The demand for a high resolution
geoid model has grown
substantially during the last
few decades especially after
inception of Global Positioning System
(GPS). Many countries across the
world have already developed their
own geoidal model which serve as the
means of deriving orthometric heights
from GPS observations. The impact
of GPS on surveying application is
undeniable. More so, this revolution
has not been confi ned to the surveying
community, but has extended into
mapping, navigation and Geographic
information system (GIS) areas.
During the last few years, we have
been witnessing the wide spread
adoption of GPS with an equivalently
vibrant range of accuracy requirement.
Many of these applications require
accurate vertical positions.
The task of transforming the
ellipsoidal height obtained from GPS
technique to the orthometric height
has prompted geodesists around the
world to determine the high precision
geoid undulations, for their region
of interest. In India the present day
nation wide geoid was computed a
long time back and based on astro
geodetic observations with respect
to Everest spheroid. It has various
limitations and does not have any
signifi cance as far as GPS solutions
for orthometric height is concerned.
Present study was taken up to
validate the results of orthometrc
heights derivation in a pilot project
of large scale mapping of a part
of Delhi through Airborne Laser
Terrain Mapping(ALTM) Technique.
A fairly dense gravity anomaly data
consisting of about 160 uniformly
distributed points covering a block of 1° X 1° including National Capital
Region(NCR) of Delhi was used in the
geoidal modelling process .The study
was aimed at to analyse approach
of data preparation and treatment
procedures and a evaluation of test
results obtained from the analytical
solution of Stokes’ integral with
appropriate Kernel modifi cations.
Gravimetric Geoid and GPS
The geoid can be broadly defi ned an
equipotential surface of Earth’s gravity
fi eld that closely approximates with
mean sea level (MSL) neglecting
long term effect of sea surface
topography (SST). The fundamental
relationship between the geoid and
reference ellipsoid is given as:
h = H + N
Where h -> ellipsoidal height
H ->orthometric height Geoid
N-> separation between geoid ellipsoid
termed as geodial undulation.
The relationship can be more
clearly shown in fi gure 1.
Geoidal undulation (N) is required for many geodetic and surveying
applications the most notable of
these being the need for converting
GPS-derived ellipsoidal height (h)
to orthometric heights (H). The
reference surface for orthometric
heights was tradionally defi ned by
the MSL measured at one or more
tide gauges and realized through
geodetic levelling in India for example
the datum for orthmetric height
was defi ned in 1909 using the MSL
data furnished from 9 tide gauges
sites at Karachi, Bombay, Karwar,
Beypore, Cochin, Nagappattinam,
Madras, Vishkhapatnam and
false point (Burrard, S.G, 1910).
The datum defi ned in 1909 is
still in use and suffi ce most of
the practical applications.
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