Table of contents

• Introduction

• Basics of RTK positioning and datums

• Differences between the World Geodetic System (WGS84) and local datums

• 1. Base station coordinate datum setting errors – large position offsets

• 2. Forgetting to convert to the local datum – risk of data mismatch

• 3. Confusing feet and meters – abnormal scale offsets

• Simple surveying enabled by LRTK

• For details about LRTK, see the links below.

• FAQ

Introduction

RTK (Real Time Kinematic) surveying is a method that applies real-time corrections to GNSS satellite positioning errors to achieve centimeter-level high accuracy (cm level accuracy (half-inch accuracy)). Standalone GPS positioning normally yields errors of several meters, but with RTK you can suppress on-site errors to about 1–2 cm (0.4–0.8 in), making it widely used in civil engineering and surveying. However, it is important to note that even RTK’s high-precision positioning will produce large errors in deliverables if the datum used is mismatched. In practice, when performing RTK surveying overseas (especially in the United States), failures due to differences in geodetic references that do not occur in Japan are common. This article focuses on the problem of “position offsets caused by differences between RTK and datums” and explains in detail common mistakes that occur in the U.S. and how to counter them. It is intended for those involved in international projects or surveying work overseas, and also for anyone domestically who feels unsure about handling reference coordinate systems.

Basics of RTK positioning and datums

First, let’s cover the basics of RTK positioning. RTK, short for Real Time Kinematic, is a technique that obtains highly accurate positions by performing differential comparisons of GNSS data between a reference station (base station) installed at a point with known coordinates and a rover at the point to be observed, and applying real-time corrections to positioning errors. The base station is installed at a point whose accurate coordinates are known beforehand, and the rover receives the correction information and applies it to its own positioning. With this mechanism, positions that would be off by several meters in standalone positioning can be reduced to within a few centimeters with RTK.

On the other hand, a geodetic datum is a reference framework for expressing positions on the Earth. Simply put, it determines “which reference the coordinates are defined on,” and if the reference ellipsoid or origin differs, the numeric latitude/longitude or coordinates for the same point will change. For example, in Japan before 2002 the old Tokyo Datum was used, and the difference from the current world geodetic system around Tokyo was about 450 m (1,476.4 ft) to the northwest. If coordinates defined in different datums are compared as-is, positions on a map can diverge by hundreds of meters. Fortunately, Japan is now unified under the Japanese Geodetic Datum 2011 (JGD2011), so within the country datum mismatches are no longer a major concern. However, looking overseas each country still often uses its own local datum, and if these differences are ignored in RTK surveying they can produce fatal offsets.

Differences between the World Geodetic System (WGS84) and local datums

The World Geodetic System (WGS84), the basic coordinate system for GNSS, is a global reference frame developed by the United States. Latitude, longitude, and height output by GNSS receivers are basically expressed in the WGS84 reference. However, many countries have historically adopted local datums optimized for their regions, and slight differences exist between those local datums and WGS84. For example, the U.S. North American Datum 1983 (NAD83) is a datum referenced to the North American continent; its reference ellipsoid and origin differ from those of WGS84. NAD83 and WGS84 were nearly identical when established, but WGS84 has been updated over time to reflect Earth-center-based references (including plate motions), so currently there is a difference of about 1–2 m (3.3–6.6 ft) between the two in the continental U.S. In other words, if U.S. topographic or survey maps are based on NAD83, plotting WGS84 coordinates as-is will place them about 1–2 m (3.3–6.6 ft) off on the map [note: the difference varies by region and epoch]. In applications requiring high accuracy, a 1–2 m offset can be a serious problem.

Datum differences also affect vertical references. Heights obtained from GNSS are ellipsoidal heights (height from the WGS84 ellipsoid), while official heights used in each country are geoid-based heights referenced to mean sea level. For example, the difference between the U.S. official vertical datum (NAVD88) and WGS84 ellipsoidal heights can be tens of meters (tens of ft) depending on location, so care is needed when handling elevation data. When using height information from RTK surveys, converting to the local vertical datum (by applying a geoid model correction) is indispensable.

As described above, when country or region differs, the reference coordinate systems differ, and overlaying coordinates obtained in the WGS84 system directly onto local coordinate data can result in offsets ranging from a few meters to, in some cases, tens of meters (tens of ft). Below we look specifically at typical datum-related mistakes that occur in the U.S. and how to prevent them.

