How to Read RTK Elevation? Organizing the Difference Between Ellipsoidal Height and Geoid Height in 5 Minutes

When a height value appears on an RTK screen, it’s tempting to accept that number directly as the “elevation.” However, the meaning of the height obtained from RTK cannot be judged correctly without checking the settings and the reference surface. Positioning by satellite directly yields the ellipsoidal height, and the elevation commonly used on site is a different value obtained by subtracting the geoid height from that. Many cases where the height numbers seem inconsistent are not measurement failures but arise from not having sorted out “which height” is being shown.

In Japan, an elevation system based on mean sea level is used, and in recent years the practice of handling elevation by combining satellite positioning and geoid models has become increasingly important. In fact, for public surveying, GNSS height surveying using the geoid model “Geoid 2024 Japan and Surroundings” and continuously operating reference stations was introduced on April 1, 2025, making how to handle height references a more central practical theme than before. To read RTK heights correctly, the quickest route is to first separate the three terms ellipsoidal height, geoid height, and elevation.

Contents

• Why RTK height displays are easily confusing

• What is elevation in the first place

• What is ellipsoidal height

• What is geoid height

• Understand the relationship between elevation and ellipsoidal height with a formula

• Points to check when reading heights on site

• Latest practical notes to keep in mind in Japan

• How to make RTK height management more practical

The main reason RTK height displays are hard to understand is that the single word “height” contains multiple concepts with different reference surfaces. Because satellite positioning calculates positions with the Earth’s ellipsoid as the reference, what is first determined is the ellipsoidal height. On the other hand, the heights used for drawings, control points, benchmarks, and height management of existing structures in civil engineering and construction are those tied to mean sea level—i.e., elevation. Both are correct heights, but because their reference surfaces differ, the numeric values do not match. If you look only at the screen numbers without sorting this out, you are likely to feel that “the same point has different heights” or that “RTK is unstable.”

In practice, discussions about accuracy tend to be conflated with discussions about types of height. Even if RTK is fixed, whether the displayed number is ellipsoidal height or elevation after geoid correction is a separate issue. In other words, good RTK status and high-precision measurement do not mean the displayed value can be used directly as elevation. To prevent misreading heights, you must first confirm “what reference surface the current value is based on,” and then compare it with design values or known point heights.

Moreover, even when two values—ellipsoidal height and elevation—are shown for the same point, it does not mean one is wrong. When reference surfaces differ, multiple “correct heights” can exist for the same point. The confusion is not caused by the difference in numbers itself but by comparing values expressed in different height systems without confirming which system each uses. For those handling RTK heights, what matters more than reading the device screen is interpreting the meaning of the numbers.

For example, when comparing the numbers on an RTK screen with heights listed in a known point results table or design documents on site, you may see large discrepancies. Rather than immediately suspecting device malfunction or correction data errors, simply checking whether elevation and ellipsoidal height have been swapped often reveals the cause quickly. If you’re unsure about RTK heights, the first thing to check is not the number of satellites or communication status but the definition of the height.

What is elevation in the first place

Elevation is not merely a number that indicates whether something looks high or low. In Japan, elevation is defined relative to mean sea level, and the surface that is conceptually extended beneath the land from that reference is the geoid. Official explanations say it is easy to imagine by thinking of a tunnel dug from the sea to under the land so that the sea water would flow in; the surface formed by that water is the image of the geoid. Elevation is the height measured from this geoid to the ground surface along the direction of gravity.

What is important here is that elevation reflects “how water would flow.” Even if the ground surface appears flat, if the gravity distribution is not uniform, the way water flows will change. Therefore, the reference surface used for infrastructure-related heights cannot be a simple geometric surface alone. Elevation is used for longitudinal road profiles, drainage planning, land development, structure installation, and as-built verification because the heights needed on site must be consistent with gravity and water flow. The emphasis on elevation in mapping, infrastructure development, and disaster prevention is for the same reason.

