Site Localization Guide: How to Align RTK to Project Coordinates

Table of Contents

• What is on-site localization?

• Differences between RTK positioning and project coordinates

• Localization procedures

• Points for improving localization accuracy

• Recommendation for simple surveying with LRTK

• Frequently Asked Questions (FAQ)

What is on-site localization?

"site localization" refers to the process of aligning positioning results obtained by GNSS (especially RTK) at a survey site with the project coordinate system used on that site. Put simply, it means converting the global coordinates obtained from satellites (worldwide latitude/longitude and height) into the local coordinate system used on construction sites and similar locations (plane rectangular coordinates or site-specific local coordinates), and correcting the offset between the two. In surveying instruments and software terminology it is also called "site calibration" or coordinate alignment, and it corrects GNSS coordinates using information from known points (points whose coordinates are known in advance).

By performing site localization, you can assign the same coordinate values to points obtained by RTK positioning as those on drawings and existing control points. This ensures that newly measured points are consistent with past survey data and design drawings, preventing positional shifts and discrepancies. Especially in infrastructure projects and civil engineering construction, observations must be aligned with reference coordinates established by national and municipal authorities (e.g., public coordinate systems) or project-specific prescribed coordinates, and localization is an essential process for accurate construction management.

Difference between RTK positioning and project coordinates

RTK positioning (high-precision GNSS surveying using Real Time Kinematic) is a technique that determines positions with an accuracy of a few centimeters (a few inches) by relative positioning between a base station (base) and a mobile station (rover). The measurement results of RTK are usually obtained as global positioning coordinates, that is, Earth-referenced coordinates. For example, typical GNSS surveying in Japan outputs latitude and longitude and ellipsoidal height (height in satellite positioning) based on the World Geodetic System (WGS84). Alternatively, latitude and longitude in JGD2011, Japan's geodetic system, may be obtained. Such global coordinate systems provide a unified representation across the entire Earth, but often differ in format from the coordinates used on-site.

On the other hand, project coordinates refer to the local coordinate system used at a construction site or within a project. In many cases, the plane rectangular coordinate system defined by the Geospatial Information Authority of Japan (the regional systems of JGD2011) or a site-specific arbitrary coordinate system is used. For example, public surveying in Japan uses a plane rectangular coordinate system that divides the country into several zones. For a site in Tokyo, you would specify a region-appropriate coordinate system such as “the plane rectangular coordinate system zone 9 of JGD2011.” In some projects, a coordinate system (local grid) with a custom origin and orientation adopted in the design drawings is used as the project coordinates.

Between such project coordinate systems and GNSS’s global coordinate systems, errors arise from planar position offsets and differences in vertical reference standards. For example, even if you project the latitude and longitude obtained by satellite positioning as-is, there can be discrepancies of tens of centimeters or more from the site’s planar coordinates. In the vertical direction as well, there are differences between the ellipsoidal height provided by GNSS and the heights used on site (such as geoid height or reference heights like Tokyo Bay mean sea level). For this reason, no matter how precisely you position with RTK, it is highly likely that the results will not match existing points on the drawings as-is. Therefore, by localization it is necessary to correct the offsets caused by coordinate system differences and unify the RTK positioning results with the local coordinate system.

Localization Procedure

On-site localization is generally performed by the following steps. RTK-capable surveying instruments and apps provide functions to execute this sequence in order to determine the coordinate transformation parameters.

• Preparation of known points: Select reference points, triangulation points, etc., located on or around the site for which the exact coordinate values are known (known points). You need points whose X, Y, Z coordinates in the project coordinate system are known. It is desirable to prepare at least three points, and preferably four to five or more. The more known points you have, the more stable the transformation accuracy becomes, and the easier it is to verify errors and eliminate outliers.

• Observe known points with RTK: Using an RTK rover receiver, measure the coordinates of each prepared known point. First set up a base station and start RTK positioning (the base station does not have to be a known point; if you use a network RTK service, a virtual reference station will be established). Place the rover on each known point and observe to obtain the GNSS positioning values for that location. Coordinates obtained directly from GNSS are basically in a global coordinate system (for example: WGS84 latitude/longitude and ellipsoidal height, or planar coordinates projected from those). For each measurement, remain stationary at the point for several to several tens of seconds and average multiple positioning results to stabilize accuracy. Record all obtained positioning coordinates so they can be compared later with the known point’s “true” coordinates.

