What is Elevation?
Explaining the Height Reference Defined by the Japanese Geodetic System
Published April 18, 2025
In construction and civil engineering sites, the term "elevation" is commonly used, but the expression "altitude" also exists, which can sometimes cause confusion.
For example, when asked, "What is the elevation (altitude) of this point in meters?", you may have found yourself uncertain about the reference height. In fact, elevation and altitude are often used interchangeably and both refer to the height above the average sea level. However, without a correct understanding of the reference for height, you may end up using incorrect height data on-site, leading to construction errors or inconsistencies in surveying.
This article will explain the definition of elevation and the height reference system in Japan (Japanese Geodetic System), offering a clear understanding. By considering the latest developments of the Japanese Geodetic System JGD2024, we aim to help you apply the correct elevation reference in your work.
Table of Contents
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What is Elevation?
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Height Reference in Japan (Japanese Leveling Origin and Mean Sea Level)
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The Relationship Between the Japanese Geodetic System (JGD) and Elevation (JGD2000 → JGD2011 → JGD2024)
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How to Determine Elevation: A Comparison of Leveling Surveys and GNSS Positioning
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Handling of Height Data on Site: Key Points to Note
What is Elevation?
Elevation refers to the vertical distance from a reference height. Simply put, elevation is the height above the average sea level. In Japan, according to surveying standards, the average sea level of Tokyo Bay is defined as 0m, and the height above this 0m level is considered the elevation.
On the other hand, the term altitude traditionally refers to the height above the average sea level of the nearest sea, but it is now commonly used with nearly the same meaning as elevation. Therefore, in everyday conversations or disaster-related information, when we say "altitude of ○ meters," we are essentially referring to "elevation of ○ meters" (height above the average sea level of Tokyo Bay).
Another similar and often confusing term is height. This term can have different meanings depending on the context. In aviation, "height of ○ meters" may refer to the height above the ground (as measured by an altimeter), but this term is not commonly used in surveying or mapping contexts. Remember, when referring to the height of terrain or land, the correct term is elevation (altitude).
Height Reference in Japan (Japanese Leveling Origin and Mean Sea Level)
In Japan, the government has established a unified height reference. The average sea level of Tokyo Bay (Mean Sea Level) is set as 0m for elevation based on surveying standards, making it possible to express height consistently across the country according to this reference. However, it is not practical to directly measure "the average sea level of Tokyo Bay" in every survey. Therefore, a reference point for height is established on land, and its height is officially defined as the baseline. This reference point is known as the Japanese Leveling Origin.
The Japanese Leveling Origin is a stone marker set at Nagatacho, Chiyoda Ward, Tokyo (in front of the National Diet Building), and is defined as being 24.3900 meters above the average sea level of Tokyo Bay (T.P. = Tokyo Peil). This value has been revised several times historically. When it was first set in 1891, the height was defined as 24.500m. However, due to ground subsidence from the 1923 Great Kanto Earthquake, it was adjusted to 24.414m in 1928. After the 2011 Great East Japan Earthquake, the height was re-measured, and the current value is 24.3900m. The change in the height of the Japanese Leveling Origin is not due to an alteration of the reference point itself, but rather an update to the "height above the average sea level" due to ground movements caused by the earthquakes.
The Japanese Leveling Origin serves as the starting point for leveling points (reference points based on leveling surveys) across the country. By extending leveling surveys from this point, the elevations throughout Japan are determined. During the Meiji era, the height of the sea surface was observed over a long period at Reiganjima in Tokyo Bay to define the average sea level as 0m, and this was used to establish the elevation of the Japanese Leveling Origin. However, because the accurate average sea level could no longer be obtained due to land reclamation and river inflows in the Reiganjima area, the observation site was moved to Aburatsubo Tide Gauge Station on the Miura Peninsula in Kanagawa Prefecture, where the sea level corresponding to Tokyo Bay's average sea level is still measured today.
While the reference average sea level has not changed, the observation site has shifted, but the baseline for elevation at 0m (the geoid) is still maintained.
In summary, Japan has defined "the average sea level of Tokyo Bay = 0m elevation" and fixed this reference surface at the Japanese Leveling Origin on land, allowing consistent height representation across the country. Thanks to this unified standard, elevation comparisons can be made on the same reference surface (geoid) across different regions and periods, making it possible to compare surveying results from various times and locations on the same footing.
The Relationship Between JGD and Elevation (JGD2000 → JGD2011 → JGD2024)
JGD (Japanese Geodetic Datum) is the name of the coordinate system used to define the position reference (latitude, longitude, and height) in Japan. The Japanese Geodetic Datum has been updated over time, with major changes occurring in 2000, 2011, and 2024. Let’s look at each of these updates and their impact on elevation.