In other words, elevation is also the height that serves as a “common language on site.” When the client, contractor, surveyor, and designer share the same height, elevation is a convenient standard to use. If you want to utilize RTK display values on site, first confirm whether those numbers have been translated into this common language. Understanding the concept of elevation is not merely memorizing terms but provides the foundation for handing over height information on site without misunderstanding.

With this perspective, it becomes clear why values obtained from RTK are not used as-is but are deliberately corrected by the geoid height to convert to elevation. Heights handled on site need to be tied to a social reference surface, not merely the vertical component of a spatial position. Therefore, elevation’s essence is not “distance to the ground” but “which reference surface the height is measured from.” Although height may seem like a single thing, in practice different types are used depending on the application.

What is ellipsoidal height

Ellipsoidal height is the height from a mathematically smooth rotational ellipsoid representing the Earth to a point on the ground. In satellite positioning, the observed geocentric coordinates are converted to latitude, longitude, and ellipsoidal height to determine position. In other words, the height GNSS directly determines is not elevation tied to mean sea level but first the ellipsoidal height. Understanding that the heights RTK refines in real time are, at base, ellipsoidal heights makes the situation easier to organize.

A strength of ellipsoidal height is its very good compatibility with satellite positioning. The Earth ellipsoid is convenient to handle as a computational reference surface and aligns well with global coordinate computations, making it suitable for consistently determining positions. On the other hand, the ellipsoid is not mean sea level. Therefore, however precisely ellipsoidal height is determined, by itself it does not have the same meaning as the elevation needed on site. Between improving RTK precision and obtaining elevation that can be read directly on site, an additional translation via the geoid is required.

From a practical viewpoint, ellipsoidal height is not an unnecessary value but the source data for obtaining elevation. Recent guidelines revise that calculations of distances on the reference surface by total stations and similar instruments use ellipsoidal height, and ellipsoidal height is shown to be derivable from elevation and geoid height. This means ellipsoidal height continues to play an important role in satellite positioning and coordinate computations. Because elevation is often used on site, understanding the ellipsoidal height that underlies it deepens the interpretation of calculation results.

If you operate without knowing this difference, you will be confused when comparing with design heights or management heights. For example, for as-built verification you want the difference from drawing heights and consistency with control point results. If the comparison target is made in elevation but only the measured values remain in ellipsoidal height, it is natural that the numbers will not match. When looking at RTK heights, it is very important to accept the fact that “the height determined by satellites” and “the height you want to manage on site” are not the same.

What is geoid height

Geoid height is the height from the ellipsoid to the geoid. It is helpful to think of it as the value that expresses how far the ellipsoid used in satellite positioning and the geoid used as the elevation reference are separated. Official explanations also state that to correctly obtain elevation, both accurate ellipsoidal height and accurate geoid height are necessary. In other words, geoid height is not a subsidiary afterthought but a central element for translating ellipsoidal height into elevation.

Geoid height becomes necessary because Earth’s gravity distribution is not uniform. The reference surface obtained by extending mean sea level beneath the land is not a simple geometric surface but one influenced by gravity. Therefore, the separation between ellipsoid and geoid varies by location. A correction that works at one point does not necessarily apply unchanged at another. Heights are not so simple as to allow subtracting the same fixed value anywhere in the country to get elevation.

For this reason, handling elevation with RTK requires a geoid model appropriate for the region. Even if a device or software shows “geoid correction applied” or “elevation display,” if it is unclear which model is used, differences may remain when comparing with known point results. When confused about heights, what you should check is not only the positioning method but also which geoid model is operating behind the scenes. This perspective makes a practical difference, especially when checking against known point results or public coordinates.

Geoid height is, so to speak, a “translation dictionary” between ellipsoidal height and elevation. Just as a different dictionary produces different translations, different models or references change how heights are read. Those experienced with RTK tend to focus on the stability of positioning itself, but regarding height, confirming how geoid height is handled is required before the numbers can be used in practice. The shortcut to reducing height discrepancies is not to avoid the technical term geoid height but to accept it as a necessary conversion step on site.

