RTK surveying (Real-Time Kinematic positioning) is a high-precision positioning technique using GPS. Because it can measure positions with errors of a few centimeters, it is widely used in surveying and civil engineering. However, have you ever experienced “I did RTK surveying but the coordinates don’t match”?

One reason for this is the difference in datums (geodetic reference systems). In this article, aimed at beginners to intermediate users, I explain what a datum is, why differences in datums cause coordinate shifts, and focus especially on the differences between two representative datums, NAD83 and WGS84. I also explain how to prevent datum-related problems in RTK surveying and introduce a smartphone-based simplified RTK surveying system, LRTK, describing how it prevents datum-setting mistakes and its advantages.

Table of Contents

• What is a datum?

• Why do coordinates shift when datums differ?

• Differences between NAD83 and WGS84

• Effects of datum differences on RTK surveying

• How to handle coordinate shifts caused by datum differences

• Preventing datum mistakes with the smartphone RTK system “LRTK”

• FAQ (Frequently Asked Questions)

What is a datum?

A datum is a coordinate system used as a reference to express positions on the Earth. Simply put, a datum is the set of rules that define how the Earth is modeled and which point is used as the origin to define latitude, longitude, and height. The Earth is not a perfect sphere but is close to an oblate spheroid (a rotational ellipsoid) slightly bulging at the equator. In a datum, a mathematical shape called an ellipsoid (its size and flattening) is defined first, and then reference points on that ellipsoid are chosen. Based on those references, the latitude, longitude, and height for locations around the world are determined.

For example, Japan’s current datum, JGD2011 (Japan Geodetic Datum 2011), adopts the GRS80 ellipsoid and was established based on domestic observation networks. On the other hand, WGS84 (World Geodetic System 1984), operated by the U.S. Department of Defense, adopts the WGS84 ellipsoid and is a global datum. Both are called “global datums” and represent the whole Earth with a single coordinate system, but they differ slightly in ellipsoid parameters and how reference points are chosen.

The important point is that even for the same physical point, the numerical values of latitude, longitude, etc., will differ depending on the datum used. A datum is like a measuring stick for position data; if you use a different measuring stick, the measured values change. Therefore, being aware of the datum used by maps and GPS devices is essential for accurate positioning.

Why do coordinates shift when datums differ?

Different datums approximate the Earth with ellipsoids of different shapes and sizes and use different origins for reference. These differences cause the coordinate values (latitude, longitude, height) for the same physical point to differ among datums.

Imagine a point A on the Earth. When measuring point A, datum X uses “ellipsoid X” and origin X for calculations, while datum Y uses “ellipsoid Y” and origin Y. If ellipsoid X and Y assume different Earth shapes or their origins are offset, the latitude and longitude of point A calculated under X and Y will differ.

A familiar analogy is differences in map conventions. Even for the same town map, one map might measure streets relative to the town hall and another relative to the station; the coordinate values for the same building would differ depending on the reference. Datum differences are similar: they result from differences in “where the Earth’s center is considered” and “which ellipsoid is used to approximate the Earth’s shape.”

A typical example of datum-related shifts is the difference between Japan’s former “Tokyo Datum” and the current global datums (such as JGD2011). The old Tokyo Datum was a Japan-specific datum with substantially different references from modern global datums. As a result, the same point could differ by about 400 m (1,312.3 ft) between the Tokyo Datum and global datums. This is an extreme example but shows that different datums can produce very large discrepancies.

Differences between NAD83 and WGS84

As a concrete example of datums, let’s look at the differences between NAD83 (North American Datum 1983), used in North America, and WGS84 (World Geodetic System 1984), used globally. Both NAD83 and WGS84 are types of geocentric global datums, but there are subtle differences.

NAD83 (North American Datum 1983):

• A datum mainly used across the North American continent. It was revised from the older NAD27 in the 1980s and is a geocentric coordinate system (centered on the Earth’s center).

• It adopts the ellipsoid known as GRS80. This ellipsoid has defined values for the semi-major axis and flattening, and NAD83 computes latitude and longitude based on it.

• NAD83 is often described as a coordinate system fixed to the North American plate. In other words, on the North American plate, latitude and longitude are defined so they remain nearly constant over time (although NAD83 itself has seen refinements, its basic concept is plate-fixed).