1. Base station coordinate datum setting errors – large position offsets

Failure example: In RTK surveying the basic practice is to set the base station at a point with accurately known coordinates. However, on overseas sites this base station coordinate datum setting error tends to occur frequently. For example, a base station’s coordinates might be entered without considering the datum, using Japanese coordinate values as-is. If known point coordinates used in Japan (in the Japanese geodetic datum JGD) are mistakenly assumed to be the local U.S. reference and entered, all measured points will be shifted by the difference between the local datum and the entered datum (such as the difference between WGS84 and NAD83). In some cases entire measurement sets can be off by tens of meters (tens of ft), and because observations may appear normal on-site the error is hard to notice. Discovering later that deliverables do not match existing map data can lead to serious trouble. Also, simple input errors in digits (for example, mistyping degrees/minutes/seconds), or confusing latitude/longitude with projected plane coordinates, can similarly cause large offsets. Extra caution is required in unfamiliar overseas environments when handling reference point coordinates.

Countermeasures:

• Use official local control points: Before installing a base station, obtain the coordinates of local known points (official benchmarks or existing triangulation points) as much as possible, and adopt them as the base station coordinates. Using official coordinates provided in the local datum as the reference prevents datum-induced offsets from the outset. If you must use an unknown point as a base station, measure multiple nearby known points for checking so you can later convert to the local coordinate system if needed.

• Double-check coordinate entry: When entering known base station coordinates into equipment, always double-check the datum and the coordinate values twice. Verify digit counts, plus/minus signs, and the coordinate system used (latitude/longitude or projected plane coordinates, and which datum), and if unsure have another operator cross-check. In the U.S., forgetting to enter west longitude as a negative value when inputting latitude/longitude is a common mistake (for example entering 100°W as 100°E by mistake). Also check for overlooked settings such as zone numbers. Use a checklist to prevent input errors.

• Validate with trial positioning: After setting the base station, it is important to immediately perform trial positioning to validate the setup. Measure a known reference point (e.g., a surveyed benchmark found in advance) with the rover and confirm that the obtained coordinates match expected values (as shown on maps or benchmarks). If an offset of more than a few meters is found at this stage, you can quickly review and correct the settings.

• Establish standard procedures: Rather than leaving such procedures to local discretion, prepare company-standard procedures and checklists. Template the cautions for base station coordinate settings and share them with overseas teams and local staff so that consistent quality is maintained regardless of who performs the work. Building organizational safeguards against human error is crucial.

2. Forgetting to convert to the local datum – risk of data mismatch

Failure example: RTK positioning results obtained by a GNSS receiver are by default in WGS84 coordinates, but there are many cases where data are delivered without converting to the local official datum. Since official geodetic datums differ by country, failing to perform this conversion can cause surveying results to not match local maps or design coordinate systems. For example, on a U.S. project a point cloud recorded in WGS84 longitude/latitude and handed over without conversion to the NAD83-based State Plane coordinate system resulted in offsets of tens of meters (tens of ft) on maps and large rework. Additionally, differences in how longitude east/west are represented and differences in projected coordinate systems (such as UTM zone numbers) can lead to simple conversion mistakes. The U.S. in particular has state-specific projected coordinate systems (State Plane) and differing zone settings, making “coordinate transformation omissions” a common cause of data inconsistency.

Countermeasures:

• Research the local official coordinate systems in advance: At project start, identify the official horizontal and vertical datums to be used locally and decide on unified rules. Determine the datum adopted by the country or state (e.g., NAD83 for the U.S., ETRS89 for parts of Europe), the coordinate system type (e.g., which UTM zone, which State Plane zone number, units used), and set a consistent reference for the whole project.

• Configure equipment to output local coordinates: Check GNSS receivers and surveying software settings and, if possible, configure them to output positioning data directly in the local datum and coordinate system. For example, if using a U.S. network RTK service, confirm that the service’s provided coordinate system matches the project's reference and set the receiver to output that coordinate system as needed. Recording data in the local coordinates from the start reduces later conversion errors.

• Use official tools for accurate conversion: If you must convert WGS84 data to a local system later, use the official transformation parameters or software provided by national geodetic agencies to ensure accuracy. In the U.S., use tools provided by the USGS or the National Geodetic Survey (NGS); in Japan, use transformation parameters from the Geospatial Information Authority of Japan. Avoid manual calculations or heuristic conversions that can introduce errors or human mistakes.

• Verify transformation results: After performing coordinate transformations, always cross-check them against local known points or official maps. For example, plot the transformed coordinates on the local topographic map to confirm they coincide with benchmarks or landmarks. Detecting offsets at this stage allows corrections before delivery. Thorough verification prevents critical errors like “data that does not match delivered plans” from occurring.

