324
May 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
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Datum at the North base end (invar baseline) of Pointe
Banda in 1921, in the village of Sainte-Marie where: Φ
o
=
– 3° 50´ 03.4˝ South, Λ
o
= + 11° 00´ 46.6˝ East of Greenwich.
The defining azimuth was from Banda to Tranchée: α
o
= 158°
59´ 56.0˝. The usual Hatt Grid had the same origin as the
Datum. The triangulation stations monumented by Lafargue
for this survey were pyramids constructed of cement, and the
south side was imprinted “M.H.A.E.F. 1921.”
There was no topographic survey of the AEF until after
WWII. During the war, the Gabon Belt Transverse Mercator
(“Fuseau Gabon”) was used such that the ellipsoid of reference
was the Clarke 1880. The unit of measurement was the
meter, the Central Meridian, (
l
o
) = 12° West of Greenwich,
and the Latitude of Origin, (f
o
), by definition was the equator.
The Scale Factor at Origin (m
o
) = 0.99931, the False Northing
= 1,500 Km, and the False Easting = 800 Km. The particular
math model for this zone was presumably the Gauss-Krüger,
identified by the French as “Projection de Gauss.”
A geodetic astronomical station in the town of Mporaloko,
east of Port Gentil, was set by the “Société des Pétroles
d’Afrique Équatoriale” (SPAFE), the Equatorial French
Africa Petroleum “Society,” (corporation). This position
was transferred to the lighthouse at Cape Lopez such that
the Cape Lopez Datum of 1951 is synonymous with the
Mporaloko Datum of 1951, where: Φ
o
= – 0° 37´ 54.2˝ South,
Λ
o
= + 8° 42´ 13.2˝ East of Greenwich. The Mporaloko 1951
Datum is currently used for most of Gabon that is south of
Libreville.
In 1954, Sauzay of the French Navy recovered two of
the monuments set by Lafargue in 1921. These two points
were Table and Babar. The coordinates adopted for Table
were: Φ
o
= – 3° 49´ 13.9˝ South, Λ
o
= + 10° 00´ 55.0˝ East of
Greenwich. Sauzay observed a solar azimuth from Table to
Babar such that: α
o
= 348° 09´ 05.5˝. The baseline distance
adopted was a mean of the new observations with invar
tapes and with those observed by Lafargue in 1921. The
Hatt Grid used for this Datum had the same origin with a
False Easting and False Northingof 10 kilometers.
In 1955, Mannevy of the French Navy reoccupied an
astro station at Esteiras for the triangulation of the Bay
of Corisco, North of Libreville. The Cadastral Service of
Libreville measured a three-kilometer baseline in Libreville
in concert with the French Hydrographic Mission. The Cape
Esteiras Datum of 1955 origin is: Φ
o
= + 0° 36´ 48.58˝ North,
Λ
o
= + 9° 19´ 19.02˝ East of Greenwich. The Department of
Public Works of Gabon (Travaux Publics du Gabon) assisted
Mannevy by building towers for the triangulation of the Bay
of Corisco. The triangulation was performed with WILD T3
theodolites using eight sets of angles. The design of the towers
and targets were patterned after those used in Madagascar.
The first computations on the Universal Transverse Mercator
Grid in Gabon were computed by Mannevy with the Clarke
1880 ellipsoid, Fuseau 32 (zone 32), where Central Meridian
(
l
o
) = + 9° East of Greenwich. The Cape Esteiras 1955 Datum
is used for Libreville and points north in Gabon.
The latest edition of “TR8350.2” by the National Imagery
and Mapping Agency (NIMA) published in July of 1997 lists
the three-parameter shift from M’PORALOKO (sic) Datum to
WGS 84 Datum as:
Δ
X = – 74 meters,
Δ
Y = – 130 meters,
Δ
Z =
+ 42 meters. Note that only one point was used to determine
the published shift, and the accuracy is stated at ± 25 meters
for each component. Such a level of accuracy is
acceptable for military artillery purposes or shirt-pocket
Global Positioning System receivers when one is working with
maps at a scale of 1:25,000 or 1:50,000. But, an uncertainty of
± 25 meters is useless for the precision necessary for seismic
geophysical exploration or municipal mapping.
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
.
