RTK Geoid Models Explained: How to Get Correct Elevations in the Field

Have you ever been puzzled that, even after measuring positions with high-precision RTK surveying (real-time kinematic positioning), the heights you obtained did not match actual elevations? The horizontal positions may be accurate to a few centimeters, yet the heights alone can differ by tens of meters — many of you may have experienced this.

Heights obtained by RTK positioning using GNSS have a different reference from the “elevation” we commonly use, and if left uncorrected this can cause large discrepancies. The indispensable tool to bridge this gap and compute correct elevations is the geoid model. In this article, we explain the role and importance of geoid models in RTK surveying and detail the key points for obtaining correct elevations. At the end, we also touch on an easy surveying method using the latest technology, LRTK. Let’s get started.

Table of Contents

• What is the geoid?

• Relationship between elevation, ellipsoidal height, and geoid height

• Why geoid models are important in RTK surveying

• How to compute elevation using a geoid model

• Points to note when using a geoid model

• Simple surveying with LRTK

• FAQ

What is the geoid?

The geoid, simply put, is a virtual water surface obtained by extending mean sea level under the continents. In Japan, the mean sea level of Tokyo Bay is used as the height 0 m (0 ft) reference (= the elevation reference surface), and the sea surface extended across the globe is defined as the geoid surface. At any point on the geoid surface the gravity potential is the same, and it is the surface on which water would rest quietly and horizontally. In other words, the geoid is the surface on Earth where water would naturally settle level.

The geoid is not the Earth’s actual topography but a gravity-based reference surface, so unlike a mathematically defined reference ellipsoid it is irregular. Compared with a reference ellipsoid that approximates the shape of the Earth (for example, the GRS80 ellipsoid used in Japan), the geoid surface has global undulations on the order of ±100 m (±328.1 ft), and there are local variations near Japan as well. In Japan, the geoid surface is generally located about 30-40 m (98.4-131.2 ft) lower than the reference ellipsoid; that difference is what we call the geoid height for each region. In other words, the geoid height is the vertical distance measured downward from the reference ellipsoid to the geoid at a given point (the difference between the ellipsoid surface and the geoid surface). This value varies by latitude and longitude and can be obtained using an official geoid model.

Relationship between elevation, ellipsoidal height, and geoid height

To understand heights in RTK surveying, let’s sort out commonly confused height terms.

• Elevation: The height from the geoid surface (mean sea level) to a survey point. This is the “above sea level X m” value used on maps and in construction, and it is a physically meaningful height managed by benchmarks.

• Ellipsoidal height: The height from the reference ellipsoid to a survey point. This is the geometric height obtained directly from GNSS positioning, measured relative to a rotating reference ellipsoid centered on the Earth’s mass (e.g., GRS80). Because it does not account for gravity variations, it can differ from elevation by tens of meters.

• Geoid height: The vertical difference from the reference ellipsoid to the geoid surface. It is the correction value that links ellipsoidal height and elevation and varies by location. Using an official geoid model, you can obtain the geoid height at any point.

These relationships can be expressed simply: elevation = ellipsoidal height − geoid height. In other words, subtract the geoid height at a point from the ellipsoidal height obtained by GNSS to compute the elevation (and conversely, add the geoid height to elevation to get the corresponding ellipsoidal height). Strictly speaking, because the geoid and ellipsoid surfaces are not perfectly parallel, there are minute differences, but for ordinary surveying they are negligible. Therefore, in practice the above relation is sufficient for height conversion.

By the way, in Japan there is roughly a 30 m (98.4 ft) difference between ellipsoidal heights obtained by GNSS (the global geodetic system) and the elevation reference (Tokyo Bay mean sea level). Applying the geoid height allows conversion between the two height systems.

Why geoid models are important in RTK surveying

So why is height correction using a geoid model so important in RTK surveying? There are two main reasons.

1\. To obtain heights that are physically meaningful Heights that ignore the influence of gravity do not correctly represent real water flow or horizontal reference. For example, if you use ellipsoidal height alone as the height reference, in theory you could have counterintuitive situations where water flows from lower to higher ellipsoidal heights. This would lead to fatal errors in infrastructure design and flood control. Therefore, elevations based on the geoid are adopted as the height standard, and gravity-based elevations are regarded as the physically meaningful heights. Using the geoid as the reference ensures that elevation differences correspond to the intuitive “water flows from high to low” direction, providing consistent and intuitive height information.

