ORDNANCE SURVEY. By this term is understood the various operations undertaken by the ordnance department of the British government for preparing maps and plans of the whole kingdom and its parts. The idea of a general map of the country to be exe cuted by the government was first proposed after the rebellion in 1745, when the want of any reliable map of the northern parts of Scotland was much felt by the officers in corn ' mend of the royal troops. Its execution was intrusted to lieut.gen. Watson, the deputy quarter-master of North Britain; but 'it was mostly carried out by maj.gen. Roy, an officer of engineers. The drawing, on a scale of one inch and th•ee-iburths to the mile, was completed in 1755; but in consequence of the war which broke out in that year, was never published. In 1763 it was proposed to extend the survey to the whole kingdom; but the first steps to effect this were taken only in 1784, when maj.gen. Roy commenced measuring a baseline on Hounslow heath, near London. This principal triangulation was designed partly for astronomical purposes, and partly as a basis for a map on a small scale. The base-line was remeasured with great care in 1791; and detail plans were com menced by officers of the royal engineers, partly for practising them in military drawing, and partly for the purpose of forming plans of some portions of Kent fur the use of the ordnance. The principal object was, however, the instruction of a corps of military sur veyors and draughtsmen, the plans themselves being regarded as of secondary import ance. In 1794 the survey for the one-inch map was begun, and some sheets were pub lished in 1795. As the series of principal, triangles were extended westward towards the Land's End, it was thought right to measure another base, for verification, on Salis bury plain in 1794; and two other base-lines were subsequently measured—one in 1801 at Mistertou Carr, and the other in 1806 on Ruddlan marsh. Though first intended chiefly as a military map, the publication of the survey soon created a desire on the part of the ' public for better maps, and surveyors were then hired to hasten its progress. This,
however, was very slow, the map being at one time entirely suspended during the war in the beginning of this century, and even the parts which were executed, having been done by contract, were found very inaccurate. In this condition the survey of England con tinued timing the first quarter of the present century, sometimes delayed by the govern ment from motives of economy, at other times urged on by the county gentlemen, who wished the map either as a hunting,-map or for local improvements.
In Scotland, the principal triangulation was begun in 1809, but was discontinued iu the following year, to enable the persons who bad been employed there to carry for ward the subordinate triangulation required for constructing the detail maps in Eng land. In 1813 it was resumed, and continued steadily up to 1819; a new base-line hav ing been measured ou Beihelvie Links, near Aberdeen, in 1817, and the great sector used at various stations, both on the mainland and in the islands. In 1820 it was again suspended, was resumed in 1821 and 1822, and anew broken off in 1S23, the large theo dolite being wanted in order to proceed with the principal triangulation in s. Britain. In 1S24 the survey of Ireland was begun, and nothing more was done in Scotland tAi 1833, except that some detail surveying for a one-inch map was continued for a few years in the southern counties. The chief strength of the surveying corps was now transferred to Ireland. A map of that country was required for the purpose of making a valuation which should form the basis of certain fiscal arrangements and other improvements which the social evils and anomalies of Ireland urgently demanded. For this map a scale of 6 in. to the mile was adopted, as best suited for the purposes in view. On this scale the whole map was completed, and published in 1845, though the first por tions were in an imperfect form, and needed revision, which was proceeded with iu 1873.