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etween 7 to 12 mm and 11 to 16 mm
for horizontal and vertical, respectively.
The RMS of fitting is 5.6 mm, 5.1 mm
and 6.3 mm for the northing, easting
and height components respectively.
Determination of Brunei
datum transformation
Once the adoption of ITRF2005 has been
completed, the next task is to define the
relationship between the local geodetic
system BT48 and the newly established
geocentric datum of Brunei Darussalam
2009 (GDBD2009). For this reason, the
old primary and secondary triangulations
of BT48 stations (see Figure 1) need to be
revisited. These triangulation stations are to
be observed using GPS technique in order
to tie with the GDBD2009 reference frame.
The most challenging task was to arrange
an expedition to visit all of these stations
(except for station B09, see Figure 1)
since they were remotely located on
the mountainous area in the Borneo
tropical rain forest. A GPS campaign
was conducted in January and March
2009. Snapshot of the location of these
triangulation stations are given in Figure 5.
During the GPS campaign, data was
collected over 48 hours duration at each
station. The GPS data was processed
using similar procedures as discussed
in Section 2.0. The only difference is
that the station coordinates of Brunei’s
CORS in GDBD2009 reference frame
were held fixed. Result from this network
adjustment has shown that the RMS fitting
is between 0.5 to 4mm and 2-10mm for
the horizontal and vertical, respectively.
The next step is to adopt the Bursa-Wolf
model to derive a single set of seven
transformation parameters between the
GDBD2009 and BT48 using coordinates
of common points. Figure 6 shows the
derivation flow of this transformation
procedure. Basically, the transformation
procedure requires an ellipsoidal height
from each station, and this creates a
problem since there are only a few stations
of BT48 with the height information. In
addition, the existing height values are
provided in ‘Mean-Sea’ Level (MSL),
38 | September 2011
which probably was updated by the local
authority using a local vertical datum.
To overcome the above problem, a
‘reverse’ height computation strategy
was implemented. There are three (3)
computation steps involve in this strategy,
which can be summarized as follow:
Step 1: Computation of
homogenous BT48 station
orthometric height (H BT48 )
As the common point’s
coordinates are available in
the GDBD2009 reference
frame, the derivation of
the geoid height (N EGM08 )
of each station from the
global Earth Geopotential
Model 2008(EGM08)
grid can be carried out.
Since the ellipsoidal height
(h GDBD2009 ) is available, the
computation of H BT48 for
each station is given by,
H BT48 = h GDBD2009 – N EGM08 (1)
Step 2: Computation of BT48
station geoid height (N BT48 )
To compute the N BT48
value, a multiple regression
equation produced
by the U.S. Defense
Mapping Agency (DMA) – ‘local geoid
height on Timbalai 1948 datum’ (see
DMA, 1987) has been employed. The
regression equation is given by,
N BT48 = -1.703 - 6.806.U – 7.143.V +
18.663 U 3 + 23.300.UV 3 -13.211.U 5 -
10.642.U 4 V - 1.909.V5 - 36.586.U 3 V 5 -
28.381.U 3 V 7 (2)
Fig 4: Combined Daily Repeatability
(a) (b) (c) (d)
(e) (f) (g) (h)
Fig 5: Some of the Primary Triangulation Stations: (a) Bukit Agok; (b) Bukit Telingan; (c) Bukit
Tunggulian; (d) Bukit Bedawan; (e) Bukit Sagan A; (f) Bukit Miri; (g) Bukit Lambir and, (h) Bukit
Bub Rumah.