Understand the relationship between elevation and ellipsoidal height with a formula

The relationship between elevation, ellipsoidal height, and geoid height is very clear in a formula. Elevation = Ellipsoidal height − Geoid height. Once you internalize this formula, it becomes immediately clear what to check when confused by RTK height displays. You convert the ellipsoidal height determined by satellite positioning into the elevation used on site by reflecting the region-specific geoid height. That is the basic idea.

For example, suppose the ellipsoidal height at a certain point is 92.380 m (303.084 ft) and the geoid height at that point is 37.120 m (121.785 ft). The elevation in that case is 55.260 m (181.299 ft). If 92.380 m appears on the screen and that is the ellipsoidal height display, it is natural that it differs from the elevation of 55.260 m. Conversely, if the display shows elevation after geoid correction, it is easier to compare directly with drawings and known points on site. The issue is not that numbers differ but that you do not know what is being displayed.

Another important point is that the accuracy of elevation is not determined by ellipsoidal height alone. Official materials also state that the precision of elevation determined by satellite positioning is governed by both the accuracy of the ellipsoidal height and the accuracy of the geoid height. In other words, even if RTK fix rates and reception conditions are good, if the geoid model used or the height reference settings are inappropriate, the result may not be adequate as the elevation you want. Sites that carefully manage heights especially need this two-step perspective.

Understanding this by formula changes how you view RTK heights. What was once lumped together as “height doesn’t match” can be decomposed into questions such as “Is the ellipsoidal height correct?”, “Is the geoid correction appropriate?”, and “Is the comparison target using the same elevation system?” By breaking down the problem, identifying the cause becomes faster. Sites strong on height are not necessarily those that handle equipment best but those that can verbally explain differences in reference surfaces.

Points to check when reading heights on site

The first thing to confirm when reading heights with RTK is whether the displayed or output value is ellipsoidal height or elevation. Even if the screen simply labels it “Height,” internal settings or output formats may distinguish them. For comparison with known points, construction management, as-built verification, geotagged photos, or point cloud registration, correct judgment is impossible unless the type of height being compared matches. Height checking work starts by aligning definitions before looking at the numbers.

Next, check whether geoid correction is applied and which geoid model is used. For public surveying in Japan, GNSS height surveying using “Geoid 2024 Japan and Surroundings” was introduced on April 1, 2025. Therefore, if you want to be consistent with public survey results or corresponding height management, operating while ambiguous about which geoid model is used is risky. Some software or devices may expect older models or different file formats, so it is necessary to include checking the file format compatibility in pre-inspection.

Even more important is the era of the known points or results you are comparing against. National control point results are distinguished between the pre-revision “Geodetic Results 2011” and the post-revision “Geodetic Results 2024.” When height figures do not match, the cause may be not a measurement failure but that the comparison target was created under the old results or the new results. Especially for projects spanning multiple years, reuse of past data, or checking against existing drawings, failing to indicate which height system the values were created in will spread confusion downstream.

Also, how height columns are labeled and noted in shared on-site documents should not be overlooked. If the height column in coordinate tables, photo management tables, point cloud results, or as-built reports is simply labeled “H” or “Height,” someone later may confuse ellipsoidal height and elevation. Keeping the height type, the geoid model used, and the reference results together makes documents strong for handover and rechecking. Many height problems occur not during observation but during information sharing, so it is safer to assume that.

In remote islands or special regions an additional caution is required. Official Q&A explains that some remote islands cannot be converted using a geoid matched to the Tokyo Bay mean sea level alone, and when converting ellipsoidal height to elevation an additional datum correction amount must be used together with geoid height. Islands south of the Tokara Islands or south of Hachijō Island require reading heights under such additional conditions. This point can be overlooked in mountain or urban sites, but for projects covering wide areas it is practical knowledge to keep in mind.