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JGD2000 (Geodetic Results 2000):Previously, Japan used a local coordinate system called the "Old Japanese Geodetic System (Tokyo Coordinate System)." However, around the year 2000, with the widespread adoption of global positioning systems (GPS), Japan switched to a global coordinate system. This system, JGD2000, uses the GRS80 ellipsoid, which is nearly identical to WGS84, and is a geocentric coordinate system. The shift to JGD2000 resulted in a significant change, with a shift of about 400 meters in the previously used latitude and longitude values, but the height reference (mean sea level 0m) remained the same, based on the average sea level of Tokyo Bay. In other words, while the horizontal coordinate system was updated, the elevation reference stayed unchanged.
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JGD2011 (Geodetic Results 2011):The Great East Japan Earthquake of 2011 had a major impact on Japan’s geodetic system. The Earth's crust shifted significantly, especially in the Tohoku region, resulting in position shifts of several meters in some areas. As a result, the Geospatial Information Authority of Japan (GSI) reviewed and updated the coordinates of the reference points. The new reference system, JGD2011, uses the same ellipsoid and coordinate system as JGD2000, but the latitude, longitude, and elevation values were updated in the Tohoku region and other affected areas to reflect the correct positions and heights after the earthquake. For example, the Japanese Leveling Origin was revised to 24.3900m after the earthquake. While JGD2000 and JGD2011 showed no significant difference in regions like Hokkaido and western Japan, differences in surveying results were noted in the Tohoku area, prompting the change to JGD2011 to avoid mixing pre- and post-earthquake data.
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JGD2024 (Geodetic Results 2024):In recent years, Japan has transitioned to a new geodetic system based on JGD2024, effective from April 1, 2025. Starting from this date, the official height values of reference points (such as electronic reference points and leveling points) across Japan will be updated to the new "Geodetic Results 2024." The name of the geodetic system will also change from JGD2011 to JGD2024. The goal of JGD2024 is to resolve the discrepancies caused by crustal movements that have accumulated nationwide since 2011 and to revise the height reference using satellite positioning and the new geoid model, "Geoid 2024." This update addresses errors in leveling measurements that had accumulated, particularly in regions far from the Japanese Leveling Origin, allowing for consistent elevation accuracy across Japan. Some regions may see a change in elevation by as much as 60cm. For example, in remote areas and islands, minor errors in the old surveying network are now corrected. It is important to note that horizontal coordinates (latitude, longitude, and plane coordinates) have not changed from JGD2011 in JGD2024. The transition primarily aims to standardize the naming of the geodetic system and update the elevation reference, with the position itself inherited from JGD2011. Through this shift to JGD2024, Japan's surveying system has moved to a new height system based on satellite positioning. The official introduction of GNSS-based height measurements (described later) is expected to enhance the efficiency of surveying and construction.
How to Determine Elevation: Comparison Between Leveling and GNSS Positioning
There are two main methods for determining height (elevation) on-site: traditional leveling and GNSS (Global Navigation Satellite System) positioning. Each method has its own principles and efforts involved, so it’s important to understand their features.
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Elevation Measurement via Leveling:Leveling is a method that uses a level (surveying telescope) and a leveling staff to measure the height difference from a known point. Starting from a known benchmark (a point with a known elevation), the height difference to the next point is measured, and the process is propagated from point to point to determine the elevation of the target point. This method, also known as direct leveling, has very high accuracy—typically millimeter-level error over short distances. It is indispensable for establishing height benchmarks on construction sites and for precise public surveying. However, it is time-consuming and labor-intensive because it involves manually setting up equipment and taking readings. When connecting leveling surveys over long distances (more than a few kilometers), even small measurement errors, such as a few millimeters per kilometer, accumulate. In mountainous areas or locations with limited access, the work can be especially challenging.
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Elevation Measurement via GNSS Positioning:In GNSS (Global Navigation Satellite System) surveying, your position is determined by signals from satellites. Using a dedicated GNSS receiver, you can instantly obtain your three-dimensional position, including height, regardless of the location. This method is especially useful when there are no nearby leveling points, such as when establishing new reference points in remote areas. Additionally, high-precision positioning using multiple satellites (such as RTK-GNSS or network GNSS) now allows for height measurements with an accuracy of almost a few centimeters.However, an important point to note is that the value directly obtained from GNSS is called ellipsoidal height.
