Geodetic and gravimetric surveys : applications to planning and development

NRD/CRSU/89/7

GEODETIC'.'AND;GRAVIMETRIC:SURVEYS i : 'APPLICATIONS

1 'TQ'PLANNING'.AND'DEVELOPMENT

NRD/CRSU/89/7

GEODETIC AND GRAVIMETRIC SURVEYS :APPLICATIONS TO PLANNING AND DEVELOPMENT.

Abstract: The paper- defines geodesy and gravity

observations and refers to thf>m as tools contributing inputs to measures f$r effective and efficient planning required for the.

detection, and subsequent exploration and development,of specific natural ^sources and prevention of hazards. The o^jjects of a . geodetic framework of a -4^6. gravity observations are briefly discuss^, stressing

their complementary role in the determination

of the size and shape of the earth, its constitution and strength. Because of the heterogeneous co-ordinate systems and, hence, lack of a common geodetic datum, regional projects cannot be economically planned and implemented. Poor linkage of geodetic levelling lines and failure to connect height datums to mean sea level which, very often, is not based /' on sufficient tidal observations pose serious limitation to the preparation of qualitative topographical maps which can be used for implementing , regional projects and determination of the geoid. The paper outlines some of the visible areas where geodetic and gravimetric surveys can provide inputs , into planning and meaningful development namely:

provision of controls for mapping $nd breakdown into lower orders of the horizontal controls for cadastral and township mapping; datum unification and strengthening of national geodetic networks; monitoring of the crustal movements in order to prevent hazards;

discovery of underground watery for man's survival during spells of droughts;

complementary inputs into data Required for mineral prospection; and accurate demarcation of international boundaries, one of the pre-requisites for good neighbourliness and which can support efficient planning of

regional projects. It concludes Spy stressing

the importance of an early establishment of a common geodetic datum for Africa,

1. INTRODUCTION

In order to appreciate the contributions from geodetic and gravimetric surveys to planning and development it will be rewarding to understand the subjects which come under the two

disciplines as well as the inter-relationship with others with which many resources planners are familiar in their day-to-day activities. This then requires that the definitions of geodetic and gravimetry surveys be first . given before discussing their

applications and/or contributions to economic planning and development. It must be borne in mind, therefore, that they constitute tools or better still, inputs to measures for effective and efficient planning required for the defection, and subsequent exploration and development, of specific natural resources and prevention of natural as well as man-made hazards which, in

themselves, can place limitations on the level of socio-economic

development.

The literary meaning of GEODESY is 'dividing the earth', and its first object is to provide an accurate geometrical framework for the control of topographical and other surveys.

When talking about geodesy nowadays, it is difficult to be precise about the dividing line between it and geophysics. As a result, the following subjects are generally considered as coming within the domain of geodesy:

a) Primary or Zero-order triangulation, trilateration, and

traverse; :

b) Height determination above mean sea-l$vel;

c) Astronomical observations of latitude, longitude and azimuth;

d) Crustal movements;

e) Gravimetry which includes observations of the direction of gravity by astronomical methods for latitude and longitude;

and the intensity of gravity by pendulum or other apparatus;

f) Determination of the geoid and of the external equipotential surfaces at all heights, using data frpm (e) above; and

g) Satellite geodesy which has largely replaced the method of astro-triangulation.

Gravity (usually denoted by the symbol 'g'), on the other- hand, may be defined simply as the force with which the earth attracts any object on its surface; in so far as geodesy is concerned this represents the vector sum of the acceleration due to the attraction of the earth, and that due to the centrifugal

force arising from the use of rotating axtis. Its value can be measured in absolute terms using methods of pendulum observations whilst the relative difference of its values between one point and another on the surface of the earth is conveniently measured nowadays by gravimeters. Since about 1950, the great majority of gravity observations have been made by gravimeters, and the use of pendulum has mostly been confined to the provision of national or base stations, and of the calibration lines which are required for the effective use of gravimeters.

II. WHAT ARE THE OBJECTS OF A GEODETIC FRAMEWORK AND GRAVITY OBSERVATIONS

A geodetic framework is desirable to be able to constitute the main framework on which less precise surveys can be based and which, in turn, may form the basis for topographical and cadastral maps; assist in the determination of the fize and shape of the earth and the form of its external equipotential surfaces when combined with observations of latitude, longitude and gravity; and to detect and record movements of the e&rth's crust. Gravity observations help to throw light on the constitution and the strength of the earth; assist in the prediction of satellite orbits and in the computation of spirit-levelled heights in geopotential units; and to measure earth tides including other crustal- movements, changes in mean sea-level as we^ll as in the detection of possible changes in the gravitational constant (G) where the mutually attractive force of two particles m and M, separated by a distance r, is given by the familiar formula F=GmM/r2.