WGS84 (World Geodetic System 1984):

• The global standard datum used by GPS and many mapping services. It was developed by the U.S. Department of Defense to cover the entire world with a single coordinate system.

• It adopts the WGS84 ellipsoid, whose semi-major axis and other parameters are very close to those of GRS80, with only minute differences in flattening. In practice, the ellipsoid shapes are nearly identical.

• WGS84 is based on a global Earth-centered coordinate frame (an Earth-centered, Earth-fixed Cartesian system) and has been revised several times to align with international terrestrial reference frames (ITRF). Because it targets the whole Earth, coordinates for a given point can change over time due to continental plate motions.

So how large are the coordinate differences between NAD83 and WGS84? In short, the difference is said to range from several tens of centimeters to, in some places, about 1-2 m (3.3-6.6 ft). In particular, on the North American continent, because NAD83 is plate-fixed to the North American plate, differences between NAD83 and WGS84 have accumulated since 1984 by the amount the plate has moved. For example, in the continental United States, there are locations where the difference between NAD83 and recent realizations of WGS84 is on the order of about 1 m (3.3 ft). This magnitude of difference is significant for surveying accuracy. In applications requiring centimeter-level precision such as RTK surveying, a 1 m discrepancy is a critical error.

Moreover, although NAD83 and WGS84 are conceptually similar, precise coordinate transformation (datum transformation) between them requires a rigorous conversion formula with seven parameters. GIS and surveying software provide these parameters to convert between NAD83 and WGS84 precisely, but doing it manually is impractical. Therefore, in the field it is common practice to perform surveying in the required datum from the start.

Effects of datum differences on RTK surveying

How do these datum differences actually affect RTK surveying? In RTK surveying, it is ideal that both the base station and the rover are positioning within the same reference coordinate system. If the base station recognizes its position in WGS84 coordinates while the rover interprets results in NAD83 coordinates, there will be inconsistency between them and correct positioning cannot be obtained.

For example, suppose the true position of a point is (φ, λ) in WGS84. Even if the base station sends correction information based on that WGS84 position, if the rover interprets the corrections in NAD83, the rover’s computed latitude and longitude could be off by about 1 m (3.3 ft) from the true position. This occurs because the map reference (datum) assumed by the base station and rover do not match.

In RTK surveying, the base station coordinates are usually set from known points, or in network RTK (VRS, etc.) the system generates a virtual reference position. What matters is which datum those reference coordinates are defined in. In Japan, many RTK services and Continuously Operating Reference Stations (CORS) provide coordinates in JGD2011 (a global datum). In regions like the U.S. where NAD83 is officially used, base station coordinates may be provided in NAD83. The user’s receiver (the rover) computes its position from the received correction info and the base station coordinates, but if the receiver’s settings are not using the same datum as the base station, the resulting coordinates will be offset.

When performing high-precision positioning, differences of several tens of centimeters are not negligible. Especially in public surveying or civil engineering where measured values are compared with existing plan coordinates, datum mismatches can cause the plan and measured values not to line up, leading to serious problems. There have been cases where work proceeded without noticing a consistent ~1 m offset, and later investigation revealed the cause to be a datum setting error. In RTK surveying, checking the datum/coordinate system setting on equipment and software before starting is fundamental.

How to handle coordinate shifts caused by datum differences

To prevent “coordinates don’t match!” situations in the field, keep these countermeasures for datum-related issues in mind.

• Unify the datum used: It is most important to ensure the base station, rover, and any GNSS services use the same datum. For example, in Japan, unify on JGD2011 (global datum). In the U.S., if a project requires NAD83, set both base and rover to NAD83. Many devices let you select WGS84, NAD83, etc., so always verify this before starting surveying.

• Verify with known points: If there are known control points (points with pre-known coordinates) at the site, measure one of them with RTK before work begins to confirm that the coordinates match. If the measured result differs from the known coordinate by several tens of centimeters or more, suspect a datum setting error. A simple pre-check can reveal critical mismatches early.

• Use coordinate transformation tools: If you must handle data in different datums, use coordinate transformation software provided by national mapping agencies or GIS packages. Manual calculation is difficult, but software can accurately perform transformations like NAD83⇔WGS84 or Tokyo Datum⇔global datums. Even when using transformed data, clearly state which datum the data has been converted to.

