What is Geoid Height?
Basic Knowledge of Height Reference in Surveying
Published April 18, 2025
Have you ever found yourself wondering, "What is geoid height?" while performing GNSS surveying on-site? Many people are often confused by this technical term that suddenly appears when measuring height, and may feel uncertain, asking themselves, "What's the difference between this and the usual elevation?" In fact, geoid height is a crucial reference for obtaining accurate height (elevation) data.
In this article, we will provide a clear explanation of what the geoid and geoid height are, specifically for professionals in construction, civil engineering, and surveying. We will cover the differences between geoid height and elevation, its importance, and how to handle it in practice, from the basics. By deepening your understanding of the correct height reference, you can improve surveying accuracy!
Table of Contents
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What is the Geoid?
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What is Geoid Height?
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Difference from Elevation – The Relationship Between Ellipsoidal Height and Geoid Height
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Why is Geoid Height Important?
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Practical Use of Geoid Height in the Field
What is the Geoid?
In simple terms, the geoid is the "hypothetical surface extending the mean sea level onto land." In Japan, the average sea level of Tokyo Bay is used as the reference for height 0m (i.e., the reference surface for elevation), and the geoid is defined as the extension of this sea level surface downward through the Earth.
When considering the entire Earth, the sea level (mean sea level) varies in height due to differences in gravitational strength at different locations, resulting in a bumpy surface. This uneven gravitational surface is the geoid and serves as the "zero height" reference surface.
At any point on the geoid, the gravitational potential is equal (i.e., it is gravitationally level), meaning water remains stationary on the geoid surface. In other words, the geoid is the "surface where water naturally settles horizontally" on Earth.
For example, even in areas that appear flat topographically, water will flow slightly if the distribution of gravity is uneven. Therefore, the geoid, which reflects both the topographical elevation and gravitational differences, is used as the reference for elevation. By defining the geoid as the 0m reference surface, the direction of height—where "water flows from higher to lower points"—aligns accordingly.
It is important to note that the geoid is not the shape of the Earth itself but rather a hypothetical reference surface influenced by gravity. Unlike the mathematically defined smooth rotational ellipsoid, the geoid is uneven. Compared to the reference ellipsoid, which is close to the shape of the Earth (described later), the geoid surface has undulations of about ±100m globally, and even within Japan’s landmass, this difference is about 30-40m.
This "discrepancy from the ellipsoid" is referred to as geoid height.
So, what exactly is geoid height?
What is Geoid Height?
Geoid height refers to the difference in height between the previously mentioned geoid surface and the reference ellipsoid (or geodetic reference ellipsoid) used in surveying. The reference ellipsoid is a rotational ellipsoid that approximates the shape of the Earth and serves as a virtual sphere, providing the basis for GNSS positioning and coordinate systems on maps.
For example, in Japan's geodetic system, the GRS80 ellipsoid is used (which is almost identical to WGS84), and satellite positioning calculates height based on this ellipsoid surface. The height directly obtained by the GNSS receiver is the vertical distance from the Earth's ellipsoid to the survey point, which is referred to as ellipsoidal height.
On the other hand, the elevation (height above the ground) that we commonly use is based on the geoid. In other words, elevation is the height from the geoid surface (mean sea level) to the survey point. Geoid height acts as a bridge connecting the two, and it is defined as the difference between ellipsoidal height and elevation. In other words, geoid height is the vertical distance between the reference ellipsoid and the geoid. It indicates how much the geoid is above or below the ellipsoid surface, and this value varies from location to location.
In Japan, the geoid surface is generally positioned several tens of meters below the reference ellipsoid. Therefore, the geoid height is a positive value (meaning the ellipsoidal height is greater than the elevation). For example, around Tokyo Bay, the geoid height is approximately 30 meters, while in mountainous areas, it can exceed 40 meters. By using the geoid height value, it is possible to calculate the actual elevation (height above sea level) from the ellipsoidal height obtained through satellite positioning.
Difference from Elevation – The Relationship Between Ellipsoidal Height and Geoid Height
Now, let’s clarify the relationship between elevation, ellipsoidal height, and geoid height. Below is a summary of each term:
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Elevation (Orthometric Height): The height from the geoid surface (mean sea level) to a point on the Earth’s surface. This is the commonly used height in maps and construction, often referred to as "height above sea level." It is a gravity-based height reference, managed through benchmarks like leveling points.
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Ellipsoidal Height (Ellipsoidal Height): The height from the reference ellipsoid surface to a point on the Earth's surface. This height is directly obtained through GNSS positioning and is a geometric height based on an artificial rotational ellipsoid. Because it is measured from a smooth ellipsoid centered on the Earth’s center of mass, variations in gravity are not considered.
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Geoid Height (Geoid Height): The difference in height between the reference ellipsoid surface and the geoid surface. This is the displacement between the two height reference surfaces and acts as a correction value connecting ellipsoidal height and elevation. It varies depending on the latitude and longitude of the location and is calculated using a geoid model.
