JAPANESE GEODETIC DATUM

Survey-II

Group: 5

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WHAT IS A GEODETIC DATUM?

A geodetic datum is an abstract coordinate system with a reference surface (such as sea level) that serves to provide known locations to begin surveys and create maps.

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TYPES OF DATUM:

1. Horizontal Datum:

The horizontal datum is the model used to measure positions on the earth.

2. Vertical Datum:

A vertical datum is used for measuring the elevations of points on the sea level.

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The Geospatial Information Authority (GSI) of Japan is responsible for the

establishment of the Japanese Geodetic Datum

(and also its revision).

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HISTORY OF JAPANESE GEODETIC DATUM (JGD):Until the enforcement of the amended Survey Act on April 1, 2002, the local geodetic reference system had been used in Japan, based on the origin point at the former Tokyo Astronomical Observatory, Azabudai in Tokyo, whose longitude, latitude and azimuth were determined by astronomical observation carried out in the 19th century, and based on the Bessel Ellipsoid adopted by the Meiji Government. 6

After 2002 the Japanese Geodetic Datum 2000 (JGD2000) was established in accordance with ITRF94 (epoch 1997.0) and GRS80 ellipsoid.

ITRF stands for International Terrestrial Reference Frame.

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EARTHQUAKES AND THEIR EFFECTS:

Japan is located on the plate boundary and is frequently suffered from large earthquakes such as the 2011 off the Pacific coast of Tohoku Earthquake. Due to this Earthquake:

“Owing to accumulated strains and crustal deformations caused by plate subduction, some areas have had considerable gaps between static JGD2000 and actual coordinates estimated by GNSS observations.”

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As a result of the 2011 Earthquake,

JGD-2011 was established, which is still used today.

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DIFFERENT GEODETIC DATUM USED IN JAPAN:

1. The Tokyo Datum

2. JGD 2000

3. JGD 2011

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THE TOKYO DATUM:In 1874 USA determined the longitude difference between Tokyo and Nagasaki. The Geographic Department of the Interior Ministry also carried out the determination of the longitude difference between Nagasaki and Tokyo from 1879 to 1880. The average of both of these was considered the Longitude of Tokyo.

Concerning the latitude of the origin point, the value determined by the astronomical observation at the Tokyo Naval Observatory in 1876 was adopted as the standard until the Survey Act Amendment.

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JGD 2000:In 1998 March, the new policy, which includes the adoption of a Geocentric Geodetic System, the Japanese Geodetic Datum 2000, and the establishment of realization of geodetic results or the Geodetic Coordinates 2000, was decided.

The Aim was to provide a standard for positioning at any time, at any location, and at the required accuracy.

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Geocentric System takes the center of Earth as its reference point.

Geocentric Datum are more compatible with satellite positioning systems, such as GPS, than are local Datum.

Geocentric Geodetic System was adopted because:

1. Needs for accuracy of the geodetic datum, which has been rapidly increasing under the circumstances of rapid progress on survey technology.

2. Difficulty in maintaining a big number of control points.

3. Adoption of a geocentric geodetic system recommended by the international organizations.

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And so considering all these points the ITRF94 fixed at the epoch of 1 January 1997 0:00 UT was used which used the GRS80 Ellipsoid to establish JGD2000.

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JGD 2011:

Events that led to the establishment of this system:

In 2011 Pacific coast of Tohoku Earthquake stuck Japan after which there was seen a difference between the points established by JGD2000 and those by GNSS. This led to the establishment of a new Geodetic Datum i-e JGD2011.

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TOHOKU EARTHQUAKE:

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The solution was to calculate new coordinates of GNSS-based Control Stations on newest International Terrestrial Reference Frame (ITRF2008), and then the coordinates of triangulation stations were calculated through re-survey and re-calculation using correction parameters. The new set of revised survey results together with existing results in un-revised areas are now called Japan Geodetic Datum 2011 (JGD2011).

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VERTICAL DATUM:

Two systems were used in Japan for this purpose:

1. Tokyo Bay Mean Sea Level

2. Japanese Vertical Reference System 2000

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TOKYO BAY MEAN SEA LEVEL:

Survey Act Article 11 describes that position should be principally represented by:

1. Geographical Latitude

2. Longitude,

3. Height above the Mean Sea Level.

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The “Mean Sea Level” of the article is defined in the Cabinet Order of the Survey Act, which describes that the numeric value of Nihon Suijun Genten(the origin point for the vertical datum of Japan) is 24.4140 meters above the mean sea level at the Tokyo Bay.

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JAPANESE VERTICAL REFERENCE SYSTEM 2000:In the Japanese vertical reference system in 2000, the Orthometric Height System is adopted, which displaced the normal orthometric height system used in the former vertical system, the Japanese vertical reference system, in 1969.

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ORTHOMETRIC HEIGHT:

Orthometric Height is defined as the length along a plumb line between a point and the standard equipotential surface (Geoid).

While a general method for determining an orthometric height is a leveling survey, a sum of measured values at each measuring point with a leveling survey without any gravitational correction is not coincident with the orthometric height, because of the effect whereby equipotential surfaces are not exactly parallel.

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END

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