• Share datum knowledge within the team: It’s important that not only surveyors but the whole site team understands basic datum concepts. If other departments or external parties use survey results later, lack of datum awareness can lead to misinterpretation. Clearly document and notify others, for example, “All project coordinates are in the global datum (JGD2011).”

By implementing these measures you can prevent coordinate shifts caused by datum differences and conduct RTK surveying with confidence. High-precision RTK can be ruined by a single datum-setting mistake, so keep these basic precautions in mind.

Preventing datum mistakes with the smartphone RTK system “LRTK”

Finally, I’ll introduce an easy-to-use smartphone-based RTK surveying system, “LRTK”, suitable for beginners. LRTK is a next-generation surveying tool that attaches a compact RTK-GNSS receiver to a smartphone, aiming to make centimeter-level positioning accessible to anyone. LRTK includes features to prevent datum-setting mistakes and simplifies surveying workflows.

Automatic datum handling in LRTK: The dedicated LRTK app can automatically convert and display measured coordinates in the prescribed datum. For example, when used in Japan, LRTK internally acquires WGS84 positioning data and automatically converts it to Japan’s official datum, JGD2011, and presents that to the user. This means users do not have to consciously set the datum and can always obtain coordinates unified in the global datum (JGD2011). The app also allows one-touch display of WGS84 latitude/longitude, geoid height, or Japan’s plane rectangular coordinates as needed. Because LRTK automatically computes appropriate datum values, beginners need not worry about datum differences.

Advantages as a simplified surveying tool: By pairing with a smartphone, LRTK achieves high-precision positioning without the expensive, complex traditional surveying equipment. With only a receiver weighing a few hundred grams and a smartphone, positioning tasks on site can be completed. By tapping the “Measure” button in the app, latitude, longitude, and height are recorded and data are immediately shared to the cloud. The UI is designed to minimize complex settings, reducing not only datum selection mistakes but also procedural errors in surveying. True to its claim as an easy-to-carry, all-purpose surveying tool, LRTK excels in on-site usability.

Using a smartphone RTK system like LRTK allows high-precision positioning without worrying about datum differences. Because the system covers the specialized knowledge required, beginners can start RTK surveying with confidence.

FAQ (Frequently Asked Questions)

Q. How much do coordinates actually shift when datums differ? A. It depends on the datums, but famous examples include a roughly 400 m (1,312.3 ft) shift between Japan’s old Tokyo Datum and global datums. Even between relatively close datums like NAD83 and WGS84, differences of about 1 m (3.3 ft) can occur in North America. In RTK surveying, which demands centimeter-level precision, a 1 m difference is unacceptable.

Q. Are WGS84 and JGD2011 the same? A. Strictly speaking they are different datums, but for practical purposes they can be considered almost the same. JGD2011 is Japan’s realization of a global datum and uses a geocentric reference compatible with WGS84. The ellipsoids differ only slightly, and in typical GPS usage in Japan you can use JGD2011 without concern.

Q. Which datum should I use when starting RTK surveying? A. As a rule of thumb, use a global datum. This refers to WGS84 or compatible systems such as JGD2011. Many commercial GNSS receivers and services default to global datums. However, some countries or projects may require NAD83 or other regional datums, so follow the project specifications.

Q. Will RTK work if the base and rover use different datums? A. The system may produce a solution (fix), but the reported coordinates will be incorrect. Corrections sent from the base station are based on that base station’s datum. If the rover interprets results in a different datum, offsets of several tens of centimeters to several meters can occur. To obtain accurate results, the base and rover must use the same datum.

Q. Can a smartphone really achieve centimeter-level surveying? A. Yes. By attaching a high-precision GNSS receiver to a smartphone and using network RTK corrections, you can achieve accuracy comparable to dedicated surveying equipment. In fact, LRTK can further refine averaged standalone positions to the millimeter level in some cases. High-precision positioning that once required specialized hardware is now achievable with a smartphone and a small external device.

Q. When should I be particularly careful about datum differences? A. Be especially cautious when comparing old and new maps or handling foreign surveying data. For example, if an old topographic map uses the Tokyo Datum, its coordinates can be greatly offset from modern global datum coordinates. Overseas, regional datums such as NAD83 in North America or ETRS89 in Europe may be used. Always confirm the datum of your data sources and transform them appropriately before use.