• Calculate coordinate transformation parameters: For multiple known points, compare the “coordinates observed by GNSS” with the “correct coordinates of the known points” and compute the transformation parameters needed to make them coincide. Specifically, determine the planar two-dimensional translation amounts (ΔX, ΔY), rotation angle (θ), and, if necessary, scale factor (S). These are generally computed by the Helmert transformation (a simplified version of the 2D seven-parameter transformation). If you have only one known point, you can apply translation (shift) only; with two points you can do shift + rotation; with three or more points you can include shift, rotation, and scale in the correction. For the vertical direction, compute the difference between ellipsoidal height and the site’s orthometric height (geoid height difference) to obtain the vertical correction. Software that calculates the transformation parameters will also compute residuals (offsets) at each known point, so check those values.

• Apply the transformation parameters: Apply the calculated planar and vertical transformation parameters to the RTK surveying system. Specifically, go to the “coordinate transformation” or “localization” menu in the GNSS receiver or controller settings and input/register the parameters obtained in step 3. Many modern devices and apps will automatically perform parameter calculation through application if you input the known point true values and observed values. After applying the transformation, RTK positioning results will be converted in real time to the project coordinate system (site coordinates). In other words, all points subsequently acquired by the rover will be displayed and recorded in the same coordinate system as the known points.

• Validation of results: After applying localization, perform validation work as a precaution. Re-measure known points or additional checkpoints and confirm how closely the displayed coordinates match the known coordinates for those points. Also compare distances and bearings between known points to check for any differences before and after the transformation. If the corrected coordinates fall within the allowable tolerance, localization is successful. If deviations are large, re-investigate the sources of error (check for input mistakes, poor GNSS reception, etc.). If necessary, add other known points and recalculate, or exclude points with clearly large errors and recompute the parameters. If validation finds no issues, you can proceed with normal surveying tasks such as measuring unknown points and stakeout (guidance to design coordinates).

This is the basic localization workflow. In other words, it is the adjustment process for aligning GNSS global coordinates with the local coordinate system used on site. Once the coordinate systems are aligned, survey data within the same site can be handled according to a unified reference, making subsequent analysis and construction management smoother.

Points for Improving Localization Accuracy

When performing on-site localization, pay attention to the following points to obtain accurate results.

• Secure as many known points as possible: At a minimum three points, but it is safer to use four or more known points if possible. The greater the number of points, the more the influence of outliers is reduced and the more stable the transformation becomes. Public surveying standards also recommend using three or more known points.

• Arrange known points so they surround the site: Ideally, known points should be placed around the perimeter of the survey area in a balanced way. When reference points surround the entire site, corrections can be applied accurately anywhere within the area. Conversely, if known points are concentrated in one location, correction accuracy may decrease at distant locations. For large sites, place points appropriately for each area.

• Quality control for GNSS measurements: When observing known points with the rover, it is important to maintain good satellite reception. Measure in locations with clear sky visibility and observe in environments with minimal multipath (errors due to reflections) and radio interference. Taking multiple measurements at each point and averaging them, among other measures, is also effective in reducing measurement error.

• Also pay attention to vertical reference: In addition to plan localization, be sure to perform vertical correction. To convert GNSS ellipsoidal heights to site elevations, you can apply geoid height differences obtained using the Geospatial Information Authority’s geoid model (GSIGEO2011, etc.). Some surveying instruments can automatically apply geoid correction. If the vertical reference is not aligned, it will affect vertical construction accuracy, so be careful.

• Verify the coordinates of known points: Confirm in advance that the coordinate values of the known points you will use contain no errors. Coordinates transcribed from old drawings or past data may include errors due to differences in geodetic datums (such as confusion between the old datum and the World Geodetic System). As needed, obtain the latest control point results from municipal or surveying offices, or re-measure distances between known points in the field, so that you use reliable coordinates.

• Installation of the base station: Keep the base station (fixed station) in place as much as possible and operate it under the same position and conditions even after performing localization. If you have to move the base station or re-install it on another day, it is advisable to redo or check the localization each time. If possible, install the base station on a known point from the start, set that coordinate, and start RTK positioning; in theory this allows immediate positioning in site coordinates without localization (even in this case, the verification described below is necessary).

• Thoroughly verify the results: After applying the transformation parameters, always re-measure the known points and confirm they properly match. Even if some error remains, determine whether it is within an acceptable range for the work. Setting up independent check points allows objective accuracy verification. If you have any doubts, do not force the work to proceed; investigate the cause and reapply corrections as needed.

By following the points above, you can perform coordinate alignment via localization with high accuracy. Especially if you are attempting GNSS localization for the first time, don’t rush—carry out repeated validations and make sure to reliably achieve the required accuracy.

