East African geodetic networks :ECA what next?

l-MTED NATIONS

ECONOMIC AND SOCIAL COUNCIL

GENERAL

SCA/NRD/CART/48 5 Nbveirfcer 1986 Originals ENGLISH

BDCNCMEC COMMISSION FOR AFRICA

Sixth United NaUons Regional Cartographic

Conference for Africa

Addis Ababa, Ethiopia, 10-21 November 1986

EAST AFRICAN GBCDETIC NETWORKS - WffiT NEXT?

B.Ee F0K4STCN, DIRECTOR OF OVERSEAS SURVEYS ORCNANCE SURVEY OF GREAT BRimiN, UNITED KINOXM

BCA/lffiD/CaRP/48

B E FURMSTON, Director of Overseas Surveys Ordnance Survey of Great Britain, United Kingdom

I. The African Doppler Survey (ADOS) has received wide support from African ind donor nations and there are now a considerable number of stations in the continent with coordinates derived from Precise Ephemeris Transit satellite

fixations.

>. At the 3rd Symposium on Geodesy in Africa, held in April 1986 in Yamassoukro, several papers turned attention beyond the ADOS campaign itself to the development, 3f a datum and figure of the earth which may fit the geoid in Africa more cJnsely than those currently in use - and to the possibility cf carrying out a readjustment :>f the existing geodetic data to provide a homogeneous, and more accurate, set 3f coordinated data for existing stations of national frameworks.

CURRENT ADJUSTED VALUES

3. The British Directorate of Overseas Surveys (DOS) was responsible for a great deal of the observation, and adjustment, of the geodetic framework of East and Central Africa. No comprehensive report of the field operations was produced but details of progress are contained in DOS Annual Reports. A pape* setting out the sequence of adjustments and their results was presented to the Symposium

Geodesy m Africa' hold in Khartoum in 197*1(1) No major adjustments of data concerning East or Central Africa have been carried out by DOS or, it is beli-ved by any other organisation, since that time.

«• „ t"he AfriCan adjustments of DOS are certainly capable of improvement.

Windsor s paper points out sowe of the constraints and comments:-

"Were these adjustments to be undertaken today (i.e.197*0 with the large electronic computers available, they would be done very differently. The results overall would be better, particularly if the secondary breakdowns between the

mam networks were included in the overall adjustment".(1)

The major constraint, which was particularly ssvere in the earliest work, wa*

the sheer voxume of tin* consuming effort in large adjustments vhich forced work to be broken down into stages. It is difficult now to recall, o- to imagine, the application and determination necessary to compute, with a hand operated mechanical calculator, the long strings of cross-multiplications necessary to enter correctly each figure in a desk sized matrix of squared paper. There werr often four or wore computers working -ound one -.uch sheet and there was certainly no question of repeating trial solutions in search of an optimum solution. Even after electronic computers were in use the limited memory and processing speed

of those available to DOS in the I96o's still limited the size'and comity of

Lh r w obsen'atl0nS and the vulnerability o.f their tap., readers sometimes

led to the repetition of many hours of work.

DCA/NRD/CART/48

?age 2

. The result, »f course, was the -Q-Uai ad3ust»ent »f £

.riangulation (later incorporating ^"^^^^a -preliminary work on

.hrough the 30th Arc, Uganda, Kenya and Norther" ^"^ J to m very

,he remaining networks in Tanzan^ ^o«ed that ^ey «ere no g g disappointing ..

comfortably into the space that had been left ^e^d^tfflents of compiex

£■£. Le^vefus - ^wild^o - ways L, which errors adulate

n survey networks.

..

