Geodesy in the Ivory Coast

•

h ~ITED NATIONS

~CONOMIC

,

.AND SOCIAL COUNCIL

Distr.

LIMITED

E/CN •14/CART/189 6 September 1966 EftaLISII

Original I FRENCH

ECONa~IC C~M\ITSSION FOR AFRICA Second United Nations Regional Cartographic Conference for Africa Tunis (Tunisia), 12-24 September 1966 Provisional. agenda item 7

Submii;1;€d by th" Government of the Republic of the Ivory Coast

rU-5 M66-1080

·.~~" . . k

,

\,

, t

• ^{r _}

EjCN.14jCARTj189

GOEDESY IN THE IVORY COAST

11

There does not exist at prese~t in the Ivory Coast any geodetic control network, determined with the required precision to serve as a fulcrum for precise homogeneous large-scale surveys.

Since 1960, the date when the:eogra"hicol Institute of the Ivory Coast came into being, this problem has been the subject of sever,l investigations.

The exploitation of aerial photographic coverage on the scsle of 1:25,000, which it is planned to cerry out systematicol'y in accor~3nce

with a progressive programme in the cOurse of the next few years, will make it absolutely necessary in the future for a regular homogeneous geodetic network control to be carried out.

A number of triangUlations and local traverses have been carried out in the Ivory Coast at the request of diff'rent departments.

The following are worth noting - the triangulations carried out in

Abidjan (181 points), Bouake (20 points), Man (19 points), Daloa (15 pCl'ts), Grand-Bassam (15 points), etc. the elements of which were calculated

simply in the plan.

The coastal geodetic traverse, vmich is 200 km long stretching f-om the Ghana border to Grand-Lahou, and carried out from 1961 to 1963 by the

~entral Hydrographic Service of the French Navy as part of the hydrographic mission of the South Atlantic (calc'jlated on the MTU oroj ection},

The sides measuring 3 to 11 km were measured by Tellurometer and periodically reorientated by astrocomical azimuths.

Tn 1965, the geodetic minor co-itro l of the SOGETEC, for the cadastral survey along the co~munication routos, in a quadriloteral bounded by Abidjan, Babou, Toumodi and Kotobi.

968 km long, it cont2ins 14 tr,verses divided up into 10 knots calculated on the MDJ system_

The longer sides which are very variable according to the terrain, have been measured with a distomat, the shortest sides measuring about· 100 metre••

It is worth quoting, by way of rominder, the former geodetic chain known as the "pr'mordinl chain of Guinea ond the Uaper Volta" which was

~b$erved from 1911 to 1941 along the 10t~ parallel, and crosses the Ivory Coast in the regions of Odienne, Boundiali and Korhogo.

11

E/CN.14!CART!189 Page 2

i. If

The time has now come to put on absolute stop to this scettered ty~e

of work and formulate a homogeneous geodetic programme which will obviously take a long time to be carried out.

As a first priori ty, it was ap~'ropriate to study first the conditions for carrying out the first chain of the first order situated in the

neighbourhood of the 5th meridian.

This chain ha~ in fact the following Advont~ges ;

(1) Since it skirts the volley of BandamQ, it would link the centres of Abidjan, Bouake and Korhogo which are regarded by the Cadastrd and Agricul··

tural Services as priorities, both for carrying out aerial photographic coverage on the sC3lc of 1:25,000, os for using thom to advantage in cartography and c"dastral work.

(2) Situated in the centre of the country it would provide excellent sinews for the genoral future of geodesy in the territory, the basic centre of which would be situated at BOUck".

(3) It would be capable of being used for scientific purposes concurrently with the Africen chain.

(4) It would meke it possible or the local triangUlations with which it links Up to be adjusted.

(s)

The following paragraphs pr-oviJe a brief study of a first draft of this chain, which would be the besic ele~ent in the primordial geodesy of the Ivory Coast.

FIRST DRAFT OF 111E GEODETIC CHAIN OF lliE 5111 MERIDIAN

From the pr~liminary survey corried out with the aid of a map, it appears that the Ivory Coast, in the median part of the north to the south, is a

slightly undulating plain rising from sea-level towards Abidjan to 400 metres high, the northern frontIers. At intervals, well defined hummocks dominate the mean level of 150 to 200 metres. ThGse hummocks whieh are fairly regUlarly distributed, would be the perfect choice, ~bviously intended for the geodetic points of the first order.

•

\

»

hqe 3

The first draft encourages the prospect of 30 geodetic points ot

distances of 20 to 75 km, according to the lend relief, with sides averaging 45 km in length.

During the months of May and June 1966, three reconnaissance circuits were carried out in the whole of the zone earm"rked for the meridian chain.