Recommendation for Simplified Surveying Using LRTK

In recent years, LRTK (a high-precision positioning system) has appeared as a tool that makes RTK surveying easier. LRTK combines a smartphone and a compact GNSS receiver to provide centimeter-level positioning comparable to traditional, expensive surveying instruments. It is used by mounting the smartphone and GNSS unit on a monopod (pole), allowing a single person to easily perform control point surveys and on-site coordinate checks. Because a dedicated app guides the procedures, it is also easy for beginners to use without specialized knowledge.

Field localization with LRTK is also extremely simple. On the dedicated app you select the regional coordinate system (e.g., "Plane Rectangular Coordinate System ● zone" or any local coordinate system), measure and register three or more known points, and the calculation and application of the coordinate transformation parameters are performed automatically. The error for each point (transformation residual) is displayed instantly, so you can verify accuracy on-site while completing the coordinate alignment. Whereas the calculations used to be cumbersome, with LRTK the app handles everything, so the user only needs to carry the device to the point to be measured and press a button.

Furthermore, LRTK includes functions for storing and sharing the acquired high-precision positioning data in the cloud and for loading drawing data to display it in AR on-site. By using localization to align data with site coordinates, you can, for example, display target points from design drawings in AR at the actual location and intuitively identify those spots. Tasks that previously required veteran surveying skills—such as staking and as-built verification—are becoming something anyone can perform with a smartphone in hand when using LRTK.

Aligning RTK to project coordinates is an important step in the digitization of surveying and construction. By using a simple surveying tool like LRTK, that step can be greatly simplified, contributing to improved efficiency and easing labor shortages. If you are facing challenges with site localization, why not consider trying a new surveying method using LRTK?

Frequently Asked Questions (FAQ)

Q. How many known points are required for localization? A. Basically, we recommend three or more points. With at least two points you can correct for planar shift and rotation, but with three points you can also perform scale correction for precise coordinate alignment. With only one point you can only correct for translation and cannot correct for orientation (angle) error. Public surveying standards also recommend using three or more known points, so secure three or more whenever possible. The more points you have, the more stable accuracy verification and error averaging become.

Q. What should I do if there is only one known point? A. If there is only a single point, you will apply only the planar shift (and height offset) at that point. Concretely, imagine adding the single-point difference to all coordinates to align them. However, in this case the rotational difference between the site coordinate system and the GNSS coordinate system is not corrected, so over a wide area a slight displacement in the east–west and north–south directions may occur. If you must align using one point in practice, it is advisable to choose a control point whose site axes are as nearly parallel as possible to the latitude/longitude axes of the World Geodetic System and to, if possible, set up one additional simple checkpoint for verification. Ideally you should localize with multiple points, so if possible obtain information on other known points as early as you can.

Q. How do you align heights (elevations)? A. For the vertical component, the relevant task is converting GNSS ellipsoidal height to the site’s orthometric height (orthometric height or elevation above sea level). Typically, within Japan you derive the geoid height for the area from a geoid model (e.g., GSIGEO2011) and obtain the orthometric height by subtracting the geoid height from the observed ellipsoidal height. Many GNSS surveying instruments and apps will apply the geoid correction automatically and display orthometric height when you preselect the region. When performing localization you also calculate the difference from the known point’s orthometric height and apply a vertical offset. As a precaution, on sites with significant terrain relief it is advisable to verify heights at multiple locations to ensure there are no errors due to local geoid variations.

Q. Is localization necessary when the base station is set on a known point? A. If the base station (reference station) is placed beforehand on a known point whose coordinates are known in the project coordinate system, and you enter those coordinate values into the controller and start RTK positioning, the coordinates obtained by the rover will be close to the site coordinate system from the outset. In this case, large coordinate shifts will not occur, so you can reduce the effort required for localization. However, depending on the equipment or positioning service, small offsets may occur, so it is desirable to perform a verification survey at another known point as a precaution. On the other hand, for network RTK (virtual reference point), since the reference station is set automatically, you should generally consider localization necessary.

Q. If you set the site's coordinate system directly on a GNSS receiver, is localization unnecessary? A. Modern GNSS receivers and surveying apps include a feature that lets you select the local plane rectangular coordinate system in advance and display positioning results directly in that system. If used correctly, this feature can produce observations that are very close to the site’s coordinate values. However, you should be aware of the initial positioning conditions and small discrepancies with known points. For example, if the reference station’s initial coordinates contain a slight error, that error will be reflected in the displayed coordinates. Therefore, even when operating equipment with the site’s coordinate system selected, it is prudent to check important control points against their on‑site coordinates and, if necessary, perform fine adjustments via localization. In short, the field rule is: “don’t rely entirely on automatic conversion—always check against known points at the end.”