Using Benchmarks with RTK: Procedures to Correctly Tie to Existing Control Points

Table of Contents

• The importance of RTK positioning and existing control points

• What is a benchmark (existing control point)?

• Benefits of using existing control points in RTK surveying

• Preparations for correct tying - Confirm coordinates and geodetic datum of existing control points - Configure RTK base and rover

• Survey procedures to tie to existing control points - Step 1: Selection and preparation of known points - Step 2: Observe known points with RTK and compare - Step 3: Apply coordinate corrections (localize) - Step 4: Survey and record new points - Step 5: Accuracy check and adjustment

• Notes when tying to existing control points - Beware of shifts due to different coordinate systems - Consider vertical datum (geoid)

• Recommendation for simple surveying with LRTK

• Frequently Asked Questions (FAQ)

The importance of RTK positioning and existing control points

RTK surveying is a revolutionary method that provides centimeter-level (half-inch accuracy) high-precision positions in real time. However, no matter how accurately RTK positions are obtained, if those coordinates do not match the surrounding reference system they are of limited practical use. In the field you must align measurement results with previous survey results and the coordinate systems used in design drawings. Therefore, tying to existing control points (reference points with already-known accurate coordinates) is important. Using raw RTK results (latitude/longitude, etc.) as-is may not align with existing drawings or other reference points, so it is essential to follow proper procedures to tie to existing points and unify coordinates.

What is a benchmark (existing control point)?

A benchmark (existing control point) is a reference point established during past public surveys or construction. Names vary—triangulation stations, leveling benchmarks, construction control points—but all are points whose accurate coordinate values (X, Y, Z or latitude, longitude, elevation) are known. Examples include triangulation stations and CORS installed by the Geospatial Information Authority of Japan (GSI), construction control points established by municipal or private entities, and known points associated with boundary stakes. A benchmark serves as a coordinate anchor in surveying; tying new survey results to these existing points maintains consistency in the coordinate system. For example, in railway or road maintenance, mileage markers or existing control stakes function as benchmarks and bridge old and new data.

Benefits of using existing control points in RTK surveying

Tying RTK surveys to existing control points offers the following benefits:

• Consistency with existing data: New survey points will match the coordinate system of past survey results and design drawings, enabling smooth data integration. Survey results can be overlaid immediately on existing CAD drawings or GIS data without extra transformations.

• Survey efficiency: You can compare and correct results against existing points on site, avoiding later coordinate transformations. If you can measure directly in a public coordinate system (e.g., Japan’s JGD2011 plane rectangular coordinate system) from the start, electronic deliverables and reporting proceed smoothly.

• Assurance of accuracy and reliability: Using known points as references allows on-site checks of positioning accuracy. You can verify how well RTK results align with existing references and quickly detect errors due to device settings or datum mismatches. If discrepancies with existing points are within acceptable ranges, the measurement results can be considered reliable.

• Future-proofing as an asset: Recording data in absolute coordinates ensures the reference remains stable for future renovations or other projects. For example, if crack locations captured during inspections are recorded relative to existing control points, their positions can be accurately reproduced in other drawings or systems even after many years.

Thus, to fully leverage RTK’s high precision, tying to benchmarks—existing control points—is essential.

Preparations for correct tying

Proper preparations before starting surveying are indispensable to correctly tie RTK positioning results to existing control points. Specifically, confirming the coordinate system and device settings are key. Below are the items to check as pre-survey preparations.

Confirm coordinates and geodetic datum of existing control points

First, confirm the coordinate values and geodetic datum of the existing control points you will use. Obtain the official coordinates from control point descriptions or construction documents. In Japan, many coordinates are based on JGD2011 (Japanese Geodetic Datum 2011). If coordinates are given in a plane rectangular coordinate system, check which zone they belong to; if given as latitude and longitude, plan to convert them to a plane coordinate system later. Also confirm whether elevation values exist. Leveling benchmarks will have orthometric heights, but GNSS-derived heights are ellipsoidal heights, so you must consider the difference (geoid height). Key points:

• Understand the reference coordinate system: Verify the coordinate system of the existing control points (e.g., WGS84, a particular JGD2011 zone) and match the RTK output settings.

• Obtain official coordinate values: For triangulation stations, consult GSI results; for construction control points, check design documents. Record X, Y, Z (or latitude, longitude, elevation) accurately.

• Check vertical datum: If elevations are provided, verify whether the GNSS receiver can apply a geoid model (e.g., GSIGEO2011). If it cannot, at least know the geoid height to subtract later.