.0 Tanzania. Host agree, in broad terms that be - value, .houl^ ^ ^

Several studies have been aade of the DOS -£*££ SoulfresuTt "of^

, large simultaneous adjust.e^t but no =«^rete ^tepS ^^ ^ ^^^ Yanoussoukro

,nd. A recent paper presented to the Jra ajrapo d fi 1(J obserVatxonS>

:arries criticism a good deal further, =1»™ment model(s)"(2). Although the

;rroneous reductions and even "adequate adjustment^elW ^ ^ ^ ^^

■.tatistical tests set out apply fully only to ver> xa 6 £ calculations

confusion

^iperie^Lrff^^

corrections to observed quantities and

. Windsor explains that all the later af ^^^

iven in her paper were completed by varxat*™° measured observations ixed points held as known Thus both Uk ^^ dlstortionS caused by this

ouS reasonably be allotted there «as no other

. it is worth noting that the role * n 19*6, was to produce topographic maps

otential and to support the -Pi—at

ntirely due to be foresight of Brigadier

hat the establishment of a 5^/^/

dequate, homogeneous »»PPin*

Hotted for this purpose, ay be deficient for modern

s still adequate as a control ^ o.nmonwealth countries of East and

The scale and

hose in the framewor. of rror of the accepted values

urposes is some 1:20,000 in places

^uSTms^ fxrst Director,

_{Hotxne D^ ^ uisite to}

^^Argument that funds were

currently accepted DOS adjusted values

ourren > w ^^ adequate and

h^ogeneous .apping th.-oughout the

icaE It has als0 proved satisfactory

ri ^ £caie fcy ^^ surveyors,

n errors are, * ^ here the scaie scientific

GWARDS A HEW ADJUSTMENT?

derived positions

or the computation of "ie?1"08"^' BOr"

he survey framework certaxnly exxsts It a retrograde step, however,_{a retrog ^ ^}

Bra/NKD/CAKr/48

Page 3

networks and cadastral values could be iustified rh ^,

aim for a set of scientific valUPi tJ v! e better oourse would be <-°

Europe. Ordnance Survey adjusters o5 th^R ^T " Satisfact<"y solution in have improved scale errors to something t ^ "etWOrk (0SSN7° and 0SSN80^

values obtained from these adiusSnt^happr°aohlnS Part Per million and the needed and for projectssuch as thT **" "^ Hhere ereat accuracy is

for North Sea oL fnttlxtTtlZ ^u^i^d^T"0"*^ ^ V3lU6S

blocks, the periphery of each block LiZ?,^ f ™ WaS 0OI°Puted in

readjustments show significant etan»~ ?* °°ntr0;1 the next" Becent

Spain, Northern Norwafan^^ Italy du1 to'"f^fusl^^^pted values in Portugal,

which passed systematic errors Into the outivL Of/he 6arUer c™Put^ion?

Europe is, however, still usually based o^ the olTTl Natl°nal nappl"S ^

so. iy Dasea °" the old daturas and may well remain

:ostofa neTh^o^neius^^ °f HOTk and "» very

established by DOS, let alone tt ^'"'^f' °f £Ven those f^^works

technology it is not now ttecomputatxon ofT °f ^"^ Wlth mode- computing which is the major problem Sere are holt S °f V6ry large adjustments involved all of which wlll re™^ anaiv"^ In T* ** qUantities »f data

t

of which wlll re^ anaiv^ In T

f d h

s »f data

not an exciting task, the ultimate 'f f" a"d che<*inS- Although

pains taken at this stage. ° the Wh°le ^^ Hln d*P<«d on the

r^^ ^re^ ^T

secondary work providing croL'connect ons CTn^ oT"* °f ^

measurements than the wntetir* ^ ■ yns' Ine^ often contain more EDM

later, after EDM cLe fn'oienerL'use'^TheT' TT the "°rk "aS «"««-t8ken

p^^SL^f^oiva^te^ °f °b—ions will mean

purpose, it is SU8gestJ thSgt triL adjuit^ntf^ L^ D°S obse^a«ons for this traverse between Doppler station^%!nf ?f °Uld be aade of chai"= and/or

means that the qua!??,VtL ^r"^^' "L™88"1?" Ik iS ^ ** *™ -ch

positional closure on previously f[™li stations Eeparated "•« "e bias effect of

pabl1^\L1iT^rcan:ndBurO " •» a «*- *«

concepts and the computer ^ ^sf £"• re-adJustfflent^ the technical

are still working on an adjustment, which'started ?h- ^ taSk " EuroPea" nations

considerable cost. In considerin, ju^t^tLI ^^^^^^^^

SOS - SAVE THE OLD

s^vey f^r^ IITIT TnTeT ^ ^ "^ °f the °" s^«ons of the

engaged on mapping projects have encounte^/^M WS *"* °SD SUrVe^ parti-

destruction of, both pillars and ™TstaWols ^ °f ""^^ ^^> and

ECA/NRD/GART/48 Page 4

15 In North East Tanzania, between June 1983 and April 198*, in the course of reconnaissance for the establishment of ground control for 1:50,000 mapping 82

trigs were visited.