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Ellipsoidal Height refers to the height measured from the Earth's reference ellipsoid (such as GRS80), which differs from the elevation (height above mean sea level) that we typically use. To obtain elevation, the geoid height (the difference in height between the reference ellipsoid and mean sea level) must be subtracted from the ellipsoidal height.Fortunately, the Geospatial Information Authority of Japan (GSI) has developed and published a precise geoid model, and by using this model, it is easy to convert ellipsoidal height to elevation. For example, using the "Geoid 2024" model provided by GSI allows for accurate elevation values that align with the latest JGD2024 to be obtained through GNSS.The main advantage of GNSS positioning is its efficiency in obtaining height data over wide areas in a short amount of time, especially in regions without nearby leveling points or for large-scale surveys. However, the drawbacks include the need for specialized equipment and positioning environments, as well as the effort required to convert the data into elevation (through applying the geoid model). Despite these challenges, thanks to recent technological advancements, GNSS-based height measurement (GNSS leveling) has become practical, and the GSI has begun using it for public surveying.
In summary, if accuracy is the priority, leveling surveys are preferred; if efficiency is the focus, GNSS positioning is the better choice. In actual fieldwork, a combination of both methods is often used to leverage their respective advantages. For example, GNSS can be used to roughly determine the height from a known point near the site, and then short-distance leveling surveys can be performed from that point to establish precise height references. With the improvements in GNSS elevation measurement in the new JGD2024, there may be more instances in the future where GNSS alone is sufficient for determining height. However, for precise millimeter-level height determination, traditional leveling surveys are still indispensable. It is crucial to select and combine the most appropriate methods based on the site conditions and the required level of accuracy.
Handling Height Data on Site: Key Points to Be Aware Of
When dealing with elevation and height data, be sure to pay attention to the following key points:
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Verify the Standardization of Reference:
Always be aware of the reference surface used for height data. Even though the unit "m" is used, there can be a difference of several tens of centimeters between the Tokyo Bay Mean Sea Level reference (used for JGD2011) and the new JGD2024. For example, around 2025, you may find a mix of JGD2011 and JGD2024 elevation data. When comparing old drawings with the latest survey results, always verify the reference system (which geodetic datum is being used) and, if necessary, apply conversion parameters or correction software (such as PatchJGD) provided by the Geospatial Information Authority of Japan (GSI). When using network-based RTK-GNSS positioning, ensure that the service has been updated to JGD2024 to avoid mixing height data based on different standards. -
Don't Confuse "Ellipsoidal Height" and "Elevation":
GNSS devices and 3D design data often output height as ellipsoidal height (the height measured from the reference ellipsoid). In Japan, ellipsoidal height is typically 30–40 meters higher than elevation. If you mistakenly use ellipsoidal height as elevation, this could lead to significant errors. Always ensure you know the reference for the height data you are using and apply the geoid model if necessary to convert it to elevation (correct height). -
"Altitude of x meters" Expression:
The "altitude of x meters" you often see on local signboards or hazard maps is generally used interchangeably with elevation. While in the past, some remote islands used local sea levels as the reference, most areas now use the Tokyo Bay Mean Sea Level as the standard. Therefore, altitude = elevation in most cases. However, it is important to be cautious with older documents, as different standards may have been used in rare cases. -
Utilize Official Leveling Points and Known Points:
When establishing new height benchmarks on-site, it is recommended to use nearby official leveling points (known elevation points) if possible. You can use the GSI’s "Reference Point Results Viewing" service to find the elevations of nearby leveling points (already updated for JGD2024). From there, you can transfer the height using leveling surveys, ensuring higher reliability. When using GNSS, it is recommended not to trust the standalone measurement immediately but to verify the device’s height accuracy using nearby electronic reference points or known points. -
Impact of Crustal Movements and Subsidence:
Japan is an earthquake-prone country, and local ground movements can occur due to large earthquakes or volcanic activity. After past disasters, official elevation values have been updated (for example, in the Tohoku region after the 2011 Great East Japan Earthquake), so it is important to stay updated on the latest information. For instance, after the 2011 earthquake, some areas along the Tohoku coast experienced subsidence, making the land lower than what was previously recorded on maps. While public works projects resurvey the area, it’s important to be cautious when referencing old topographic maps or data.
This article has explained the basic concepts of elevation, the height reference system in Japan, and the changes in the Japanese Geodetic System. Elevation is commonly used in daily work, but behind it, there is a unified reference surface (Tokyo Bay Mean Sea Level) and the Geospatial Information Authority of Japan's decades of surveying efforts, allowing for consistent elevation representation across the country.
With the latest JGD2024, satellite-based positioning has become mainstream, making it possible to efficiently and accurately obtain elevation data. By applying the knowledge shared in this article, be mindful of the differences in standards when handling height data on-site to ensure error-free surveying and construction. A proper understanding of the correct elevation reference is the foundation for safe and reliable operations.
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