Configure RTK base and rover

Next, configure RTK equipment to align with the use of existing points. RTK is broadly divided into the method of setting up your own base station or using network RTK (leveraging existing base station networks). In either case, coordinate settings are critical.

• Using your own base station: Set another GNSS receiver on a known point and operate it in base station mode. Enter the exact coordinate values of the existing control point into the base station (latitude/longitude and ellipsoidal height, or plane rectangular coordinates). If you enter the base station coordinates incorrectly, all measurements will be shifted by that error, so input carefully. Also prepare the communication method for delivering corrections to the rover (Wi-Fi, low-power radio, etc.) and verify proper connectivity.

• Using network RTK: Arrange a subscription to a public or private RTK correction service (VRS, etc.) and configure the rover as an Ntrip client. Enter the Ntrip server information provided by the service (URL, port, mountpoint, login ID/PW) so that correction data can be obtained. Confirm that the service’s coordinate system matches that of your existing points. Many network RTK services in Japan (for example, GSI’s CORS network) distribute coordinates in JGD2011. If your receiver or app can output plane rectangular coordinates, set it to the corresponding zone.

• Positioning mode and quality check: After connecting and configuring devices, start satellite tracking and check the RTK solution status. If corrections are applied successfully, the solution should transition from Float to Fix. A Fix solution indicates integer ambiguity resolution and centimeter-level accuracy. Always ensure you have a Fix before starting work, and verify accuracy using known points (see steps below).

With these preparations, the RTK system is set up to operate on the same coordinate basis as existing control points. Next, we’ll go through the actual procedures to tie to existing points.

Survey procedures to tie to existing control points

Below are step-by-step procedures to correctly tie RTK survey results to existing control points. The description assumes network RTK, but the general flow also applies when using your own base station.

Step 1: Selection and preparation of known points

Select available known points (control points) at the site. Ideally, have multiple known points (preferably three or more) near the site. Public survey standards recommend integrating coordinates using three or more known points, although smaller sites may only have one or two. You can proceed with a single known point, but securing multiple existing points increases reliability.

Locate the physical markers or stakes of the selected known points and make sure they are accessible for measurement. Some points may require vegetation clearing or uncovering covers. If the marker has a benchmark plate or reference mark, take care to set the pole or antenna directly over that mark. As a rule, the more evenly distributed the known points are around the survey area—ideally surrounding it—the more stable the subsequent correction calculations will be.

Step 2: Observe known points with RTK and compare

Place the rover (RTK antenna) on each prepared known point and observe. With network RTK, the rover can obtain positioning without a local base because virtual base information is provided. At each known point, remain stationary until an RTK Fix is obtained and record the measured values. Many RTK apps and devices let you save measured coordinates on site.

Compare the coordinates obtained by RTK at each known point with the official known coordinate values you recorded. For example, compute the differences between the measured (X, Y, elevation) values for Triangulation Point A and the published coordinates for Point A. If the differences are very small, the system is likely operating correctly and the coordinate systems match. As a general guideline, with reference points about 2 km (6,562 ft) away, horizontal errors of 1–2 cm (0.4–0.8 in) and vertical errors of 2–3 cm (0.8–1.2 in) are considered acceptable (based on public survey standards). Differences within this range can be judged as within RTK precision.

If differences are larger, be cautious. Possible causes include device setting errors (incorrect base station coordinates, wrong datum) or poor measurement conditions (signal blockage or multipath). First, immediately check settings and, if necessary, re-measure. If multiple known points were measured, analyze the pattern of discrepancies. If all points show a similar shift in the same direction, a datum mismatch is likely. If errors vary point by point, random factors like signal conditions may be responsible.

Step 3: Apply coordinate corrections (localize)

If a systematic offset is identified from comparing known points, perform coordinate correction—commonly called localize or site calibration—to align the GNSS-derived coordinates with the local coordinate system.

When multiple known points have been observed, localization calculations can derive two-dimensional translation (shift), rotation angle, and scale. A common method is the Helmert transformation, which computes transformation parameters that minimize differences between measured GNSS coordinates and the official coordinates of known points. With three or more points you can correct shift, rotation, and scale comprehensively; with two points you can usually correct shift and rotation; with only one point you can apply a simple shift (and vertical offset).

How you perform localization depends on your device or software. Many surveying software packages and modern smartphone apps automatically compute and apply correction parameters when you input the observed coordinates and their true values. For example, the LRTK app lets you register multiple known points and perform a one-touch coordinate correction (“apply coordinates”). If you must do it manually, you can compute average ΔX, ΔY (and ΔZ) and apply a parallel shift as a simple correction.

