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1/2PHOTOGRAMM ETRIC ENGINEERING & REMOTE SENSING M a r c h 1 9 9 9 2 33

C O L U M N

G r i d s & D a t u m s

C L I F F O R D J . M U G N I E R

IRELAND

The contents of t his column reflect the view s of the author,

who i s responsible for t he facts and accuracy of the data

presented herein. The contents do not necessarily reflectthe official v iew s or policies of the American Society for

Photogrammetry and Remote Sensing and/ or the Univer-

sity of New Orleans, nor that of The Ordnance Surveys in

Great Brit ain, Ireland or Northern Ireland.

This m onths top ic features both

the Republ ic of I re land and North-

e rn Ir e l an d . Th e i s lan d was i n v ad ed

by the Cel t s around 500 BC, an d was

conver ted to Chr i s t i an i ty in the 5 th

cen t u ry A D by Sain t Pat r ick . Boun d-

a ry m ap s i n I r e l an d were m ad e t o

accompany the ter r iers (proper ty

records) of the surveys in 1636-40 by

orde r of Lord St rafford, Viceroy of

I re land . In 1654-59 , the Down Sur-

v ey co m p r i s ed m ap s o f t h e t o wn -

lands averaging 300 acres each and

baronies to ta l ing over two- th i rds of

the su rface of Ireland , about 20,000,000

acres . The Ordnan ce Survey was es -

t ab l is h ed i n 1 7 9 1 t o p ro d u ce n a t i o n -

al mapping , th i s included a l l o f I re-

l an d . Th e n eed fo r an accu ra t e m ap

of Ire land was brou ght to the fore inthe 1800s by problems w i th a local

tax , know n as the Coun ty Cess . In

1824, the comm it tee chai red by Tho-

mas Spring Rice recommended to the

(British) House of Common s that a su r-

vey of Ireland was requ ired to provide

a definitive indication of acreages and

ratable values for the pu rpose of estab-

lishing local taxes in Ireland . That

same year, Colonel (later Major Gen-

eral) Thomas Colby of the Ordn ance

Survey received orders to proceed

with the work of t r iangulat ion and Six

Inch (6 = 1 mile) topographical sur-

veys for all of Ireland .

React ion to the Engineers by

t h e l o ca l p eo p l e was m i x ed b u t

general ly they were regarded wi th

s u s p i c i o n . A p a r ti cu l a r n u i s an ce

was the removal by local people

of the poles , se t up a s t argets on

mou ntains , before the surveyors

h ad a ch an ce t o o b s e rve t h em

and , in one case , the observers

were a t t acked . However , by con-

t ras t , in Glenomara, Coun ty Clare ,

t h e p eo p l e c l i m b ed t h e m o u n t a i n

wi t h t h em i n a g rea t c rowd , w i t h

f lu tes , p ipes and f iddles , t reat ing

the bui ld ing of the t r igonomet r i -

cal s ta t ion as a fes t ive occas ion .

The in i t i a l basel ine for the I r i sh

survey was se lected by Colby a t the

Plain of Magelligan n ear Lough Foyle

in Lond ond erry . Colby decid ed to

u s e t h e p r i n c i p l e o f co m p en s a t i o n

for the manufacture of the basel ine

m eas u r i n g ap p ara t u s . Th e p r i n c ip l e

was to use two metal bars , one of

b ras s an d o n e o f i ro n , wh i ch w ere

p l aced 1 1/8

inches apar t bu t jo ined

r ig id ly to each o ther a t thei r centers .

Th e b a r s were a l l o wed t o ex p an d o r

cont ract f reely, wi th p ivoted s teel

tongues f ixed to both bars a t thei r

en d s . Th es e were m ark ed wi t h s il -

v e r p i n s , an d a l t h o u g h t h e l en g t h o f

t h e b a r s ch an g ed wi t h t em p era t u re ,

t h e d i s t an ce b e tween t h e t wo p i n s

remained con s tan t . S ix se t s of bars

were m ade, each jus t over 10 feetlong . The basel ine of near ly 8 mi les

was measured over 1827-1828. The

Lough Foyle Base was remeasu red in

1960 us ing e lect ronic d i s tance mea-

sur ing equipment , wi th a d i f ference

found to Colbys or ig inal m easure-

m en t o f o n l y 1 i n ch !

The One Inch Map (1 = 1 mi le)

of Ire land w as cas t on the e l l ipso i -

dal Bonne pro ject ion in 1850 which

was the pro ject ion in vogue through-

out Europ e a t the t ime. Also used

for the inch series and some 9 in ch

sheets , the Lat i tude of Origin (No

) =

53 30 North , and the Cent ra l Merid-

ian (8o) = 8 00 West of Greenw ich .

The Ai ry 1830 el l ipso id w as used

wh ere a = 6 ,377,563.396 meters and1/

f= 299.3 . The rad ius of the mean

paral le l used for the Bonn e was

13,361,612.2 feet .

The topographic surveying de-

m an d ed g rea t e r accu racy t h an t h e

m et h o d s u s ed for t h e On e In ch M ap .

Colby i s sued an in s t ruct ion (speci f i -

ca t i on s ) i n wh a t b ecam e k n o wn asthe Colonels Blue Book. Note that

i n t h e Un i t ed S t a t e s, t h e cu r ren t

(1999) speci f icat ions for note keep-

ing for acceptance of data by the

Federal Geodet ic Cont ro l Commit tee

i s based on the Blue Book. How-

ever , Colbys ann ual repor t s w ere

al so termed the same. In 1838, wi th

the I r i sh survey on a f i rm foot ing,

Co l b y r e tu rn ed t o En gl an d an d t u rn -

ed his attention to the survey of Great

Bri tain. Sir Thoma s A. Larcom, KCB

was the Officer in Charge a t Mount -

joy, Dublin from 1828 to 1846. The

f inal cos t was 820,000 (more than

twice the or ig inal es t imate of t ime

and money) , bu t the Survey of I re-

land served as a model for the re-

maind er of Great Br i ta in .

In 1858, Captain Alexand er Ross

Clarke (the same fe l low that com-

pu ted h i s e l l ipso id s of 1858, 1866,

and 1880) , se lected the observat ions

t o b e u s ed i n t h e ad j u s t m en t .

Clarkes in ter locking netw ork of wel l condi t ioned t r i angles i s now

known as the Pr incipal Tr iangula-

t ion of Ireland (1824-1832). Clarke

r igorous ly ad jus ted th e observat ions

by the meth od of l eas t squares in 21

i n d ep en d en t l y co m p u t ed b u t co n -

n ec t ed b l o ck s w i t h t h e a i d o f an av -

erage of 8 comp uters (persons) . Note

that in 1881, Colonel A. R. Clarke,

R .E. received a forced re t i remen t

f rom the Ordn ance Survey of Great

Bri ta in ra ther than accept a pos t to

t h e i s l an d o f M au r i t iu s wh i ch was

th i s co lumn s topic l as t month .

Clarke was 52 years o ld a t the t ime.

For the remain ing 30 years of h i s

l i fe , h e p u b l i s h ed n o fu r t h e r s c i en -

t i f i c work .

In the m eantime, some form of

framework was requi red on w hich to

co n t ro l th e n ew m ap p i n g a t th e

scale of s ix inches to one mi le . S ix

CONTINUED ON PAGE 235

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