Finally, it is important each time to verbalize what height you actually need on site. The required output differs depending on whether you want to compare with drawings, check consistency with known points, record pre-construction conditions, or use the height for point clouds and photos. Sites that master RTK heights are not merely those that read numbers but those that can separate tasks that require elevation from those that can work with ellipsoidal height. Making that distinction can greatly reduce unnecessary re-measurement and unexplained discrepancies.

Latest practical notes to keep in mind in Japan

Japan’s practical approach to heights has been significantly reorganized in recent years. The country is advancing the transition to an elevation system based on satellite positioning, and the idea of obtaining elevation using continuously operating reference stations and geoid models has begun to be institutionalized. As a result, the relationship between ellipsoidal height and elevation, which used to be treated as separate things, is now handled more directly in practice via geoid height. Understanding RTK heights thus approaches understanding survey practice itself rather than mere device knowledge.

One oversight common during this transition is how to read results tables. Official explanations indicate that for the transitional period, CRN station results tables will be treated in a way that allows conversion between elevation and ellipsoidal height, and that ellipsoidal height entries will be removed from results tables a few years later. In other words, the assumption that results tables will naturally include ellipsoidal height as before may no longer hold. Sites that use known points as a basis for work should share in advance how to read results tables and how to compute and extract heights.

Also, geoid models themselves have been updated. The “Geoid 2024 Japan and Surroundings” available after April 1, 2025 uses a changed format from previous models, and users are advised to confirm that their software can read it correctly. This is not merely file management: even if you think you are performing the same calculations on site, different underlying models or formats can produce differences in results. If you seriously want height consistency, you should include the version of the baseline data used in computations among the items you manage, not only observation conditions.

Even for projects not classified as public surveying, using old elevation values is not immediately prohibited; however, official Q&A requests that you be able to confirm whether new or old elevation is being used. This approach is useful in private construction and maintenance as well. If you continue to use existing results depending on site conditions, explicitly stating “which height is being used” helps prevent trouble later during comparison or renovation.

Seeing these changes, it becomes clear that what is truly necessary for RTK height management is not memorizing difficult theory. What is needed is an attitude of checking one by one which reference heights, which model, and which results you are comparing. If you understand the difference between ellipsoidal height and elevation, you can more easily reorganize on-site verification procedures even when regulations are revised or models are updated. The first step to becoming an engineer who does not get confused by heights is to accurately grasp the meanings of the terms.

How to make RTK height management more practical

The conclusion to remember when looking at RTK heights is very simple. Satellite positioning directly determines ellipsoidal height, and the elevation you want on site is obtained by subtracting the geoid height from that. Therefore, when reading the height on an RTK screen, most confusion can be avoided by checking “Is the current value ellipsoidal height or elevation?”, “Which geoid was used for that conversion?”, and “Is the comparison target in the same height system?” Thinking of height understanding not as abstruse theory but as learning a verification sequence makes it easier to put into practice.

On top of that, if you want to quickly perform control point surveys, known point checks, and local coordinate confirmations on site, it is important to choose tools that make handling position and height easy on the spot. LRTK is presented as a centimeter-level (half-inch accuracy) GNSS terminal that attaches to a smartphone, and it is well suited for daily on-site checks. High-performance equipment alone does not ensure correct reading of height numbers, but if you understand the difference between ellipsoidal height and elevation and have an environment where you can immediately confirm the reference on site, simple surveys and daily height checks become much easier.

By utilizing LRTK, an iPhone-mounted GNSS high-accuracy positioning device, it becomes easier to verify position and height information obtained on site immediately. For those who have been confused about how to view elevation, switching to an operation where you check “what reference is this height based on” rather than simply looking at numbers greatly increases the value of RTK. Using LRTK with an understanding of the difference between elevation and ellipsoidal height facilitates efficiency in control point surveys and local coordinate checks, as well as precision management for everyday simple surveys.