3. Confusing feet and meters – abnormal scale offsets

Failure example: Alongside datum differences, a key point to watch overseas (especially in the U.S.) is differences in length units. U.S. surveying and design drawings commonly use the imperial system (feet, yards) rather than the metric system. As a result, mistakes frequently occur where values are mistaken for feet when they are meters, or vice versa. For example, if coordinates on a design are shown as “1000” and they are actually in feet but are interpreted as meters in the field, a scale difference of about 3.28 times will occur. Since 1000 ft ≒ 304.8 m (1,000 ft), it would be like treating a true distance of 304.8 m (1,000 ft) as 1000 m, causing large misalignment. Conversely, delivering survey results in meters without converting them to feet can lead recipients to place them on drawings with a discrepancy of over three times. Such unit conversion mistakes are basic oversights but occur surprisingly often in international projects. Note also that there are subtle differences between the international foot and the U.S. survey foot (one foot being 0.3048 m or 1200/3937 m), and some states historically used the survey foot. Mixing units can accumulate errors at the ppm level, so be cautious (the U.S. has been moving to adopt the international foot in place of the survey foot since 2023).

Countermeasures:

• Clearly indicate and confirm units: When handling survey data and drawings, always confirm whether the values are expressed in feet or meters. If units are not noted on the drawings or coordinate tables, ask for clarification early and do not proceed while ambiguous. When delivering coordinates, clearly label the units so the recipient cannot misinterpret them.

• Check software settings: Verify the coordinate unit settings of surveying equipment and CAD software, and set them to the local usage units. For example, some U.S.-spec software defaults to feet, while equipment brought from Japan may always output meters. Switch settings as needed to avoid input/output unit mistakes.

• Use precise conversions: When converting units, apply exact conversion factors. The conversion between feet and meters is 1 ft = 0.3048 m (exact). When performing calculations in Excel or similar tools, be mindful of decimal places to avoid rounding errors. Use software that provides automatic unit conversion if available. Also check whether special cases such as the U.S. survey foot apply in the project state.

• Standardize units within the team: Unify and communicate the units to be used among project members. For example, decide rules such as “horizontal positions in international feet, heights converted to meters for delivery,” document them, and ensure everyone follows them. When working with local staff, align on unit handling ahead of time to overcome language barriers.

So far we have reviewed mistakes and countermeasures related to datum differences and unit issues. RTK surveying itself is extremely precise, but it is essential to align reference coordinate systems correctly and to reliably perform coordinate and unit conversions. Neglecting these steps can render centimeter-level accuracy useless and lead to misaligned positions. With careful preparation and verification, you can prevent accuracy problems and troubles overseas and successfully carry out RTK surveys across borders.

Recently, however, solutions have emerged that can reduce these complexities. One such solution is LRTK, which combines a smartphone with a compact high-precision GNSS device to allow anyone to perform centimeter-level positioning easily.

Simple surveying enabled by LRTK

LRTK is an all-in-one surveying system in which a dedicated ultra-compact RTK-GNSS receiver is attached to a smartphone and high-precision positioning can be performed with one-touch operation via an app. Its major feature is that it allows on-site acquisition of accurate global coordinates (WGS84) immediately without the complicated preparation of base stations or specialized settings. Acquired position data are automatically saved to the cloud and can be shared with the office team in real time. LRTK supports multi-GNSS and multi-frequency signals, so stable positioning is expected even in mountainous or overseas areas with poor network coverage. The device is designed to be intuitive and requires no complex operation, so multiple users can operate it without advanced technical training, greatly reducing the training burden on local staff.

Such smartphone × compact GNSS-based simple surveying devices like LRTK are an attractive new option for solving challenges commonly encountered when introducing RTK overseas (datum differences, communication infrastructure limitations, human errors). Even those who feel uneasy about operating RTK overseas can simplify and reliably carry out complex RTK surveying by leveraging modern solutions such as LRTK. Use technology to upgrade on-site surveying workflows to the next level.

The LRTK series provides high-precision GNSS positioning for construction, civil engineering, and surveying, dramatically improving surveying accuracy and work efficiency on site. It is compatible with the Ministry of Land, Infrastructure, Transport and Tourism’s i-Construction initiative and is an optimal solution to support digitalization in the construction industry.

For details about LRTK, see the links below.

