, values converted in real time to a plane rectangular coordinate system may be obtainable. In any case, at this stage compare the “RTK‑measured coordinate values” with the “true coordinate values (reference values)” for each known point. For heights, if necessary, convert ellipsoidal heights to orthometric heights using a geoid model.

• Calculation of transformation parameters: From the differences obtained at multiple points (offsets between known point coordinates and measured coordinates), calculate transformation parameters such as 2D translations (ΔX, ΔY), rotation angle θ, and scale factor S. Typically, methods such as the Helmert transformation (2D seven‑parameter coordinate transformation) are used to compute optimal parameters that minimize offsets across all points. If only one known point is available, only horizontal translation can be corrected; with two points, translation plus rotation can be corrected. With three or more points, a comprehensive coordinate transformation including scale correction can be performed, so it is ideal to compute using three or more points. For height differences, an average offset can be calculated from multiple known heights or official geoid conversion values can be used for correction.

• Application of parameters: Register and apply the calculated transformation parameters to the RTK system (receiver or controller software). After this, all points measured by RTK will be automatically output as coordinates in the local coordinate system with the transformation applied. For example, by entering and enabling shift and rotation values in a field controller’s “site calibration settings,” newly measured points will be recorded in the same coordinate system as the design drawings. If the initial settings are correct, site survey outputs can be used directly for electronic deliverables without the need for post‑processing coordinate alignment.

• Accuracy check (verification): After applying the calibration, remeasure other control points or known points on site to verify how closely the transformed coordinates match the known values. For example, compare distances between multiple known points or observe an additional known point to check errors. If discrepancies exceeding the allowable range are found, exclude outlier points and recompute, or add other known points as needed and perform calibration again. Such verification ensures confidence for subsequent main surveying tasks.

These are the basic steps for on‑site calibration in RTK positioning. Key points are to prepare points with accurate reference values in advance, configure correct coordinate system settings on the equipment (datum, plane zone, geoid correction, etc.), and verify offsets with a sufficient number of points to obtain stable transformation parameters. By following these practices, survey data obtained on site can be integrated with existing coordinate systems with high accuracy, greatly simplifying subsequent data processing and quality control.

Network RTK and smartphone RTK

When operating RTK surveying, there are broadly two approaches: installing your own base station on site, or using network RTK to receive correction data via the internet (using a reference station service). Traditionally, it was common to place an expensive GNSS base station from a surveying equipment manufacturer on site, but in recent years the development of reference station networks provided by private and public entities (such as Virtual Reference Station, VRS) has advanced, and network RTK has become widespread, allowing real‑time corrections without placing a base station on site. By using network RTK, centimeter‑level positioning is possible simply by obtaining correction information distributed via mobile communication from a service (Ntrip caster). For example, you can contract with the Geospatial Information Authority of Japan’s Continuously Operating Reference Station (CORS) network or high‑precision positioning services offered by telecommunications carriers, and connect the site rover to those services. In network RTK, the reference station network is typically operated in a predefined public coordinate system (for example a specific plane rectangular coordinate zone of JGD2011), so obtained position information will also be provided in that coordinate system. However, because the actual site may not use the same coordinate system, local fine adjustments (on‑site calibration) using site known points are often still necessary.

On the other hand, an approach gaining attention for making RTK easier to use is the so‑called smartphone RTK. Smartphone RTK refers to surveying methods that use a small GNSS receiver paired with a smartphone or tablet via Bluetooth. By attaching a pocket‑sized RTK‑capable receiver released in recent years to a smartphone, you can use your smartphone as a high‑precision positioning terminal without the conventional fixed surveying equipment. In this method, the smartphone serves as the user interface and communication device, while the compact receiver performs high‑sensitivity positioning. On site, the smartphone’s mobile communication can connect to network RTK correction services without special radio equipment, allowing you to start centimeter‑level surveying conveniently without carrying heavy tripods or large batteries.

The advantages of smartphone RTK solutions are low cost and ease of use. Compared to dedicated surveying instruments, the initial introduction barrier is lower, enabling field personnel to conduct RTK surveying without complex operations or advanced expertise. Many smartphone apps support coordinate system settings and calibration procedures, and some products allow completion of on‑site calibration simply by following guided steps in the app. In other words, even non‑RTK specialists can easily achieve high‑precision positioning and coordinate alignment on site.

Simple surveying with LRTK

One notable solution for enabling such simple surveying is LRTK. LRTK is a modern surveying system composed of an ultra‑compact RTK‑GNSS receiver that can be attached to a smartphone and a dedicated app. By Bluetooth‑connecting a compact device equipped with an internal battery and high‑performance antenna to a smartphone, the smartphone instantly becomes a centimeter‑accuracy surveying instrument. LRTK generally operates using network RTK; by connecting through the smartphone’s mobile communication to the Geospatial Information Authority’s electronic reference station data or commercial correction services (Ntrip distribution), it obtains a real‑time FIX solution (high‑precision solution). If you select the regional coordinate system in the dedicated LRTK app (for example: a specific plane rectangular coordinate zone or a geoid model), the positioned coordinates are automatically converted into that coordinate system and displayed and recorded in the site’s reference system.

