Update to the Geoid-based vertical datum

GSI has updated the geodetic datum to the latest "Japanese Geodetic Datum 2024" realized by GNSS CORS, triangulation points and benchmarks with revised vertical survey results of them on April 1, 2025.
This update aims to eliminate distortions among survey results due to long-term accumulation of crustal deformation and enables us to get orthometric height more quickly by GNSS observation and newly implemented "Geoid 2024: Japan and its surroundings". We expect that the new geoid-based vertical datum will contribute to improving efficiency and productivity in public surveys and construction, and also to developing new services.

In general, survey results of national control points such as GNSS CORS, triangulation points, benchmarks have dislocations from actual coordinates caused by long-time accumulated crustal deformations. Furthermore, conventional leveling method has a disadvantage that the error increases with distance, that is the height of control point far from vertical origin has much more error. To solve these problems, GSI rebuilt the vertical datum to geoid-based one and updated the vertical survey results of control points on Apr 1, 2025.

A height obtained directly from Global Navigation Satellite System (GNSS) is an ellipsoidal height, which is a height from the surface of the reference ellipsoid. On the other hand, we usually use orthometric height from the mean sea level and "Geoid Heights" are required if we want to get such heights from GNSS observation results.
Geoid is an equi-potential surface fit to the mean sea level and its height from the reference ellipsoid is called "Geoid Height". GSI is providing a new Geoid Model "Geoid 2024: Japan and its surroundings" to give a geoid height at an arbitrary point, whose potential is equal to that at Tokyo Bay, rebuilt with airborne, ground, shipborne, sea-bottom and satellite gravity data. For details of the new Geoid Model, please visit [link] (in Japanese).

Our previous vertical datum was based on leveling results along 1st-order leveling route, and vertical survey results of GNSS CORS and benchmarks conformed to the datum.
The new Geoid-based vertical datum is based on ellipsoidal height of GNSS CORS and the new geoid model, complemented by readjusted leveling results of benchmarks referred to a marker of nearby GNSS CORS.

The new datum has advantages as follows:

• discrepancy between vertical survey results and actual orthometric heights caused by long-term accumulation of crustal deformation are solved.
• orthometric height can be obtained more quickly than before by GNSS observation and the new geoid model, which leads to improvement of efficiency and productivity in construction as well as promptness in reconstruction and recovery from disasters.
• accumulated errors propagated from the origin can be mitigated.

Previously the height systems of triangulation point network and benchmark network had been slightly different. However, this time both orthometric heights of triangulation points and benchmarks were revised with the datum updates mutually in consistent. Maximum changes of the height of triangulation points are +57cm at Oshika Peninsula, Miyagi and -67cm at Shiretoko Peninsula, Hokkaido. For benchmarks, +23cm at Oshika Peninsula, Miyagi and -40cm at Betsukaicho, Hokkaido. Extremely high discrepancy around Miyagi area would reflect the effects of the large earthquake in 2011 ("The 2011 off the Pacific coast of Tohoku Earthquake"). Widespread minus trends toward Hokkaido would come mainly from error accumulation by leveling.

When you calculate geoid height in the survey after Apr. 1, 2025, the new geoid model "Geoid 2024: Japan and its surroundings" should be used. You can download it in ISG format (filename: JPGEO2024.isg) at "Fundamental Geographic Data Download Service site (in Japanese) ". You can also use "Geoid Height Calculation Service " to calculate geoid height at each specified point.
* Please note that this time the format of geoid model was changed from GSI's own format to the common ISG format recommended by International Service for the Geoid, International Association of Geodesy. For details of the format, please see the documentation of "Geoid 2024: Japan and its surroundings". The format of GML files (250m mesh data) has not been changed. Please make sure the new geoid model can be input into your software before you use it. XML-formatted files (2km grid data) are not provided.

"Geoid 2024: Japan and its surroundings" is an on- and off-shore seamless geoid model equivalent to the mean sea level of Tokyo Bay. But it has become clear that the sea levels of some remote islands such as Ryukyu Islands, Ogasawara Islands are quite different from those in Japan's main islands. Japan's Survey Act allows remote islands having their own origin for surveys, and we decided to keep holding these exceptional origins although seamless geoid was established. Therefore, the values of ellipsoidal height subtracted by geoid height is NOT orthometric height used for surveys in those remote islands. In such cases, please use "Height reference conversion grid" newly provided for surveys for remote islands additionally with the new geoid model. The conversion grid is used to compute the offsets between the mean sea level of Tokyo Bay and those of remote islands. The relationship of heights for remote islands is as follows;

(Ellipsoidal Height at remote island) = (Orthometric Height) + (Geoid Height) + (the Offset)
Height reference conversion grid (filename: Hrefconv2024.isg) are available at "Fundamental Geographic Data Download Service site (in Japanese) ". GML-formatted files (250m mesh data) are also provided. In addition, files combining offsets and geoid model (JPGEO2024+Hrefconv2024.isg) are also temporarily provided (for old analysis software not supporting these offsets only: manual calculation with combined parameters is not recommended as mistaken choice may lead to incorrect result).

The old leveling-based vertical datums (Japanese Geodetic Datum 2011 (vertical) height, etc.) can't have the reference epochs since it takes quite long time (over 10 years) to complete a nationwide leveling survey. On the other hand, Geoid-based vertical datum can clarify the epoch for the following reasons:

• geoid model is more stable against crustal deformations.
• GNSS analysis takes shorter time than leveling for long leveling route.

As for the new vertical datum "Japanese Geodetic Datum 2024 (vertical)", the epoch of "June 1, 2024" was newly decided.
The clarified epoch of vertical datum enables us to monitor quantitative changes of the height at each GNSS CORS over time since the initial epoch, which contributes to providing secular vertical deformation model to transform from height at initial epoch to that at current epoch (and vice versa). With the secular vertical deformation model, we can deal with heights more accurately reflecting the effect of crustal deformation than before.

PCV (Phase Center Variation) correction is required in static and short-term static baseline analyses as a rule of public surveys in Japan. At the same time as the datum update, antenna phase center models were updated to those correspond to the combinations of antenna and pillar types of the stations from those for antenna types only. More accurate baseline analyses can be realized with a wide variety of antenna phase center models well-matched to the combination of both an antenna and a pillar. New antenna phase center models are available at [link].
When you use the models for surveys using GSI's GNSS CORS, please check the types of the antenna and the pillar of the CORS at the header of the RINEX file or Code List of Reference Points to apply suitable antenna phase center model.