After this step, coordinates obtained subsequently by the RTK system will be treated as corrected values in the same coordinate system as the existing control points. In other words, you will have aligned the measured coordinates with the site’s coordinate reference.

Step 4: Survey and record new points

Once localization is applied, proceed to survey your new target points. As in regular RTK surveying, set the rover over each point to be measured and record coordinates. With localization applied, the output coordinates are on the same coordinate system as the existing control points, allowing immediate comparison with design drawings and plotting on GIS maps.

When recording points, also note identifiers, names, and measurement times for later processing. Many RTK apps automatically save timestamps and quality indicators (Fix/Float, etc.). For important points, assign clear names and take photos so records are well documented.

During new point measurements, continue to monitor that RTK remains in Fix. If a measurement drops to Float temporarily, re-measure that point. Even with localization applied, the measurements themselves must remain precise—continue to monitor satellite reception and signal environment.

Step 5: Accuracy check and adjustment

After completing new point measurements, perform a final accuracy check. Specifically, return to known points and measure them again to verify how closely the localized coordinates match the official values. This is a repeat of Step 2 and demonstrates whether the corrections functioned properly.

If you used multiple known points, comparing the known inter-point distances and the measured distances is also useful. Checking how distances between adjacent control points change before and after localization helps determine whether scale errors were properly corrected.

If discrepancies of several centimeters remain, reconsider the correction calculations. If additional known points can be obtained, measure them and recompute transformation parameters, or exclude outlier points that may have been measured incorrectly and recalculate. If issues persist, consider surveying at a different time, as environmental factors (multipath, atmospheric effects) may be causing errors.

Finally, once consistency with existing control points is confirmed, finalize the survey results. For quality control, it is advisable to check a known point at both the start and end of the survey to document any drift. If an error is found late in the process, you can use the start-of-day record to assess and adjust correction parameters. Incorporating such start-end checks improves data reliability.

Notes when tying to existing control points

You can tie RTK surveys to existing control points following the above procedures, but be aware of several cautionary points—especially differences in coordinate systems and vertical datum—which are easy to overlook in the field.

Beware of shifts due to different coordinate systems

If the geodetic datum or coordinate system does not match, measurement results can shift by tens of centimeters or, in extreme cases, hundreds of meters. Japan standardized on geodetic datums (JGD2000/JGD2011) after 2002, but older drawings may use the Tokyo datum or other legacy datums, so be careful. Always confirm that the coordinate system of the existing control points matches the coordinate system used for RTK positioning. For example, network RTK typically generates virtual reference positions based on JGD2011, but local construction control points may use a custom coordinate system; localization is essential to correct that discrepancy.

Even within JGD2011, selecting the wrong plane rectangular zone number can introduce errors, especially near prefectural borders. Make sure the receiver or app’s output coordinate zone is set correctly. Datum mismatches typically appear as a consistent shift or a rotated offset across all measured points. If you observe such behavior, promptly review settings and, if necessary, recalibrate using known points.

Consider vertical datum (geoid)

RTK-derived heights are ellipsoidal heights, whereas elevations used in maps and construction (height above sea level) are orthometric heights based on the geoid. The difference between them varies regionally and is on the order of tens of meters in Japan. To convert RTK results to orthometric height, subtract the geoid height from the ellipsoidal height.

Enable a geoid model in the GNSS receiver or app (e.g., choose “GSIGEO2011”) so that heights output by the device are directly usable as elevations. If your equipment does not support geoid correction, perform post-processing by subtracting the local geoid height from the ellipsoidal heights.

Additionally, if you are using a leveling benchmark (BM) for vertical control, you can compare the ellipsoidal height measured by RTK on that BM with the known orthometric height to derive a single height offset. Applying that offset to all measured points is a simple correction that can yield acceptable accuracy for nearby areas (but note that geoid undulation varies over large areas). The important point is to understand and apply necessary corrections for vertical alignment with existing points.

By accounting for coordinate system and height datum issues, you can obtain RTK survey results that are consistent with existing control points. Finally, we introduce a tool that makes such high-precision surveying easier.

Recommendation for simple surveying with LRTK

Reading through the procedures to tie RTK to existing control points, tasks like verifying datums and performing corrections can seem complex. Consider using a tool designed to let beginners perform these advanced processes easily: LRTK. LRTK is a solution consisting of a compact RTK-GNSS receiver and a smartphone app developed by Refexia Co., Ltd., designed specifically for simple surveying.