The on‑site calibration procedure is also very simple with LRTK. Upon arrival at the site, first select the coordinate system to be used in the app (for example “JGD2011 plane rectangular coordinate zone X” and “geoid model JNGeoid2020”). Next, set the LRTK receiver over a known point on site, measure it, and input the known coordinate values into the app to register the point. By similarly measuring and registering multiple known points, the app automatically calculates shift amounts and applies the calibration (with one point, height offset and horizontal shift are applied; with two or more points, a more accurate transformation is executed). Once localization is configured, all subsequent points obtained with LRTK will be recorded in the site coordinate system. For example, on one civil engineering site, registering 2–3 control points with LRTK beforehand allowed all measurement points acquired for as‑built management to be unified in a public coordinate system and submitted directly as electronic deliverables, significantly reducing post‑processing coordinate transformation work. LRTK thus enables a new surveying workflow in which site supervisors and construction managers without specialized high‑precision positioning knowledge can quickly measure and immediately use the data themselves.

LRTK, which realizes high‑precision RTK positioning and on‑site calibration with simple operations, can be said to support the “democratization of surveying.” Even amid a shortage of veteran surveyors, introducing LRTK allows users to measure when needed, share results immediately, and move on to the next task, enabling smart site operations. For those who want to lower the RTK introduction barrier or perform quick surveys on small sites, LRTK is a powerful option.

FAQ

Q: How accurate is RTK positioning? A: Using RTK, horizontal positions are generally accurate to the order of several centimeters (under good conditions about 1-2 cm (0.4-0.8 in)). Vertical accuracy is also typically within several centimeters to less than 10 cm (3.9 in). This is a dramatic improvement over standalone GPS positioning (errors of approximately 5-10 m (16.4-32.8 ft)), and is sufficiently applicable to tasks that require centimeter‑level accuracy such as boundary surveys and as‑built management.

Q: What happens if I omit on‑site calibration? A: If you perform RTK surveying without calibration, the obtained coordinates may be offset from the site’s design coordinate system or existing control points. For example, a few centimeters of discrepancy between coordinates on the drawings and RTK‑measured coordinates can lead to errors during stakeout or as‑built checks. In particular, height (elevation) can differ by several tens of centimeters due to geoid differences, so without calibration or geoid correction, practical problems may occur. Therefore, on‑site calibration is generally indispensable to fully utilize RTK’s high accuracy.

Q: How many known points are required for on‑site calibration? A: Ideally, use three or more known points. With three points you can perform a precise coordinate transformation including shift, rotation, and scale, minimizing residual errors. With two points you can correct for shift and rotation, but cannot correct scale, so slight scale errors may remain over a wide area. With only one point you can correct horizontal shift (and height offset), but orientation and scale are not guaranteed. In reality, the number of available known points may be limited by site conditions, but it is recommended to secure multiple points and perform verification where possible.

Q: What if there are no known points on site? A: If absolutely no known points exist, one option is to survey using a coordinate system based on public control points. For example, by connecting to a network RTK service referencing the Geospatial Information Authority of Japan’s CORS network, you can directly obtain coordinates on the public coordinate system. Later, you can install control points on site and perform calibration for correction. For short‑term work, you can set temporary control points on site and assign arbitrary coordinates (a temporary coordinate system) and proceed with RTK surveying; however, this will not be consistent with other sites or existing records, so final conversion to an official datum will be necessary. In any case, it is important to perform control point surveying or calibration later and apply coordinate corrections to the data.

Q: If I use network RTK, is calibration unnecessary? A: Not necessarily. Network RTK reference station networks are typically operated in a unified coordinate system (usually a public coordinate system), and in theory if the site uses that same coordinate system you may be able to use the positioning results without calibration. However, many sites use locally adjusted coordinate systems from previous works, in which case fine adjustments are still needed. Even when using network RTK, it is advisable to measure at least one known point on site and compare with existing coordinates; if discrepancies exist, perform calibration. Also, vertical coordinates can be offset if geoid correction in the network RTK is not configured correctly, so prior verification is recommended.

Q: Can a smartphone paired with a small GNSS receiver really achieve high accuracy? A: Yes. Modern small GNSS receivers have performance comparable to surveying‑grade equipment and can use multiple satellite constellations such as GPS, GLONASS, Galileo, and QZSS. With L1/L2 dual‑frequency support enabling ionospheric error mitigation, they can achieve RTK positioning accuracy comparable to conventional fixed equipment (centimeter level). The smartphone mainly handles display, communication, and recording, so positioning accuracy depends on the performance of the connected receiver. Under correct settings and good satellite reception conditions, smartphone‑paired RTK can provide sufficiently high‑precision positioning. In practice, smartphone‑paired systems such as LRTK have demonstrated positioning accuracy approaching that of total stations on many sites. To ensure accuracy, operate in locations with good satellite visibility and perform calibration as needed.