PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
May 2016
323
T
his month’s topic features the Republic
of Gabon, the West African country that
straddles the equator. The Portuguese settled
on the Island of Sao Tomé and discovered the estuary
of the Como in 1473. The country is named after the
Portuguese word “gabão,” a coat with sleeve and
hood bearing a resemblance to the shape of that
estuary. While the French were establishing trading
posts in the 18th century, the trade in black slaves
began to flourish in Lambaréné and Cape Lopez.
At the beginning of the 19th century, the French
chased slave traders away and gained the trust of
local chiefs. The capital, Libreville, was created in
1849, and Gabon became a French colony in 1883.
From 1910 to 1958, it was part of French Equatorial
Africa or “Afrique Equatorial Francais” (l’AEF
Gabon became independent in 1960.
In March 1886, Le Pord of the French Navy used a sextant
and two chronometers to determine the position of Libreville
as: Φ
o
= 0° 23´ 15˝ North, Λ
o
= + 7° 06´ 30˝ East of Paris. In
September of 1890, Serres of the French Navy determined the
differences in longitude between Libreville and Kotonou with
the aid of a telegraph. Audoin of the French Navy established
the first classical horizontal datum in Gabon in July of 1911
at Akosso: Φ
o
= – 0° 42´ 45.9˝ South, Λ
o
= + 8° 46´ 56˝ East
of Greenwich. The defining azimuth was determined from
Akosso to station signal Alugubuna as: α
o
= 341° 47´ 04.1˝.
Akosso 1911 Datum is referenced to the Clarke 1880 ellipsoid
where a = 6,378, 249.145 meters, and
1
/
f
= 293.465. This datum
origin also defined the point of origin for the first Grid used in
Gabon, which was based on the Hatt Azimuthal Equidistant
Projection.
Hatt was the Hydrographer of the French Navy in the
late 19th century, and his projection became the standard
“local” projection for individual hydrographic surveys. This
projection became synonymous with “Systémé Rectangulaire
Usuel” (Usual Rectangular System) for French hydrographic
THE REPUBLIC OF
surveys. Note that until the late 1980’s, the Hatt Azimuthal
Equidistant projection (and Grid) was also used for the
military topographic series of Greece. Of particular note
for the Akosso Grid is that one of the points located in the
original triangulation was “Phare du Cap Lopez” (Cape Lopez
Lighthouse) where X = -10,450.02 meters, and Y = + 10,809.11
meters. Insubsequent triangulations and adjustments, this
point would gain particular prominence in Gabon.
The following year, Audoin established the Owendo
Datum of 1912 where: Φ
o
= + 0° 17´ 43.9˝ North, Λ
o
= + 9° 29´
35.55˝ East of Greenwich and the usual Grid was the Hatt
at the Datum origin. In 1914, Lafargue of the French Navy
established the Gabon River Datum at Cape Esterias as: Φ
o
=
+ 0° 36´ 48.58˝ North, Λ
o
= +9° 19´ 19.02˝ East of Greenwich.
This point would also gain prominence in the history of the
classical Datums of Gabon. Lafargue established another
The Grids & Datums column has completed an exploration of
every country on the Earth. For those who did not get to enjoy this
world tour the first time,
PE&RS
is reprinting prior articles from
the column. This month’s article was originally printed in 1998.
Photogrammetric Engineering & Remote Sensing
Vol. 82, No. 5, May 2016, pp. 323–324.
0099-1112/16/323–324
© 2016 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.82.5.323