2\. To make GNSS-derived heights practical for field use In recent years, high-precision GNSS positioning (RTK-GNSS surveying, etc.) has become widely used on civil engineering and construction sites. As noted above, heights obtained directly from satellite positioning are ellipsoidal heights, which use a different reference from traditional elevations. In practice, what is needed are elevations (heights above sea level) used in drawings and designs. Therefore, to obtain elevations from heights observed by GNSS, correction by the geoid height (application of a geoid model) is essential.

If you do not apply geoid correction, the heights may be off by tens of meters and unusable. This is not an issue of positioning accuracy but a difference in reference surfaces; no matter how accurate the GNSS is, the discrepancy cannot be resolved without geoid correction. In fact, even smartphone GPS apps can show vastly different altitudes depending on the device. Android devices often display ellipsoidal heights as-is, while iPhones (iOS) are said to internally apply geoid correction to show elevations closer to sea level; this can cause confusion where the same location shows “altitudes differing by tens of meters!” depending on the device. That demonstrates how indispensable geoid-model-based height correction is for measuring heights. For iOS, a representative global geoid model (e.g., EGM96) is probably used for height correction, but to obtain precise elevations in Japan one must apply the Geospatial Information Authority of Japan’s (GSI) geoid model. In short, using GNSS heights without knowing the geoid height is very risky; conversely, if you correctly apply a geoid model you can obtain high-precision elevations in the field without performing leveling.

How to compute elevation using a geoid model

How should you actually compute elevation (the correct height) from GNSS heights in surveying? There are two main methods.

• Apply geoid correction in the GNSS receiver Many recent high-precision GNSS devices (RTK receivers, etc.) have built-in geoid models and can convert ellipsoidal height to elevation during positioning. By selecting the geoid model appropriate for the region in the settings (for example, in Japan select “Japan Geoid 2011” or “Geoid 2024 Japan”), the receiver can automatically output elevations (heights after geoid correction). This is convenient for field surveyors because you get elevations directly without special calculations. However, be careful when entering reference point coordinates. Confirm whether the device or software expects the reference point height as an ellipsoidal height or an elevation, and don’t enter an incorrect reference-surface value. For example, if you only have the “elevation” value of a known point and you mistakenly enter that value as the ellipsoidal height for the base station, the geoid height difference will be carried as an error into the positioning results. When setting up a base station, always use height values that are consistent with the height reference you require.

• Manually convert using geoid heights If your GNSS device does not include a geoid model or you want to post-process observations to convert to elevation, you can apply the geoid height to the ellipsoidal height yourself. Using the official geoid model provided by GSI (grid data of geoid heights covering all of Japan), you can obtain the geoid height at any location. For example, GSI’s website offers a “geoid height calculation” service where you enter latitude and longitude to obtain the geoid height at that point. You can also download numeric geoid model files (such as “Japan Geoid 2011” or the latest “Geoid 2024 Japan”) and use them in GIS software. Subtract the corresponding geoid height from the observed ellipsoidal height to compute elevation (for example, if GNSS gives an ellipsoidal height of 50.00 m (164.04 ft) and the geoid height at that point is 30.00 m (98.43 ft), the elevation is 20.00 m (65.62 ft)). The official geoid models are highly accurate; the difference between heights obtained from the model and measured elevations is said to be roughly 2-3 cm (0.8-1.2 in). If applied properly, you can obtain heights comparable in accuracy to conventional leveling.

Points to note when using a geoid model

Finally, here are key points to bear in mind when applying geoid models in RTK surveying.

• Confirm the version of the geoid model: Pay attention to the version and reference frame of the geoid model you use. Japan’s geoid models have been improved from the 2000 version to the 2011 version, and the latest “Geoid 2024 Japan” offers higher accuracy. For public surveying, use of “Geoid 2024 Japan” is recommended from 2025 onward, and differences between old and new models can be several centimeters depending on the region. If a specific model is prescribed, follow the instructions and use the correct geoid model.

• Pay attention to the reference surface of reference point coordinates: When setting up an RTK base station or entering known point coordinates, always confirm whether the required height reference is ellipsoidal height or elevation. If an ellipsoidal height is required but you mistakenly input elevation, the results will include the geoid height error. The reverse is also true — be careful not to mix up reference surfaces.

• Consider GNSS positioning errors themselves: While geoid correction theoretically produces accurate elevations from ellipsoidal heights, real observations are affected by inherent GNSS errors. Factors such as satellite signal reception conditions, tropospheric effects, and multipath (signal reflection) can degrade height accuracy, so when observing under poor conditions some residual error may remain in the final results. Although introducing a geoid model enables elevations comparable to leveling to be obtained, evaluate measurement results taking observational errors at the site into account.