III. STATUS OF GEODETIC NETS AND GRAVITY OBSERVATIONS IN AFRICA

First and foremost, the word 'geodetic' as often used is associated with the high precision in which measurements have been made even though there is a scientific discipline known as geodesy.

For the purpose of this paper, therefore, w^ can safely link this attribute with methods of observations concerning triangulation and trilateration, levelling, gravity and recent techniques using Doppler Positioning methods and Global Positioning. Systems for fixation of points on the earth's surface for specific scientific undertakings, particularly those connected with the objects already mentioned in section II.

The account of the status of geodetic and gravimetric surveys in Africa presented in this paper has been based on the. study of the scanty information and data collected during the period 197 6 to 1982 in the course of preparation of the atlas of the cartographic inventory of basic data for Africa. In spite of the paucity of the data for full representation of the situation, analysis based thereon will be able to provide, at least, an insight into the overall continental situation. There is no doubt, of course, that this is far from being adequate quantitatively and

qualitatively■ from point of view of cur^fnt day- planning and development. Very many of these nets have ]$&en established during the colonial days within the limits permitted by available instrumentation at that time, and also the specific interests of the foreign powers.

Available records show that most of the classical triangulation networks in the continent consist of chains of triangles which have been observed using precise'theodolites and electronic measuring devices (EDMs). There are different reference ellipsoids which form the. basis of national co-ordinate systems.

As such there is no single homogenous co-ordinate system in the continent and hence, no common geodetic datum for Africa. This situation makes it an arduous task to undertake planning and development of projects spann ing more than one country without resorting to cumbrous and laborious exercise of co-ordinate transformation or embarking on fresh mapping. The effect of this on the implementation of such projects is their non-viability and, subsequently, abandonment due to apparent non-cost effectiveness.

In flat terrains, precise traversing using EDMs replaces precise triangulation. Approximately, about 50% of the continent has been covered by horizontal control.The accuracies attained vary from one country to another. Prior to the execution of the continental geodetic surveys there were two continental control networks:namely, the chain of first-order triangulation along the 30th Arc Meridian (from Cairo to Cape Town) which was conceived in 1880 and completed in 1954 and the first-or$er geodimeter traverse and geoidal profile along the 12th Parallel from Dakar (Senegal), through Mali, Burkina Faso, Niger, Nigeria, Cameroon to Andre at the border of Chad with Sudan, blending well with the first order triangulation which crossed the 30th Arc Meridian'survey. More recent still is the continental ZERO-ORDER geodetic network christened 'AFRICAN DOPPLER SURVEY (ADOS) whose objectives are to provide:

a) zero-order control for future geodetic networks which could support mapping;

b) control for datum unification/strengthening;

c) improved geoid for Africa; and

d) coastal calibration points for GEOS C altimetry.

To date, some 310 zero-order points had been observed in 47 countries and are being computed at five centres, two of which are located in Africa.

As for geodetic levelling, nearly the whole of the continent is covered, most of these being found in north-western., western and central parts of the continent. A large number of these are still poorly linked and, often isolated in spite of the fact that the

lines of level in adjacent countries rur^ §.|ose to each or one another. Another deficiency is that h^fht datums are not everywhere connected by precise levelling |g mean sea level and,

in cases where they are, the mean sea leve^ fias not been based on sufficient tidal observations. Available ^fcords show that the

misclosure of the Kenya - Egypt connection wjy,ch ran through Uganda

is of the order of 30 cm and that the datjp at Mombasa (Kenya), based only on a one-year observation in t% early thirties on a gauge sited on a coral reef, showed evidence" of sinking and damage

by the rough waters._/ A clear inconsistency of the values of the heights of benchmarks is seen in the discrepancy of some 6 cm

between the values of the Dar es Salaam tide gauge (Tanzania)

determined from the mean sea level observations from the Tanga tide gauge and that originally derived from observations using the same station as a fundamental benchmark. Also, according to an I.G.N. report, the elevation of the surface of lake Chad in 1954 as measured from the tide guage in Dakar was 281.68 and 281.77 m