• [LRTKとは｜LRTK公式サイト](https://www.lrtk.lefixea.com/)

• [LRTKシリーズ｜デバイス一覧ページ](https://www.lrtk.lefixea.com/lrtk-series)

For product inquiries, quotations, or consultation on introduction, please feel free to contact us via the [inquiry form](https://www.lrtk.lefixea.com/contactlrtk). Let LRTK take your site to the next stage.

FAQ

Q1. How different are WGS84 and NAD83 coordinates? A. WGS84 and NAD83 are very similar geodetic reference frames, but currently there is a difference of about 1–2 m (3.3–6.6 ft) between them. NAD83 was established in the 1980s fixed to the North American plate, while WGS84 is Earth-center-referenced and has been updated since; depending on region and epoch, WGS84 coordinates in the continental U.S. are reported to be about 1–2 m (3.3–6.6 ft) northeast of NAD83 coordinates. In high-precision surveying this 1–2 m difference cannot be ignored. Therefore, U.S. surveying results are generally unified on NAD83, and GNSS coordinates obtained in WGS84 should be converted to NAD83 before use. Note that a new North American reference frame (NATRF2022) intended to replace NAD83 is scheduled for introduction in the late 2020s, which is expected to further improve unified high-precision referencing.

Q2. Why is it necessary to check the datum in RTK surveying? A. RTK surveying itself provides high relative accuracy, but the obtained coordinates are always based on some datum. If surveying is performed using a datum different from the project’s reference, even centimeter-level observation accuracy will contain errors at the scale of the datum offset (meter-level). For example, a point cloud measured to 1 cm accuracy (cm level accuracy (half-inch accuracy)) by RTK can still be misplaced by several meters if the datum is wrong. This is not an accuracy issue but a reference error, and unless corrected in post-processing it cannot be restored. Therefore, when performing RTK surveys you must first confirm that the base station coordinate system and the output coordinate system are the correct project reference. Additionally, check that GNSS-derived heights are converted to the local vertical datum (geoid) as needed. In short, unless “which datum was used” is matched, you cannot make use of RTK’s high precision in practical applications.

Q3. Do I need to be careful about length units (feet/meters) when surveying in the U.S.? A. Yes, confirming the unit system is very important. Distances and coordinates in the U.S. often use the imperial system (feet, yards) rather than the metric system. If you mistake feet for meters, you can misplace positions by about 3.3 times—for example, interpreting 100 feet as 100 meters results in a roughly 70% difference in scale. Conversely, if you provide data measured in meters and the recipient assumes they are feet, the placement will be off by more than three times. Such unit mix-ups can be difficult to correct after discovery, so always confirm and unify the units beforehand. Also note there are two definitions of the foot—the international foot and the U.S. survey foot—but as of recent years most use the international foot where 1 ft = 0.3048 m (exact). Regardless, unify units among project stakeholders and set software input/output correctly to avoid mistakes.

Q4. What kind of product is LRTK? A. LRTK is an ultra-compact RTK surveying device used in combination with a smartphone. It consists of a small GNSS receiver attachable to a smartphone and a dedicated app, enabling easy real-time centimeter-level positioning on site. Whereas traditional RTK surveying required large dedicated equipment and base stations, LRTK completes the workflow with just a smartphone. For example, attaching an LRTK device to an iPhone and operating the app can realize high-precision GNSS positioning, 3D scanning, and AR-based position display all in one. LRTK supports multi-GNSS and multi-band signals, and works with Japan’s QZSS “Michibiki” augmentation (CLAS) as well as conventional network RTK services. In short, it is a next-generation tool that turns your smartphone into a high-precision surveying instrument, with excellent portability and rapid response that can transform field survey workflows.

Q5. Does LRTK solve datum offset problems? A. Using LRTK can greatly reduce human errors in base station setup and positioning configuration. LRTK fundamentally obtains accurate global coordinates in the WGS84 system, avoiding errors caused by incorrect base station coordinate settings. Project data can then be converted to the local datum as required; LRTK cloud services and apps can perform coordinate transformation and output settings. In other words, LRTK does not automatically correct datum differences by itself, but because it makes it easy to acquire correct absolute coordinates, it facilitates reliable downstream conversion. Traditional surveying workflows involved many manual steps from base station setup to coordinate transformation where mistakes could occur; with LRTK these processes become simpler, minimizing the risk of failures due to datum mismatches. Improvements in software are expected to further enable one-touch support for various national coordinate systems. In any case, leveraging LRTK will significantly reduce the effort and risk across the entire surveying process, including bridging datum gaps.