From this, the relationship between elevation, ellipsoidal height, and geoid height is expressed by the following formula:
Elevation = Ellipsoidal Height – Geoid Height
This formula shows that, for example, by subtracting the geoid height from the ellipsoidal height obtained from GNSS, the elevation can be calculated. In other words, by applying the geoid height correction to the ellipsoidal height, the height based on mean sea level is obtained. In practice, because the geoid surface and the ellipsoid surface are not perfectly parallel, there are slight differences, but these are negligible for typical surveying purposes.
Why is Geoid Height Important?
So, why is geoid height considered so important? There are two main reasons:
1. To Obtain "Physically Meaningful Heights"
As mentioned earlier, heights that ignore the influence of gravity cannot accurately represent real-world water flow or the slope of the level. For instance, if only ellipsoidal height is used as the height reference, variations in gravity can cause phenomena such as water flowing from lower to higher elevations. This can be disastrous for infrastructure design and flood control planning. Therefore, the geoid is used as the reference for height, and elevation is preferred. When using the geoid as a reference, the elevation difference directly represents the direction of water flow (high to low), which is both physically and intuitively consistent.
2. Practical Necessity with the Spread of GNSS Surveying
With the increasing use of satellite positioning (such as GPS and Quasi-Zenith Satellite System) in the field, GNSS surveying (including RTK-GNSS) has become widespread. The height obtained directly from satellite positioning, as mentioned earlier, is the ellipsoidal height, which differs from traditional elevation values. However, what we seek are the elevations used in topographic maps and designs, so to obtain elevation from GNSS measurements, it is necessary to correct with geoid height. Without considering geoid height, the height could deviate by several tens of meters, rendering it unusable for practical purposes. In fact, even GPS on smartphones handles elevation differently depending on the platform. Android returns ellipsoidal height, while iOS applies geoid model corrections internally to display a value closer to elevation.
If you're unaware of this difference, confusion may arise when the same location shows height discrepancies of tens of meters depending on the device. This highlights how crucial geoid height corrections are in height measurement.
In summary, the reason geoid height is important is because it is an essential correction value for obtaining "physically meaningful elevations." Especially now, with GNSS becoming the mainstream method for efficient surveying, if geoid height is correctly handled, high-precision elevation can be obtained instantly without the need for leveling surveys. Conversely, using GNSS height measurements directly without understanding geoid height is highly risky and can lead to significant errors due to a lack of understanding of the elevation reference.
Practical Use of Geoid Height in Surveying
So, how should geoid height be utilized in actual surveying work? Here are some key points and practical methods for using geoid height on-site.
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Applying Geoid Correction with GNSS Receivers:
Many modern high-precision GNSS devices (such as RTK receivers) now come with built-in geoid models, allowing for real-time conversion of ellipsoidal height to elevation. By selecting the appropriate geoid model for the region (such as "Japan's Geoid ◯◯") in the receiver's settings, the height displayed will automatically be adjusted to elevation (after applying the geoid correction). This makes it convenient for surveyors, as they do not need to worry about performing complex calculations. However, special attention should be paid when setting up reference stations, especially when inputting coordinates. It’s important to confirm whether the system requires ellipsoidal height or elevation to avoid using the wrong reference surface. For example, if the elevation of a known point is incorrectly set as ellipsoidal height in the reference station, it could introduce geoid height errors throughout the measurements. -
Manually Converting Using Geoid Models:
If your GNSS device does not have an integrated geoid model, or if you need to process existing survey data to obtain elevation, you will need to manually apply geoid height corrections to the ellipsoidal height. You can use the official geoid models provided by the Geospatial Information Authority of Japan (GSI), which offer grid data covering the entire country. For example, GSI's website has a "Geoid Height Calculation" page where you can enter latitude and longitude to calculate the geoid height for a specific location. Additionally, you can download geoid model files (such as "Geoid 2011 Japan" or the latest "Geoid 2024 Japan") and reference them in GIS software. By subtracting the corresponding geoid height from the ellipsoidal height obtained via GNSS, the elevation can be calculated. The accuracy of the official geoid models is very high, with the standard deviation between the actual elevation and geoid-based height being around 2-3 cm. If applied correctly, this method can achieve the same level of precision as traditional leveling surveys. -
Be Aware of Error Factors:
While geoid height allows for easy conversion from ellipsoidal height to elevation, there are several considerations in practice. One example is the potential confusion when setting up the reference surface at the reference station, as mentioned earlier. Similarly, you must be cautious about which version of the geoid model and reference system is being used. Japan's geoid models have been improved over time, with versions like 2000, 2011, and the newer 2024 version. Newer models generally offer higher accuracy (for example, Geoid 2024 Japan is recommended for public surveying after 2025). There can be differences of several centimeters in geoid height between old and new models in some regions, so it's important to use the correct model if specified. Additionally, GNSS positioning accuracy, atmospheric errors, and multipath effects may also impact the final elevation measurements. While geoid height corrections are theoretically accurate, it is important to account for observational errors and perform measurements with a margin of safety.
We have discussed the fundamental knowledge of geoid height, its relationship with elevation, and its importance. By understanding the simple concept that geoid height = shift in height reference surface, you can properly handle the height data obtained through GNSS surveying. In modern surveying, using geoid height for elevation corrections is now standard practice. A correct understanding of height references directly impacts construction precision and the alignment with drawings and designs. We hope this article helps you deepen your knowledge of geoid height and apply it to your daily surveying tasks.
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