and 281.88 m as measured from the tide gauges in Tunis and Pointe

Noire respectively. This discrepancy can be traced to the insufficient number of tidal observations over a long period of time to determine good mean values for the gauges. In many countries also the density of levelling n$t"s is very poor and, therefore, poses serious limitation on the preparation of qualitative topographical maps. In Africa region, there is a density of one point per 232 km2 as against ^n average requirement of one point per 4 km2. This situation is ^ven worse in the case of horizontal control where the required average density is one

point per 388.8 km2 as against one point per 4 km2 in the Africa

region (Fajemirokun, 1980) .

It is pertinent at this juncture to &eep in mind that the existing geodetic networks, be it in the form of horizontal controls by any of the preceding methods (triangulation, trilateration, astro-fixes, traverses) and vertical controls by spirit levelling and trigonometrical methods, are simply inadequate quantitatively and therefore will require massive densification in many countries for production of accurate arid up-to-date thematic maps. At the same time, one must not lose $ight of the fact that the standards of accuracies attained, in a majority of cases, fall far below modern requirements for any scientific investigation as well as the production of homogeneous map sheets that can be used for regional planning and development. On§ of the deficiencies which the ADOS programme aims at curing is the unification to modern day standards of the horizontal control networks existing

in the various countries.

With regard to gravimetric networks in the continent, there are at most 71 International Gravity Standardization Net (IGSN)__/

stations, most of which are found in th§ western part of the continent. Together with the ORSTOM net, they foxm a good framework. In the eastern and central Africa, there are fairly good gravity networks which have international connections. However,

national gravity networks in several countr^s are not and efforts

should be made to base them on the con|inental network when

established as a complementary regional p^^ject to ADOS. It is absolutely essential that the continenta% gravimetric network should be put on a uniform scale such as th§ international miligal unit so as to ensure a common system of reference which will bring about the realization of the ultimate goal of a. common geodetic datum for Africa.

IV. WHAT ROLE DO GEODETIC AND GRAVIMETRIC SURVEYS PLAY IN ECONOMIC PLANNING AND DEVELOPMENT?

To a non-expert in the fields of geodesy and gravimetry, it is difficult to see what contributions the$e two disciplines can make to measures for economic planning and development. What one should bear in mind, however, is the end product and how this can

be, or is being, used to achieve judicious economic planning of the

natural resources which are believed to be abundant in the continent. The areas in which geodetic and gravimetric surveys can provide inputs into measures for economic planning and development have been identified in the objectives of ADOS. It is only necessary, therefore, to examine how the end-products go into inputs for development planning.

It is a statement of fact that very little knowledge is known of the abundant natural endowments in th§ continent and, as a result, it has not been easy to explore, plan and exploit those natural resources meaningfully for the benefit of the teeming millions in the continent, majority of whom live below subsistence level. It is a paradox indeed that, in spite of these abundant resources, Africa remains the most economically backward of the all the continents, housing at least 28 out of the 38 Least Developed Countries in the world. The dearth of geographic information and data which are pre-requisites to judicious planning, rational development and management of the natural endowments is lamentable and has had an adverse effect on the policy and decision making with regard to the selection of priority areas of the economy to develop. Truly regional projects spanning across many national boundaries cannot be economically implemented; for lack of accurate and homogenous map sheets which are carriers of geographic information on which to base the planning and execution. The situation in the continent with regard to topographic map coverage at planning scales leaves much to be desired. It is not rewarding to undertake any fresh mapping based on those substandard horizontal and vertical controls even if the investment can be afforded. One positive contribution to production of up-to-date, accurate and reliable topographical maps is the provision of

'geodetically' surveyed horizontal and vertical control points which would support such mapping. The breakdown of these precise controls into lower orders, undertaken with necessary caution, will, enhance the production of large-scale maps which are useful for

demarcation of lots as registrable ;instalments for investment

purposes. Determination of ownership or ti^jLe to land which is an immutable property has been accorded a measyre of priority in many African countries for purposes connected with the development of agriculture and agro-related industries. With an indisputable title, the chances of securing loans, even for small scale industries, are well enhanced. In this way, one can appreciate the role which an accurate survey, based on good controls, is capable of playing in resolving possibility of litigations arising from

disputes over land ownership.