NORTH EAST TANZANIA: JUNE 83 TO APRIL 84

TRIGS VISITED BADLY DAMAGED BUT POSSIBLY RECOVERABLE

16 or 19-5%

DESTROYED AND BEYOND RECOVERY

18 or 22.0*

TOTAL DESTROYED OR AT RISK

Over one fifth were totally destroyed and beyond recovery of any accurate position (it was sometimes possible to use the position, estimated to a metre or so, for 1:50,000 mapping purposes only). Nearly another fifth were badly damaged or missing but could, if action Is taken quickly, be rebuilt accurately in their former position from buried and/or witness marks.

16. In Kenya, in July 1985, in the course of a 10 day reconnaissance for the establishment and identification of control for the air triangulation of a block o*

photography HI trigs, were visited. Nearly 30* were destroyed; over half of the

tota/were badly damaged, destroyed or missing although these could prob*** * ^

rebuilt, accurately if witness marks still exist, sometimes to within a few centimetres even if witness marks are destroyed because of their particular state.

KENYA: SOUTH NYAKZA: JULY 1985

TRIGS VISITED BADLY DAMAGED BUT POSSIBLY RECOVERABLE

21 or 51-2*

DESTROYED AND BEYOND RECOVERY

12 or 29-5*

TOTAL DESTROYED OR AT RISK

80.7*

17 In Uganda, OSD has no concrete statistics but have been told that a great

number of survey stations of all classifications have been destroyed. It is understood

that help is being sought for their replacement.

18 Most of this loss has been caused by deliberate action although this may have been based on innocent misinterpretation and/or ignorance of the ^tion of thestations. Considerable labour and ingenuity has been used to ensure that hidden, as well as surface, marks have been destroyed. In one instance a large hole 0 metres deep had been excavated in which the remains of a 1.2 metre concrete pillar

now lie.

Page 5

cannot be recovered. If S rt fC £ ?UrVe? stations ""eh do not exist

necessary to Info™ govern^ d « 1 u T^"1 aCtiO" does 3eem to »*

for infra structure, that" are ^ t f"™67 StaU°nS are Valust>le ite^

their destruction inevitably leads told r-LTl^ "* tO COOTdin^e and that Government needs to be convinced of th^hPl°n °f exPensiv^ »°rk.

before it becomes te^inaT i" to inspect %5?° "^ "^ tO repair daraa«e

regular inspection is also Jhe test ^v t, regularly and because such^

of the value placed on the concrete Mocks tnTZ ""* ^°Ple ^ally are aware

Funds invested in this wav a™T h the Way ln whioh they are used

repeating a raajor ^^'^""^J^.r'-i Pr°P°rU°n °f the °»" »^

Party offices, local councils and the "fSs^ c^t^, " ™a"Ple thrOUSh 5cbools'

ordinary people, especially those living!! central government must reach

are to understand toeir *£^£lSp'^?ZZ£X™* »»•««.. if they

" TTlTt ^^ ln the »»telllteS, the

Salaxy of receivers using several technio^f 7 f f°r flxati™ ««« a new

concept of a »U pack^e from Lxch coordin.f aV3lla"e and «" be refined.