Points selected after the study carried out on the map wore inspected on the ground, and the following conclusions were arrivelll· at :

The major difficul ty will be the forest, present everywhere throughout the length of the territory, which n,y be divided into two large s8ctors.

The portion situoted between Toumodi and the sea, the zone of dense tropical forest.

The portion situated between Toumodi an the northern frontiers of the country, the region of wooded s8vannah.

(1) In the dense forest zone, Vilere very lorgo trees Are to be found, there are nonetheless some granitic crests which are ;lmost denuded, and

summi ts where 'th e trees havo been [Frtl y cut down for ourposes of cul ti vation , Of the seven geodetic points in 'this zone, the following forecasts mol' be made

- 2 large poles, supporting 'television relvys might be fixed in torsion - 1 granitic mound might be plnced on the ground after it has been

partially clear8d of trees

- 2 wooded mounds would need 35 metre sianals ;

- 2 particularly thickly clustered summits would require a more thorough reconnaissance to assess the ,wight of tbo trees. In the case of trees which exceed the height of the geodetic signals 142 metres), the parti aI clearing of the trees in the direction of the siting will have to be envisnged.

(2) Unlike what is generally thought, the savannah in th8 nortion

inspected, is uniformly wooded except in the Qgricultural sector of Korhogo, within a radius of 15 to 20 km. The height of the trees in tho savann3h

varies from 10 to 40 metres, probably according to the nature of the ground and the height of the terrein. There docs not seem to be any rule based on latitude.

E/CN.14/CART/189 Page 4

The trees on the crests of a height exceeding 500 metres, seem to be fAirLy low (10 to 15 metres). On theso _sum~its it ought to be nossible to set up geodetic points with the aid of small signals or even Dn the ground efter1t has been cleared of trees. On the contrary in pl~ces where the savannah is flat, high signals will have to be provided (30 to 40 metres). On the 22 points to be set up in the savannah it may be possible to have:

- 11 points on the ground or on lower signals at 15 metres:

7 points on signals of 15 to 30 metres 4 points on signals of over 30 metres.

The final reconnaissance, o3rticularly in the wooded area, will often mean eutting down a lot of trees so that reconnaissance ladders may be set up.

The basic point may be the astronomic-l point determined in 1965 near

Bo~ake, almost in the centre of the country. The scaling will be earried out by neasuring soma of the geodetic sides with the Tellurometer. The chain may be orientated on orecision azimuths determined in 1965 at Bouake, Abidjan, Toumodi and Korhogo by a mission from the Institut Geographique N~tionol

Francais.

The geodetic points will hove to be represented, so that they can be easily detected. The best way would seem to be to ~lent a cement pillar which will rise above the ground by 1.50 metres at le~st, when embedded in a

reinforced concrete socket.

ESTIM~TESOF METHODS TO BE USED AND LIMITS FIXED FOR THE COMPLETION OF THE WORK Detailed reconnaissance

Fleet of car-s :

- 3 light vehicles c~pable of negotiating any terrain

3 lorries.

Equipment :

approximately 3 months ; 3 1/2 months approximately.

- 4 "Durand" type ladders for mounting at 45 - 4 "Deloez" type ladders for mounting At 30 - Equipment for reconnaisscnce and camping Estimated time for the completion of the work :

The Northern Zone of Bouakd (16 points) : - The Southern Zone of Bouake (13 points)

metres metres

"

. t

.~.,

•

,

•

E/CN;14/CART/189 Page 5

Construction and observation :

This precise estimate cannot be arrived at until the results from the detailed reconnaissance are 3v8ilable. Equi~nent ~nd observation points must be provided in the course of tho same mission, since each sicne I will have to serve at severnl points.

Fleet of cnrs :

- 4 light vehicle" for negotiating any kind of terrain - 4 lorries.

EOl.tipmen t :

3 T3 theodolites, 2 of which have Blready been acquired - 2 di s t.ornats (alre8dy on loan from the United N,-,hons) : - 5 "Bilby" typo signals 42 metres ;

- 5 "l.G.N.R.I." double type signals 24 metrGS ;

Equipment for bUilding ond camping (to be found on the spot).

EstimatGs of time limit for the completion of work:

- 6 1/2 months approximately.

HOle to be allowed for the total completion of ~ :

Thus the eXGcution of the work, using the staff con~TmGd, should be anrried out in two field camp8igns of 6 1/2 months from November to 30 June.

The computation period nay follow i~~ediately after the observqtions hove been taken. Tho totol time to be allowed therefore for the operntion should extond over two years with a few months interval when work will ce3se, between the time when the reconn~issanceis ~ade and observations.