In 1824 Colby and the Ordnance Survey were given the task of surveying Ireland. He also decided to have the work carried on under direct official supervision, and raised three companies of sappers and miners to be trained in survey duties. Later many Irish surveyors were used. It began with Colby and a small party of sappers on Divis near Belfast, in 1825. He devised a dual arrangement of brass and iron, called by him a "compensation bar", which as the "Colby bar" was used base-measurements in all parts of the world. A base-line, eight miles long, was measured under his superintendence, on the southern side of Lough Foyle, in 1827–8. When the standard yard was destroyed in the 1834 Burning of Parliament, it was restored by going back to Colby's work.