refined, if necessary, by later recalculation of "^ 5e,read ins'»taneously science fiction dream but near realty even if ft^'6" d3ta> ls "° lonser a

watch. eamy even if its size ls not yet thafc Qf a w The nd

ls not yet thafc Qf

x^has nll'ZloTnlc^Vry I'TJlteZTl0' ^V^01' °r °f —observation

are any longer required at all L'.?'* coordinated sround station"

developing countries. It may not vet L *• *' P°Se particula'- problems to

deserves debate in this forum A nwtor^f * a"SWerable but " certainly

be considered in this paper ^ °f 1SSUes are involved - only three will

foreseeable that satellite fixat.nnfn ?rfer °f ':10'°°0 «ale. It ,s certainlv

to 1:2,500 or even u^f. f tlTrell'XTxTt "" ^ ^^^ZT^

-ed of fiction and a full GPS cc^L^^~^ if

-J- Cadastral surveys have tra.-n ti™,11

Ascription of shape and relative di't.n^ C°n°entrated °" the depiction and

As a result it nas'often ZenteM^TZ s^st^ "T °°—<linat-. or position.

reproduce the observation and calculation .7 f measurement which cannot

>f 10 seconds of arc and distance correct to °rle"taUon to southing of the order

regardless of the lengtb of the ^"^^TTg i^T

ECVNRD/GVRr/43 Page 6

expediency - the practicality of extending cadas^{«££to ™« "«

(or perhaps 'readings' might be a better term snee the surveyo ^

i^i^^^^srr r/^iS r™ aPPlied *,. *

measurements within a small area.

, . t ,, *.,-. m9t,v tvn^s of normal engineering

2*1. Much the same considerations appljr to many type feasible with

survey. The setting out of roads and **"« "^«th£ LeTe'ui always be once assoc^atefwith geodetic^framework survey will be needed.

Availability

25. A question which -ust be of concern is ^ber^U of us as ^ ^ providers of land information to our countries can «to^ jr^^ .& &

system, lodged in space, which might c°^e*^* for information may not be able stile environment and many ca-is cstand by if there is even a short radius_{„ ,, .} m,,^v, o c,h/-,r't hiatus in satellite signax&»n. ,,.,„ <a-i»nsi*? for whateveri^^ »»ii«

cause -

They should be maintained, for the cost oi UT*^ ± accuracy, cadasThere will, of

Affordability

27. At present, at least, the trend is that high tecfol^ instr«nt^

that would include satellite f^o^^ul^^^e^ive" «iet»?ve to p

they are utilised ^ten^^^' / *yhi_h[Y educated and skilled manpower- The

and their use is often dependent on nignxy e ^f ^ craftsmen and

manpower is limited, and considerably »"e£ consideration, but those unskilled labour of older techniques, willc^ x® ^^^^ ing countries. The

must come from a very limited SUPW includes a very nigh proportion of installing high ^chnology c™ ^ ^ foreSeeable future.

28. creat strides have been .ade in —.of ^^SSSL^S^l

to protect systems from outside climate and ^"d^°ns; N* t0 dust is hard on tropical climate of Africa with extra.es of_^xdity from ^> r essential electronic based systems. Mechanisms such as tape ™s

for portability, cannot be ^^/^^J^fbe needed and that this is both

equipment suggests that repair and maintenance wi exchange costs, unless tLe consuming and expensive, again with a bias to foreign * wcrkillg facilities are made available reasonably close to, and ace

areas.

Page 7

29.

use.

?!ADJtJSTMENT?

carrv

™) ^ frl°a" (There are already „- L^aineworks cf «t least ro.i. h " un"kelJ that this cost ct h d"contin»«ies on the

wholly to sa^"f.fe^JUStffient irrelevant. A Stnh f b««Wow, from

SCA/NRD/CART/48

References

1.

Overseas Surveys",

"The Adjustment of Main Survey Networks in Africa by the Directorate

Kiss L M Windsor directorate Paper presented to the 1st Symposium on Geodesy in Africa Khartouni 1974 Subsequently published in African Geodetic Journal Vol 1 No 2.

2- "Testing of Homogeneity in Geodetic Networks" S B Mattey Paper presented to the 3rd Symposium on Geodesy in Africa

Yamoussoukro 1986

3- "The African Arc of the 30th Meridian". K F Rain^ford

Empire Survey Review Vol 11 No 82 (1951)