1.4 Primary Level of Inclination of Geoid Correction
The
primary level of inclination of geoid inclination used for geoid correction has
been calculated in the order of 1) > 2) > 3).
1) Calculation of geoid undulation of
WGS84 coordinate system
By using the
program gGeoid 96h developed by Geographical Survey Institute, Ministry of
Construction, the geoid undulation of WGS84 coordinate system has been calculated respectively at the points
approximately 500 meters north, south, east and west to the station datum at
each observation station, so that all the surveying points at every observation
station are within its range.
Figure 3 Map of geoid undulation calculation
points
2) Determination of the geoid undulation
of GRS80 coordinate system
Since the
ellipsoidal element and central position of both WGS 84 and GRS80 coordinate
systems did not differ significantly, the same values calculated in 1) as the
geoid undulations of WGS 84 coordinate system are used as the geoid undulations
of GRS80 coordinate system.
3) Calculation of the primary level of
inclination of geoid undulation
As shown in
Table 7, the primary level of inclination of geoid undulation (component xi for
northsouth direction and component eta for eastwest direction) have been
calculated respectively on the basis of the difference of the geoid undulation
at both ends of eastwest direction and northsouth direction (relative geoid
undulation difference) at each
observation station which respectively is one kilometer (1000m) long.
Table 7 Primary level of inclination of geoid
undulation at each station

Koganei station 
Kashima station 
Miura station 
Tateyama station 
Component
xi for northsouth direction 
11.4 seconds 
16.4 seconds 
1.9 seconds 
4.5 seconds 
Component
eta for eastwest direction 
17.4 seconds 
24.3 seconds 
9.3 seconds 
13.6 seconds 
4) Verification of the geoid undulation
by gGeoid 96h
At Kashima
station, the geoid undulation at point (X8) has been determined by calculating
the difference between the relative ellipsoidal height by GPS survey and the
orthometric height by leveling (first class leveling). The discrepancy between
this value and the geoid undulation by gGeoid 96h has been confirmed to be just
8mm and therefore these two values are thought to be consistent.