There is a degree of difficulty in relating GPS heights obtained
from Global Positioning System (GPS) surveying to the existing
national leveling networks in most countries where GPS survey
is used alongside traditional survey methods. Heights provided
by GPS relate to the surface of an ellipsoid, which approximates
the shape of the earth, but does not coincide with the geoid.
GPS can only be used in most countries to measure
the heights meaningfully only above a reference datum if the
relationship between the ellipsoid and the geoid is established.
The vertical datum is the reference used for high precision height positioning
measurement. To realise this vertical datum, ground points with known height values
referenced to the vertical datum are established in many countries.
In Singapore, the official Singapore's vertical reference datum used for cadastral
surveys is the Singapore Height Datum, which can be referenced using Singapore's network of control points, individually known as Precise
Levelling Benchmarks (PLBMs).
In order to achieve consistency for all height positioning activities on
land in Singapore, to be aligned with Singapore Height Datum, all levelling
surveys should be linked to PLBMs that are aligned to the Singapore's official height datum.
With the increasing use of GPS in positioning activities, it is now possible to
obtain surface planar coordinates (X and Y) as well as a GPS height (Z). However,
the height coordinate from GPS is referenced to a mathematical model known as an
Note that a GPS receiver on a ship at sea level (tides not considered) during
the course of a long voyage, may indicate height variations, This is because GPS
satellites can only measure heights relative to a geocentric reference ellipsoid
as they orbit about the center of gravity of the Earth. As a result, GPS height
must be corrected to obtain geoidal height while in traditional leveling, the height
obtained will always be geoidal height.
A Geoid is a model of the equipotential surface of the earth's gravity field that
is best approximated by the mean sea level over the oceans which extends hypothetically
beneath all land surfaces (DMA, 1997)
SGEOID09 is the geoid model for Singapore. A good geoid model allows us to determine
geoidal heights using Global Positioning System (GPS) (which yields ellipsoid height).
Geoid model can be used for applications where precise real-time measurement of
height is required. These applications include:
- Engineering surveys
- Vertical deformation monitoring
- Asset Tracking
- Coastal monitoring
Some actual uses of Geoid Models in the world are:
- Engineering deformation monitoring for reservoir embankment
- 3-dimensional coordinates for stations of the London Water Ring
Main project (Dodson, 1995)
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