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First
Order Gravity Survey
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It
was initially carried out in 1988 and completed in April
1992 with a total of 180 points in Peninsular Malaysia (See
map
9). The distribution of gravity control
points is between 30 – 50 km along the Precise Levelling
Network with a misclosure of 30 µgals, this network is known
as the Peninsular Malaysia Gravity Standard Network 1989
(PMGSN’89). PMGSN’89 consists of standard benchmarks
and other relevant stations and is based on the International
Gravity Standard Network 1971 (IGSN71).
In
1998, a first order gravity survey network, named as the
Sarawak Gravity Standardisation Network (SSGSN) was established
in the state of Sarawak with a total of 56 points consisting
of standard benchmarks and benchmarks (map
10). The interval between each station
is between 15 – 25 km. The reference datum was obtained
from the Sabah and Sarawak Gravity Base Network UTM90, which
was linked to IGSN71 (map
11). Another first order gravity survey network
of 40 stations will be established for the state of Sabah
by the end of 1999.
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| Second
Order Gravity Survey |
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Second
order gravity survey was implemented so as to densify the
points in between the first order points. These points were
positioned in between the first order gravity points at an
interval of 5 km for low elevation and 1 km for elevation
higher than 100 m above Mean Sea Level. The rejection criterion
for misclosure is 50 µgals. To date, there are 1242 second
order gravity points (map
12). These surveys were conducted for the purpose
of applying orthometric corrections to the levelling network.
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| Third
Order Gravity Survey |
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DSMM
commenced the third order gravity survey in 1994 with the
objective of establishing gravity points in gridded form at
a density of 25 square kilometer per station. The allowable
misclosure is at 100 µgals. A few problems were encountered
during the field operations mainly due to inaccessibility
such as rough terrain, swampy areas, bad and damaged road.
Other problems are the unavailability of updated topography
map and dense vegetation that hindered the progress of the
fieldwork. To date, a total of 1410 third order gravity survey
points has been obtained. The Minerals Department Malaysia
(MDM) has also contributed another 3895 gravity points in
grid form at a density of 25 square kilometer per station
with the level of accuracy of 100 µgals. Similar data have
also been obtained from UTM, comprising altogether 2921 stations,
of which 1969 points are common with the data included in
the DSMM and MDM data set.
Map 13 shows the distribution
of the third order gravity points.
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| Sea-borne
Gravity Data |
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Following
the request from DSMM, Bureau Gravimetrique Internationale
(BGI) had subsequently provided 39,000 off-shore gravity values,
extending from -4° to 13° in latitude, and 94° to 110° in
longitude. Out of these, 29,662 ship-borne gravity measurements
were extracted, and from that 7,263 points were retained by
DSMM in the gravity database (map
14 )
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A
total of 27,122 free air anomalies, at a grid spacing of 2
minutes (about 3.7 km), covering an area of 0°N to 10°N and
98°E to 106°E were extracted from the global anomalies file
gridded on the Mercator projection (Sandwell, 1982 and 1984;
Smith and Sandwell, 1995). Geosat/ERM, Geosat/GM, ERS-1
and ERS/GM contributed the data from which the grid were derived.
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| Air-borne
Gravity Survey |
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An
Air-borne gravity survey project is being planned to cover
inaccessible areas especially those in the hilly, mountainous
and difficult terrain. This project is programmed to commence
under the 8th Malaysian Development Plan in the
year 2001.
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| Gravity
Database |
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DSMM
had set up a gravity database, the contents of which consist
of collected data from various organisations in the country
that had conducted gravity surveys independently for various
purposes. Among those that had contributed to this database
are DSMM itself, Minerals Department Malaysia (MDM) and Universiti
Teknologi Malaysia (UTM). DSMM had also obtained sea-borne
gravity data from BGI and satellite altimetry data from Geosat/ERM/GM
and ERS-1/GM. In merging data from different sources, the
problem of different formats had to be overcome. To maintain
the database in an orderly manner, the gravity data and free
air anomalies are expanded back into observed gravity based
on IGSN’71. The information is then stored in the format
of latitude, longitude (both in decimal degrees), observed
gravity (mGal with the constant mGal subtracted) and height
of station (m).
In
Peninsular Malaysia, the gravity database is made up of 14,942
terrestrial and sea-borne data (originally, there are 9,648
terrestrial and 7263 sea-borne data, totaling to 16,911 but
minus 1969 duplicate points on land). 27,122 gravity values
derived from satellite altimetry are then added, increasing
it to a total of 44,033 gravity points.
Map 15 shows the distribution
of these data.
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| Geoid
Computations |
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The
Investigation of the Malaysian Height Datum (MHD) project
was first proposed to DSMM in mid 1992 by the University of
New South Wales, Australia (UNSW). After a review and detailed
analysis of the available data as well as intensive discussions,
the project was finally agreed upon in October 1993. The aim
of the project is to use all current data relating to height
control in Peninsular Malaysia such as tide gauges, conventional
levelling, GPS heights and gravity data to produce an optimum
height datum, height control system and geoid for the region.
The
gravimetric geoid heights were evaluated by RING Integration
method by employing gravity data in and around Peninsular
Malaysia and the value obtained were compared with the GPS/Levelling
via BAYCON (Bayesian Least Square and Constraint) adjustment.
Attempts
had been made to produce the best possible geoid for Southern
Peninsular Malaysia. Estimation of the precision of the gridded
N data are mainly based on the results of the control, gravity
coverage and topografic relief, as tabulated in (
table 1),the precision
varies from 1.5 ppm for flat areas and 9 ppm for mountainous
regions. Thus, for areas with sparse gravity data, more measurements
need to be made and a reasonable good DEM had to be produced
for the mountainous areas.
The
important outcome from this optimisation adjustment are that
the levelling network is internally consistent and that there
were no gross errors in it. However UNSW identified two important
aspects which need to be resolved if the country wants to
fully use GPS technology for heighting and positioning:
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Datum
problem caused by quasi-WGS84 status of datum used in
the existing GPS network in Peninsular Malaysia.
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Significant
variations in the quality of the geoid heights caused
by the scarcity of gravity observations in some parts
of this region.
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When
good coverage of gravity data becomes available, a more precise
geoidal map can be produced to achieve centimeter level accuracy.
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