844
November 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
tion that were never connected to form a consistent system.
The basis of all subsequent control and mapping prior to the
1990s was based on yet another survey.
In 1934, Captain V. E. H. Sanceau, R.E., performed the
re-triangulation of Mauritius. He originally intended to use
Connal’s work as the basis of his survey in that “more com-
plete records existed than of the work of the Colonial Survey
Section.” Eventually, only the latitude and longitude of Le
Pouce and the azimuth Le Pouce – Corps de Garde (Lacaille’s
Rock) were retained. The only addi- tional points originally
used in Connal’s survey that were occupied by Sanceau were:
Signal Mountain, Lagrave (the grave), Montagne Blanche
(white mountain), Nouvelle Decouverte (new discovery),
Montagne Cocotte (chuck-chuck orhen mountain), Montagne
Bambou (bamboo mountain), and the West Peak of the Fay-
ence Mountains. The new triangulation of Mauritius estab-
lished the Le Pouce Datum of 1934 with an origin of: Φ
o
=
20° 11´ 42.25˝ South, Λ
o
= 57° 31´ 18.58˝ East of Greenwich.
The ellipsoid of reference was the Clarke 1880, where a =
6,378,249.145 meters and
1
/
f
= 293.465. Sanceau’s triangu-
lation also established a new Grid based on the Lambert
Conical Orthomorphic (Con- formal Conic) where the scale
factor at origin was 1.0 (tangent conic), and the False Easting
(FE) and False Northing (FN) were 3,500,000 feet each. The
unit of measure was the yard, where 1 meter = 1.09362311
yards. Captain Sanceau also produced
The 1” Topograph-
ical Map
.
In 1943, the United States Army, Office of the Chief of En-
gineers issued tables on the Mauritius Zone with the FN and
FE being 1,166,666.67 yards. It was noted that the “new” FN
and FE of the Ordnance Survey maps were equal to 1,000,000
metres on the Blue Grid.
According to D. Ramasawmy, Chief Surveyor:
During 1962-64 the Directorate of Overseas Sur-
veys (DOS) UK un- dertook a Tellurometer traverse
using most of Captain Sanceau’s trigonometric points
and at the same time establishing about 60 new ad-
ditional points. During the period 1967-69 with the
assistance of DOS, the control points were increased
by a further 80 points. During the same period, a pri-
mary level network was established for the island.
Formerly, all color maps of Mauritius were pro-
duced by DOS and later on, the French Institut Géo-
graphique National (IGN) also produced some maps.
The scale of these maps were 1:100,000 (one sheet
covering the entire island) and 1:25,000 (13 sheets
covering the whole island). It was only during the ear-
ly seventies that the first 1:2,500 maps of the urban
areas were produced by DOS from 1968 aerial pho-
tography. IGN did some 1:2,500 mapping of the North
from1973 photography and a portion of the southern
part from 1974 photography in 1981. From 1981 on-
wards most of the 1:2,500 mapping is being done by
the Survey Department, Mauritius.
From 11 July 1994 to 16 September 1994, Professor Peter
Dare of the University of East London led a group of stu-
dents for a GPS survey of Mauritius. Seventeen old points
were occupied, and the transformation developed by Dare
was published as a three-parameter shift from the Mauritius
1994 Datum (WGS 84 ellipsoid) to Le Pouce 1934. Those pa-
rameters are: ΔX = +770.126 meters, ΔY = –158.383 meters,
ΔZ = +498.232 meters, and the r.m.s. of each component is
±0.250 meters. Dare remarked that the shift components
were in “acceptable agreement” with the WGS 72 parameters
published by the British Hydro- graphic Office in 1982. Note
that Dare used the “Leica SKI
™
” software for his students’
analysis, and a
left-handed
coordinate system is supported
by that software.
In November of 1997, the Chief Surveyor requested assis-
tance from the University of New Orleans with respect to the
relation between the Mauritius Datum of 1994 and the Le
Pouce Datum of 1934. He also requested software for a new
Grid System. Ramasawmy suggested a “UTM projection,”
with false coordinates of 1,000,000 meters in each component.
The “UTM projection” was not used be- cause of my “well-
known” disdain for military Grids being used in standard (or
especially in “modified”) form for civilian applications. Based
on my direction, David Fabre, a UNO graduate student, pre-
pared the transforma- tion software for the Republic of Mau-
ritius Oblique Ste- reographic Grid. Fabre also “volunteered”
to perform a geodetic analysis to determine a seven-param-
eter Bursa- Wolfe datum shift. The seven
(right-handed)
parameters from Mauritius 1994 to Le Pouce 1934 are: ΔX =
–91.824 meters, ΔY = –292.222 meters, ΔZ = –115.604 me-
ters, ΔS = 23.15822 X 10
6
, R
x
= + 1.01”, R
y
= –19.74”, R
z
=
–22.14”. The residuals with respect to: Latitude = ±0.12 me-
ters, Longitude = ±0.15 meters, height = ±0.50 meters.
As a computational “test point” for the reader, coordinates of
Fort George, Martello Tower are provided. For Lindsay’s 1874
position:
f
= 20° 08´ 45.92˝ South,
l
= 57° 29´ 26.39˝ East. For
Connal’s 1876 Cassini-Soldner position: N.18,484 ft., W.12,686
ft. For L. Coghlan’s 1877 position:
f
= 20° 08´ 45.92˝ South,
l
= 57° 29´ 26.39˝ East. For C. Sanseau’s 1934 position:
f
=
20° 08´ 36.2866˝ South,
l
= 57° 29´ 02.1478˝ East, h = + 11.6
meters. For Dare’s 1996 position:
f
= 20° 08´ 47.3129˝ South,
l
= 57° 29´ 27.4235˝ East, h = – 7.4 meters.
The Grid developed by Fabre is based on the Oblique Ste-
reographic Double projection, as similarly used by the Ca-
nadians in the Maritime Provinces. The origin of the Mauri-
tius Stereo Grid is at Le Pouce with a scale factor at origin
equal to unity, False Easting = 2,000,000 meters, and False
Northing = 500,000 meters. The rationale for the disparate
numbers for the false origin is that I am a proponent of the
U.S. Coast & Geodetic Survey (NGS) preference for a differ-
ent number of digits in each rectan- gular component to avoid
blunders of reporting. Note that a reason for choosing a ste-
reographic projection is that it is ideally suited for regions
that are circular in shape.