In Africa, there are nearly as many geodetic datums as there are countries, each adopting its own local origins for both vertical and horizontal controls. Simply seated, it has neither a

common geodetic datum nor a common height datum. The resultant

effect is that most of the existing map sheets, probably produced during the colonial era, have been based on different ellipsoids of reference, the most common among these being Clarke 1866 and 1880. There are equally seven projections in use in the continent.

Consequently, it has not been possible to unify the various datums

to achieve a common geodetic datum for Africa. The 30th Arc

Meridian triangulation network has revealed some discrepancies in.

the value of derived co-ordinates in som@ countries in eastern Africa. Even if the original field books are available, it is unlikely that the source of the error causing the weakness in the

strength of the figure will be detected, Strengthening of the

networks will be possible by undertaking fresh geodetic observations and carrying out a rigorous adjusment using modern methods of computing. Geodetic surveys contribute to the quality of the controls on which reliable and up-to-date maps,sine qua non

to economic planning, have to be based.

By 1988, the observations of ADOS have been completed and the computation of the Doppler positioned points is still in progress.

However, the computed co-ordinates are not immediately useful for mapping purposes as they still have to go through some

computational process for the geoidal heights which will lead

subsequently to the determination of the ellipsoid of reference

that will fit the continent and, hence, result in the establishment of a common geodetic datum for Africa. The 'contemplated regional project on the establishment of an African Gravity Standardization

Network (AGSN) whose principal objectives are: (a) to establish absolute gravity .stations for gravity calibration lines; (b) prepare gravity anomaly maps for general and regional uses; and (c) to contribute to the data for the International Absolute Network.

Let us come to the most obvious applications . of geodesy, particularly the precise determination of elevation above mean sea

level, and see what contributions it can make to planning and development. Africa has at least at least 170 rivers and 52 lakes, the largest being Lake Victoria which measures 68,800 km2. Africa

7

is believed to have about half of the tota^ potential hydropowver

of the world. There is an impressive estimate of mineral deposits

in the continent - oil, gas, coal, vanadium, bauxite, uranium, copper, gold, manganese and phosphate rock etc.

It is relevant to note that even though th$re are many rivers and lakes in the continent, about 80 % of the rainfall over Africa does

not reach the rivers and lakes but lost through seepage and

evaporation. This makes the availability of water per unit area the

least in the world and the total surface water resources are not.

evenly distributed. The importance of planning for the optimum

ustilization of the available surface waters is obvious, and the need for geodetic levelling in this respect can hardly be over

stated. By tying up the water level data to geodetic levelling, it is easy to determine the slope of a river. It is easily recalled

that the hydrometeorological- survey of lakes Victoria, Kyoga and Mobutu-Seseko which was conducted in 1967-1972 brought home the lesson of establishing a common geodetic levelling network around lake Victoria in order to provide a common geodetic datum for all the gauges.

In order to delineate accurately the catchment areas of river

basins and,subsequently, have a good interpretation of the hydrological data, contoured maps based on. consistent levellings

are essential. They are equally useful topis in the feasibility

study of hydro-electric schemes and the construction of irrigation and drainage systems.

Until very recently it was believe^ that Africa was not subject to earthquakes and tremors as violent and as frequent as in other regions of the world. Contrary to this belief, Africa does experience serious hazards such as flooding, tremors and earthquakes. It is still green in memory the eruption at Lake Nyos in the Cameroon which claimed several lives including livestock.

The measurement of the movements of the earth's crust is of paramount importance in order to avert this type of hazard. This is equally important in the construction of large dams and other major engineering constructions. In order to ensure that structures which are placed on the ground are secure from collsal damage and destruction, it is important that a programme of geodetic and other geophysical measurements are undertaken from time to time to monitor crustal movements where they are known, or likely, to

occur.

Another important application of gravimetric surveys is in mineral prospect ion. It is known that a good knowledge of the variation of gravity over parts of the earth's surface has helped in understanding the internal structure of the earth. As one of the geophysical tools for prospecting, gravimetric surveys are capable of providing rich data which are relevant to geological structures with which mineral deposits are associated. It is unfortunate, however, that most of the existing gravimetric survey networks in Africa are not on a common datum (inhomogeneous) and this situation

■J \

has affected the extension of gravity network in Africa. It is .;■■ ' pertinent to note that gravity measurement^ lose their values for .;■] lack of precise levellings. It is in realization of this fact that

:;;:] the African Gravity Standardization Network has been conceived to 0 contribute more to the planning process in Africa. The successful f,{ implementation of this regional project should be seen as

■i;i value-added venture to the planning and development process in

C] Africa.