With LRTK, you can attach a pocket-sized receiver to your smartphone and start surveying. Complex device setups are guided by an intuitive UI in the app, and tying to known points (localization) is performed by following on-screen instructions. For example, entering the true coordinates of measured control points and enabling “coordinate correction” will automatically transform subsequent positioning data to match those control points without requiring the user to perform specialized calculations.

Moreover, by using the LRTK app and cloud services, you can instantly share coordinate data, point clouds, and photos captured in the field with the office. Uploading data to the cloud on the spot allows remote colleagues or clients to review results in real time. This workflow efficiency is one of LRTK’s strengths, supporting not only measurement but also downstream data utilization and on-site digital transformation.

In short, adopting LRTK for simple surveying makes advanced RTK workflows—tying to existing points and achieving high precision—accessible to anyone. Even without expensive surveying instruments or extensive expertise, you can perform high-accuracy surveys using a smartphone and a compact device. If you feel RTK is intimidating, start with simple surveying using LRTK and experience improved efficiency and accuracy in the field.

Frequently Asked Questions (FAQ)

Q. Why is it necessary to tie RTK surveying to existing control points? A. Tying to existing control points aligns new survey data with the coordinate systems of past survey results and design drawings. Using raw RTK data as-is may result in discrepancies with existing drawing coordinates, but aligning to benchmark control points facilitates smooth data integration. Comparing with existing points also allows verification of positioning accuracy, increasing confidence in the measurement results.

Q. Where can I obtain coordinate values for usable existing control points? A. For triangulation stations and CORS established by the Geospatial Information Authority of Japan, you can obtain coordinates (latitude/longitude or plane rectangular coordinates) from GSI’s website or published results. Construction control points are listed in design documents or provided by the client. Known points associated with boundary stakes may be held by land surveyors or survey companies as part of their survey results. In any case, use officially approved accurate coordinate values.

Q. What if there are no known control points near the site? A. If no existing points are available, you can establish a temporary control point yourself. For example, perform averaged RTK positioning for several minutes at a well-exposed location and use that point as a provisional control point. The LRTK app provides features to average multiple observations to obtain a high-accuracy coordinate. Treat such a point as a “temporary benchmark” for subsequent surveying. Note that such coordinates may deviate slightly from official references, so where possible verify later against a distant known point (e.g., a triangulation station). In some cases, PPP (precise point positioning) can determine absolute coordinates without nearby benchmarks.

Q. If RTK measurements differ from a known point by more than 5 cm, what might be the problem? A. First consider coordinate system or configuration mismatches. For example, an incorrect coordinate entered for the base station or a confusion between global and local datums can cause such discrepancies. A uniform shift suggests this cause. Poor satellite reception or other environmental factors might also increase errors. First recheck device settings (base coordinates, coordinate system, geoid correction). If settings are correct, inspect the surroundings for reflective surfaces or obstructions and, if necessary, re-measure at a different time. If the issue persists, perform site calibration using multiple known points to determine shift corrections.

Q. Does network RTK automatically provide values in a public coordinate system? A. Basically, yes. Network RTK (Ntrip/VRS) uses data from multiple reference stations with known coordinates (such as GSI’s CORS network), and positioning results are generally provided in Japan’s official geodetic datum (JGD2011). Therefore, in theory rover measurements are already in a public coordinate system. However, receiver-side settings or slight regional differences can still produce centimeter-level offsets. Thus, even when using network RTK, it is recommended to verify results at a nearby known point. Don’t accept automatically obtained coordinates uncritically—confirm at least once with a control point.

Q. How accurate are heights (elevations) from RTK surveying? A. Vertical errors are generally larger than horizontal ones and are typically on the order of a few centimeters to around 10 cm. This is because ionospheric and tropospheric effects cannot always be fully eliminated, but with proper procedures you can achieve practically usable accuracy. It is important to correctly convert ellipsoidal heights to orthometric heights using a geoid model. Even if RTK ellipsoidal heights contain, for example, 10 cm of error, correcting against a known leveling benchmark preserves relative elevation relationships. Height accuracy can also be improved through longer static observations; for example, LRTK’s averaged positioning over a set period enhances accuracy.

Q. Do I need to perform localization (coordinate correction) for every site? A. If the site’s control points and coordinate system are the same each time, you do not necessarily have to recompute from scratch every time. However, we strongly recommend always performing an initial verification measurement on a known point before starting RTK surveying. Even with the same settings and region, device condition and satellite geometry can cause small errors. Performing a known-point check and applying localization if necessary ensures consistency for each survey. Once localization is done, it can be reused within the same site, but you should redo it when moving to a different site with a different control system. From a quality perspective, make it a habit to verify at known points rather than assuming—this practice maintains data integrity.