254
April 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
electronic computer was the ultimate prize of a private geodetic
survey corporation. Sometimes those computers had as much
as 8K of ferrite-core memory. The equivalent British machine
used in Hong Kong was the I.C.L. 2970 computer.)
In the 1960’s, the aphylactic projections were still commonly
used cartographic projections, including the polyconic used in
the U.S., but they were a nightmare for the control surveyor.
When I used to do control surveys with a T-2 theodolite and
an electronic distance measuring (EDM) instrument or an
invar tape, a significant correction for systematic error was
(and still is) for the “scale factor” — the difference between
true (geodetic) distance and map (grid) distance. Although
such computations are straightforward with a conformal
projection,
with an aphylactic projection the scale factor
varies as a function of the azimuth of the line being
measured
.
Things were just dandy for the cartographer and
the photogrammetrist with such grids, but the field surveyor
was perpetually immersed in exasperating daily calculations
because of the HK63 Grid.
Ten years later in Boston, my brain used to go numb just
punching out square roots of the diagonals of variance-
covariance matrices one or two days a month for photo-block
merges. Cadastral survey computations were pure drudgery
in Hong Kong back in the 60’s on a
daily
basis! In line with
the metrication policy of the 1970’s, the British Imperial
(Cassini-Soldner) Grid was converted to metric units of
measure in 1975-77 with the Grid origin further shifted 3,550
meters to the west such that False Easting = 33,016 meters,
and False Northing = 15,240 meters. Some 3,000 sheets were
also converted to 1:1,000 scale. A Photogrammetric Unit was
formed in the Lands Department in 1976.
With the introduction of EDM instruments to Hong Kong
in the late 70’s, the distances between hilltop triangulation
control points were resurveyed in 1978-79 to improve the
consistency and accuracy of the control network. In this
resurvey and adjustment, a new geodetic datum called Hong
Kong 1980 (HK80) was adopted. The definition of the new
datum was now referenced to old Trig 2 on Partridge Hill
where: Φ
o
= 22° 18´ 43.68˝ N, Λ
o
= 114° 10´ 42.80˝ E, and the
azimuth was re-referenced from old Trig 67 (now lost) to Trig
94 turned as 292° 52 58.4”. The Hayford (International) 1909
ellipsoid was adopted where a = 6,378,188 meters and
1
/
f
=
298. The
conformal
transverse Mercator projection (Gauss-
Krüger) was used for the new rectangular grid system and
was known as the Hong Kong 1980 Grid. The new False
Easting = 836,694.05 meters and the new False Northing =
819,069.80 meters were referenced to the new datum origin.
The scale factor at origin was unity (m
o
= 1.0), the same as the
implicit definition of any Cassini-Soldner Grid.
Since the miserable aphylactic grid was gone forever, the
Cadastral Land Surveyor’s life was lifted from “sweat shop”
conditions. The reobservation of the control network of Hong
Kong had been accomplished by laser Geodimeter
®
model 600
positioning of survey points by trilateration rather than by
microwave Tellurometer
®
or by Electrotape
®
observations
because of nearby “radio jamming.” A total of 162 lines had
been measured, and the standard deviation of the combined
network residuals was 0.45 parts per million.
In 1990, the Survey andMapping Office, Lands Department,
started to apply theGPS technique for fixing positions of survey
control points. A territory- wide observation on a network of
15 stations (12 of which are at existing trig stations) was
carried out by the No. 512 Specialist Team, Royal Engineers
of the U.K. Military Survey using GPS (NAVSTAR) and
Doppler (TRANSIT) satellite techniques. The network was
adjusted with high accuracy results. This survey provided a
rigid link between the local HK80 Geodetic Datum and the
global WGS84 Datum. Currently, the WGS84 Datum is used
with a UTM Grid for tourist and hiking maps. The cadastral
and engineering surveys continue to use the HK80 Datum for
day-to-day use, although aided by differential GPS.
In Hong Kong all heights and levels on land refer to the
Principal Datum which was formerly known as the Ordnance
Datum. The HKPD was originally determined by observation of
the tides from 1887-1888. A later observation of the full metonic
cycle (18.6 years) was performed by the Royal Observatory
from 1965-1983 at North Point, Victoria Harbor. “Mean Sea
Level” is approximately 1.23 meters above HKPD. The original
monument was “Rifleman’s Bolt,” a copper bolt fixed in the
Hong Kong Naval Dockyard by personnel from H.M. Surveying
Vessel “Rifleman” in 1866. It is now preserved for its historical
value at the eastern wall of Blake Block in H.M.S. Tamar.
The Chart Datum, formerly known as the Admiralty Datum,
is approximately the level of Lowest Astronomical Tide and is
adopted as the zero point for Tide Tables since 1917. The Chart
Datum is approximately 1.38 meters below HKPD. The Royal
Engineers determined WGS84 ellipsoid heights in 1991. In
general, the WGS84 heights are 2.4 meters higher in the west
and 0.4 meters in the east of Hong Kong. The accuracy of this
geoid separation is estimated to be better than ±0.15 meters.
The contents of this column reflect the views of the author, who is
responsible for the facts and accuracy of the data presented herein. The
contents do not necessarily reflect the official views or policies of the
American Society for Photogrammetry and Remote Sensing and/or the
Louisiana State University Center for GeoInformatics (C
4
G).
This column was previously published in
PE&RS
.
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