■• '■!

V-] The ' GREAT AFRICAN DROUGHT' of 1983-1985 which resulted in a

'■'-\ . famine which claimed several thousands of lives including livestock .;.;■] As still.green in everyone's memory. It goes without saying that the very survival of man and his livestock and other activities such as mining and industries in which he can engage can be linked with the discovery of reliable underground water resources. Data from gravimetric surveys, used along with geophysical interpreted data, help to indicate the distribution of density in relation to depth. Geodetic levelling provides the complementary geographic information to map correctly discovered acquifers and to correlate water-table data.

Disputes on international boundaries in many African countries have become a menace and these have tended to defeat the philosophy of African Unity as nationals across such borders have been alienated and often engage in strifes that wantomly destroy lives.

The most recent example is the holocaust between the nationals of the Republics of Senegal and Mauritania. These disputes have arisen from the improper definition of the common boundaries between the adjacent countries. This is due, of course, to the poor quality of the surveys which must have been based on very low order controls.

Peace among neighbours is a veritable parameter of development, particularly when considering regional projects. Good national geodetic networks will go a long way in defining unambiguously such boundaries. This is particularly so when th®re is a common geodetic datum from which co-ordinates of fixed points on the ground are .determined.

V. CONCLUSION

The importance of precise measurements yielding reliable controls - vertical and horizontal - cannot be over-stated in every scientific endeavour connected with the planning and development of natural endowments and prevention of hazards. It is therefore important to pursue relentlessly the laudable idea of establishing a common geodetic datum for Africa whose ground work has started with the establishment of zero-order ground positioned points by Doppler methods. Any gaps identified in the national geodetic networks should be filled before embarking on determination of a common geodetic datum for the continent. The on-going endeavour at regional level to establish an African Gravity Standardization Net, a necessary complement to the ADOS project, should be followed-up

with resolve to achieve the final objective of establishing

■J ' '. homogeneous net for the continent. The^e is, therefore, an

■j. - imperative need for homogeneous co-ordinate system in both '■■! elevation and planimetry for Africa to-be aljle to execute regional

:| projects spanning across many national frontiers. This will provide

;'j the much-needed tool for integrated approach to economic planning t:\ and development.

10

20° 50' 40° -

30'

20°

10°

30'

40c

AFRICA

HORIZONTAL CONTROL (Existing)

LEGEND

Trianguiotion and Triloterotion

30th Arc Meridian Triongulatron

12th Parallel Traverse

Presise Traverse

Areas with Astro—geodetic Points

800 1600 Kms

30°

2OC

10©

10°

20°

30'

fft« bamditirs irrd nt officltt tndoiscmtnl o- Ltl fiauiftes ft !•

ifiow/i on this map do rvt imply epltnce by (At Unit id Nttloni.

p«f rOigtalMion d.i U.uani Until.

i

Compiled on the boses of:

Notional Reports for IUGG I Incomplete)

National Reports for UN Cartographic Conference (incomplete)

"inventaire de conovas geodeslques d'Afrique"by IGN

"World Cartography", UN Publications ^40°

20° 10' 20° 30( 40°"

CART-89-6 (2/89/200)

20' 30( 40' 50' 60<

40'

Tunis Cosoblonco

' AFRICA

VERTICAL CONTROL (Existing)

30° Al«xondrio 30'

20' 2Or

Ookor

Conakry1

10'

30'

PRIMARY LEVELLING TIDE GAUGE/HEIGHT DATUM

0 400 1200 2000

40'

KILOMETRES

COMPILED ON THE BASES OF :

- National Reports for IUGG {Incomplete ), ..-National Reports for UN Cartographic

Conference (Incomplete),

- Cartographic materials from IGN,

- 'World Cartography" UN Publications

I ho boundaries and names shown on this mop do not imply olticial endorsement or acceptance by the UnkedNations.

301 Capetow

40 <

2OC IOC 2OC 30' 40'

i :. '

. r .

CAPE VERDE IS.

GAMBlAi

GUINEA BISSAU ~V SIERRA LEONE

ANNEX 1:

VERIFIED STATUS-ADOS POINTS November 1986

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