Simple surveying with LRTK

For those who want to perform RTK surveying more easily, there is a cutting-edge solution called LRTK. LRTK is an ultra-compact RTK-GNSS receiver that attaches to a smartphone; combined with a dedicated app, it enables one-person centimeter-level positioning. LRTK is designed to allow anyone to perform RTK surveying with just a smartphone, removing the need for expensive surveying equipment and specialized expertise. Its use is expanding across applications such as stakeout in civil engineering, as-built control, and infrastructure maintenance inspections. With LRTK, site supervisors and surveyors — and even those without surveying expertise — can perform accurate positioning on demand.

Using LRTK, handling heights is also very simple. The LRTK app automatically converts to elevation during positioning. For example, heights in the Japan Geodetic Datum (JGD2011) and geoid heights are calculated and displayed in real time, so users do not need to think about height conversion. Plane coordinates are also compatible with public coordinate systems (plane rectangular coordinate systems), and obtained data can be saved and shared to the cloud on the spot. With a pocket-sized LRTK device and a smartphone, surveying tasks that formerly required two people can be completed by one person. LRTK, which provides immediate access to coordinate data including heights based on the correct geoid model, will greatly improve field productivity and surveying accuracy. Experience the unprecedented simplicity of RTK surveying with LRTK, which balances ease of use and accuracy.

FAQ

Q. What is the difference between elevation and ellipsoidal height? A. Elevation is the height referenced to mean sea level (the geoid), while ellipsoidal height is the height referenced to an artificial reference ellipsoid. Because the reference surfaces differ, the two values at the same point can differ considerably (in Japan, ellipsoidal heights are typically about 30 m (98.4 ft) higher than elevations). Therefore, you cannot use GNSS-observed heights (ellipsoidal heights) as elevations without conversion.

Q. Why is a geoid model needed for RTK surveying? A. Because heights obtained by GNSS surveying are ellipsoidal heights, not elevations. To use them in practice you must correct ellipsoidal heights by the geoid height to obtain elevations. Without geoid-model-based correction, RTK surveying heights can differ from actual elevations by tens of meters.

Q. Can GNSS produce elevations with the same accuracy as leveling? A. If appropriate geoid correction is performed, GNSS can yield heights that are nearly equivalent to leveling. When using GSI’s geoid model, the difference between GNSS-derived heights and heights from leveling is reported to have a standard deviation on the order of 2-3 cm (0.8-1.2 in). If sufficient satellite positioning conditions are available, RTK-GNSS alone can practically achieve comparable elevation accuracy.

Q. Which geoid model should be used in Japan? A. The current recommendation is the latest “Geoid 2024 Japan.” Use of Geoid 2024 Japan is recommended for public surveying from 2025 onward. However, follow any geoid model specified for your work. If you need to maintain consistency with older survey results, you may use the earlier “Japan Geoid 2011” model.

Q. Does LRTK perform geoid correction automatically? A. Yes. LRTK’s dedicated app implements Japan’s geoid model, and the heights shown in the positioning results are automatically converted to elevations (heights after geoid correction). Users do not need to perform manual corrections and can obtain correct elevation data on site.

Q. If I measure heights with RTK-GNSS, is leveling unnecessary? A. RTK-GNSS can provide elevations equivalent to leveling in most cases, so in practice the need for leveling can often be omitted. However, in situations requiring extremely high precision at the millimeter level, or when GNSS reception conditions are poor, or when measuring very small height differences between closely spaced reference points, leveling remains effective. It is also desirable to use both GNSS and leveling to cross-check important control points for quality assurance.

Q. How are geoid heights computed? A. Geoid heights are derived from detailed observations of the Earth’s gravity field. GSI creates precise geoid models by combining gravity measurements and leveling results taken across the country with ellipsoidal height data from satellite positioning. Satellite-based gravity observation data are also utilized, and the accuracy of global geoid models continues to improve. Thus, official geoid models can reproduce actual mean sea level undulations to centimeter-level accuracy.

Q. Where can I obtain geoid model data? A. You can download geoid model data for all of Japan from GSI’s website. For example, the “Fundamental Geospatial Data Download Site” provides the latest geoid models (grid data in JPGIS format, etc.). GSI also offers a “geoid height calculation” service where you can look up the geoid height for a location by place name or latitude/longitude.

This article introduced key points for obtaining correct elevations in RTK surveying and presented the latest RTK solution, LRTK. By correctly understanding and applying geoid models and adopting these modern tools, you can greatly improve surveying efficiency and accuracy in the field. It will also help boost on-site work productivity — please try using them actively on your sites!