Early Irish surveyors cast long shadows

Early Irish surveyors cast long shadows

The view from above the summit of Mount Brandon in Ireland • Photo by Berx22 (CC BY-SA 4.0)

Originally published 2 September 2003

DINGLE, Ire­land — On the sum­mit of Mount Bran­don, near my sum­mer home on Ire­land’s Din­gle Penin­su­la, is a square con­crete pil­lar about 4 feet high.

The pil­lar was put in place some­time dur­ing the 1960s by the Irish Ord­nance Sur­vey, the nation­al map­ping agency, for mount­ing a theodo­lite, a tele­scop­ic sur­vey instru­ment. Sim­i­lar pil­lars can be found on oth­er sum­mits through­out Ireland.

The first car­tog­ra­phers to car­ry theodo­lites to Irish moun­tain­tops were agents of the British Ord­nance Sur­vey, dur­ing the 19th cen­tu­ry, when Ire­land was a British colony. They had no con­crete pil­lars then; they mount­ed their instru­ments on tem­po­rary tripods and braced them against the wind.

In 1824, the British Par­lia­ment autho­rized the map­ping of Ire­land on a scale of 6 inch­es to one mile. At this scale, a map of Ire­land would cov­er half a foot­ball field. No such large-scale coun­try­wide sur­vey had ever before been attempt­ed, any­where in the world.

The first step in this gigan­tic project was a “pri­ma­ry” trigono­met­ric sur­vey that would define pre­cise­ly the gen­er­al form of the island. Forty-five hill­top loca­tions scat­tered about Ire­land were select­ed to be the ver­tices of a spi­der web of tri­an­gles. If the angles of a tri­an­gle are mea­sured — with theodo­lites — the rel­a­tive sizes of the sides of the tri­an­gle can be com­put­ed. If every tri­an­gle shares one or more sides with oth­er tri­an­gles, all that remains is to mea­sure one side of one tri­an­gle for the whole web to be determined.

To this end, a base line near­ly 8 miles long was laid out along the shore of Lough Foyle in Coun­ty Der­ry, and mea­sured with unprece­dent­ed pre­ci­sion. From there a net of inter­lock­ing tri­an­gles was cast over Ireland.

For decades, British sur­vey­ors lugged their mas­sive theodo­lites to the tops of moun­tains, and wait­ed for clear weath­er that would enable them to catch a glimpse of reflect­ed sun­light — from an instru­ment called a helio­stat — or, at night, the gleam of a lime­light lantern, from oth­er moun­tain­tops dozens of miles away.

It was dif­fi­cult and dan­ger­ous work. Some­times sur­vey­ors were blown out of their tents by rag­ing gales. One man was killed.

In 1840, a sur­vey team reached the sum­mit of Mount Bran­don. They peered through their instru­ment to oth­er hill­top sta­tions, on the Iver­agh Penin­su­la across Din­gle Bay, at Taur Moun­tain to the east on the Cork-Ker­ry bor­der, and even as far away as Ben­corr in Connemara.

From the sum­mit of Bran­don, on a rare clear day, I have caught a glimpse of the Aran Islands, 65 miles away. But see­ing a glint of reflect­ed sun­light or a lantern’s beam from a Con­nemara moun­tain­top 30 miles fur­ther on strikes me as a near­ly mirac­u­lous act of patience with the Irish weather.

When the pri­ma­ry trigono­met­ric sur­vey of Ire­land was com­plete, each tri­an­gle was sub­di­vid­ed into a net of small­er tri­an­gles, then the area with­in each small tri­an­gle was plot­ted by men on the ground with mea­sur­ing chains.

The maps that result­ed from this project are beau­ti­ful, and remain today the rep­re­sen­ta­tion of their island upon which the Irish project their human affairs. When we bought our prop­er­ty in Ire­land, a map based on the 19th-cen­tu­ry sur­vey was used to denote the trans­ac­tion, the bound­aries hav­ing remained essen­tial­ly unchanged for more than a century.

Map­mak­ers no longer clam­ber to moun­tain­tops to do their sur­veys. The brac­ing of theodo­lites in howl­ing winds has been super­seded by map­ping tech­nolo­gies based on earth-orbit­ing satellites.

Today, any hill­walk­er with a pock­et-sized GPS (Glob­al Posi­tion­ing Sys­tem) device can know where he or she is with an accu­ra­cy equal to that of the 19th-cen­tu­ry sur­vey­ors. With a few clicks of a mouse I can see on the screen of my com­put­er a satel­lite pho­to­graph of almost any place on the earth­’s surface.

Still, we have those con­crete pil­lars on the moun­tain sum­mits to remind us of the inge­nu­ity and phys­i­cal sta­mi­na of the ear­li­er sur­vey­ors, who worked with far more basic tech­nolo­gies. To para­phrase Isaac New­ton: if we see fur­ther than our pre­de­ces­sors, it is because we stand on the shoul­ders of giants.

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