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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.
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.
The Geospatial Information Authority
(GSI) of Japan is responsible for the
establishment of the Japanese Geodetic
(and also its revision).
HISTORY OF JAPANESE GEODETIC
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
ITRF stands for International Terrestrial
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
As a result of the 2011 Earthquake,
JGD-2011 was established, which is still used
DIFFERENT GEODETIC DATUM
USED IN JAPAN:
1. The Tokyo Datum
2. JGD 2000
3. JGD 2011
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
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.
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.
Geocentric System takes the center of Earth as its
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
3. Adoption of a geocentric geodetic system
recommended by the international organizations.
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
Events that led to the establishment of
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
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
Two systems were used in Japan for this purpose:
1. Tokyo Bay Mean Sea Level
2. Japanese Vertical Reference System 2000
TOKYO BAY MEAN SEA LEVEL:
Survey Act Article 11 describes that position
should be principally represented by:
1. Geographical Latitude
3. Height above the Mean Sea Level.
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.
JAPANESE VERTICAL REFERENCE SYSTEM
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.
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