62
February 2018
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
datum point for the county as a whole. Sufficient comparison
be-tween the astronomical and trigonometrical, or geodetic,
values have not yet been obtained to warrant an immediate
change.”
The astronomical values observed for (G.C.S. 121) Leigon
were: Φ
o
= 5
o
38´ 54.39˝ North, and
l
o
= –0
o
11´ 52.65˝ West
of Greenwich. The corresponding geodetic coordinates of the
same Accra Datum of 1929 origin point that were computed
from the tri-angulation were:
f
o
= 5
o
38´ 52.270˝ North, and
l
o
= –0
o
11´ 46.080˝ West of Greenwich. In 1928, as other
observations were being taken at Leigon, it was decided to
observe an azimuth there to test an intermediate side of the
triangulation. The azimuth observed from Leigon to station
(G.C.S. 113) Asofa was: α
o
= 264
o
48´ 48.78˝. The ellipsoid of
reference is the War Office 1926 (McCaw 1924) where a =
6,378,300 meters (20,926,201.2257 feet), and
1
/
f
= 296. Note
that this conversion from feet to meters is specific to Ghana,
and the elevation of Leigon is 147.46 meters. As of late 1996,
the transformation parameters from Accra Datum of 1929 to
WGS 84 Da-tum were based on three collocated points where:
Δ
X = –199 m,
Δ
Y = +32 m,
Δ
Z = +322 m.
Capt. Wood continued, “For the purposes of cadastral work
geo-graphical coordinates are inconvenient, and in their
place plane rectangular coordinates on the transverse merca-
tor (sic) projection have been adopted. The whole Colony has
been placed on the same origin, the central meridian being
the meridian 1
o
W and the origin of the X coordinates 4
o
40´
N. 900,000 is added to all Y coordinates, in order to avoid
negative coordinates, and the maximum scale error has been
reduced in the customary manner by reducing the scale of the
projection by 1/4000, so that the scale error nowhere exceeds
this value except on the extreme edges of the Colony.”
For instance, “Leigon (G.C.S. 121). District Accra. Locali-
ty west of road junction at 8th mile Acra-Dodowa road. Ap-
proach through low bush from junction of Aburi and Dodowa
roads. Bare hill with good all round view. Concrete pillar 14˝
high by 9 1/2˝ by 7˝ with iron pipe as centre mark. Beacon
double quadripod of squared timber, 17´ high, centered over
old mark.” The geodetic coordinates of Leigon equate to Colo-
ny coordinates of X = 356,084.33 ft., Y = 1,192,117.91 ft. As I
always point out; if your double- or quadruple-precision soft-
ware does not exactly match the local transformation results,
your software is wrong for that country. Colony coordinates
are truncated Gauss-Schreiber Transverse Mercator trans-
formations that are similar to those used in the United States
for the NAD 1927 State Plane Coordinate Systems. They are
not computed the same way as the Gauss- Krüger Transverse
Mercator transformations normally done with other Grid sys-
tems such as the UTM and the NAD 1983 State Plane Coor-
dinate Systems.
According to J. Clendinning in Empire Survey Review,
January 1934: “The Akuse-Obuasi, Obuaasi-Nsuta and
Akuse-Apam chains were computed in terms of geographical
coordinates which were later con-verted into rectangular co-
ordinates on the projection system; but all re-cent work, both
triangulation and traverse, has been computed directly on the
projection system, the necessary corrections to distances and
bearings as measured on the ground being applied to enable
this to be done.” In that same paper, Clendinning also com-
mented on the surroundings of the Gold Coast in that “The
denseness of the forest and the topographical features of the
country naturally determine the na-ture of the survey.” With
that ter-rain in mind for Ghana, the
Textbook of Topographi-
cal Surveying
, Her Majesty’s Stationery Office, London, 1965,
pp. 221, 222 offered the following description: “311. The rope
and sound traverse.
In West Africa, the minor bush tracks maintain their gen-
eral direction for considerable distances but wind about just
sufficiently to pre-vent a sight being taken down the track.
A system of survey which has been used in these conditions
is as follows. The bearings of the traverse legs are observed
from a compass station to sound. This sound may be either a
call or a whistle from a forward chainman. The length of the
legs is measured with a rope. This rope is made of a length
allowing for the wind in the path. Thus it is normally found
that a rope 310 ft. long pulled tight along the path is equiva-
lent to 300 ft. measured direct along the leg of the traverse.
The actual allowance must be found by experience of local
conditions. The length of the rope should be checked and ad-
justed be-fore and after work and in the middle of the work.
Using this system, minor bush tracks can be traversed rap-
idly and it is possible to work through track-less bush doing
only a minimum amount of cutting. When under-growth is
thick or thorny a shorter rope (100 ft.) may be easier to ma-
nipulate. The accuracy of this work is greater than would be
expected. There appears to be no systematic error in the ob-
servations of bearing and the errors consequently tend to can-
cel out during the traverse. For work at a scale of 1/125,000,
traverses should not be carried for more than five miles by
this meth-od. Heights are observed by aneroid barometers.
On the Gold Coast, an average of five miles per day can be
maintained, and distances of 15 miles have been achieved on
single days. If, however, work is continued into the afternoon,
as is necessary for such an output, it is necessary to re-run
later that part of the aneroid heights which was observed in
the period when the diurnal wave is unreliable.”
In the Canadian cartographic journal, a letter to the editor
told how they were cutting costs by eliminating the forward
chainman. “They tied the rope to the tail of a pig and sent it
down the path. When the rope became taut, the tail of the
pig was pulled and it squealed so they could observe a bear-
ing on that sound. They were working on further improving
the method by breeding a special pig that would, in addition,
monument the spot when its tail was pulled.”
In 1977, the control network was readjusted including ex-
tra observations and an improved computation technique. As
part of the readjustment the datum was changed. The new
Datum was Leigon Datum of 1977, in which the old geo-
graphical values for (GCS 121) Leigon were held fixed at the
old values and the War Office ellipsoid was replaced by the
