The Down Survey was a cadastral survey of Ireland, carried out by English scientist William Petty in 1655 and 1656. It was created to provide for precise re-allocation of land confiscated from the Irish.

The survey was apparently called the "Down Survey" by Petty, either because the results were set down in maps or because the surveyors made use of Gunther's chain, which had to be "laid down" with every measure. At the time of its creation, it was considered one of the most accurate maps, and the first British imperial survey of an entire conquered nation.

In August 1649, the New Model Army, led by Oliver Cromwell, went to Ireland to re-occupy the country following the Irish Rebellion of 1641. This Cromwellian conquest was largely complete by 1652. This army was raised and supported by money advanced by private individuals, subscribed on the security of 2,500,000 acres (10,000 km²) of Irish land to be confiscated at the close of the rebellion. This approach had been provided for by the 1642 Adventurers' Act of the Long Parliament, which said that the Parliament's creditors could reclaim their debts by receiving confiscated land in Ireland.

The Act for the Settlement of Ireland 1652 provided for the confiscation and re-distribution of the lands of the defeated Irish, mostly Confederate Catholics, who had opposed Cromwell and supported the royalists. Parliamentarian soldiers who served in Ireland were entitled to an allotment of confiscated land there, in lieu of their wages, which the Parliament was unable to pay in full. Lands were also to be provided to a third group, settlers from England and America. The dispossessed landholders were to be transported to Connacht and to some counties in other provinces.

To facilitate the re-distribution, an accurate survey of the lands was required. Benjamin Worsley, the Surveyor General, had made a survey in 1653. Petty challenged Worsley's direction of the new survey, on the basis that Worsley intended to map only territorial boundaries, to the exclusion of the administrative boundaries introduced from 1520s for local government. The Civil Survey which preceded the Down Survey was not a mapped survey, but provided detailed descriptions of boundaries and valuations of holdings, and its data was used as input to Petty's survey. William Petty, then physician-general to the Irish armies, on a leave of absence from his position as Professor of Anatomy at Brasenose College, Oxford offered to undertake a new survey which would be concluded quickly – within thirteen months, more cheaply than the surveyor-general's proposals, and with a general map of the country. The Government signed a contract with Petty on 24 December 1654.

The survey employed about a thousand men and was performed with the promised rapidity, not by introducing new scientific methods, but by careful direction of the numerous subordinates among whom the labour was apportioned. Instead of using skilled surveyors, he completed the project using the now-unemployed – and cheap – soldiery. To enable unskilled soldiers to complete the task properly, Petty designed and built some simple instruments. The soldiers were only required to note the position of natural features and then use the chain provided to measure distances. Skilled cartographers then laid the information collected onto gridded paper at a central office in Dublin.

The method used was one of surveying the boundaries of parishes, the block of townlands inside those boundaries was not usually detailed. The scale used was generally 40 Irish perches to an inch (sometimes 80 perches), one perch equalling 21 feet (6.4 m), giving a scale of 1:251.43. This land survey method was used widely in rural Ireland up to the nineteenth century and sorting out the precise details was left usually to the legal profession. As a result, the Down Survey is considered to be about 87% accurate.^{[citation needed]}

Profitable and unprofitable land were distinguished, and there were abbreviated captions for arable, meadow, bog, woodland, mountain and several kinds of pasture, with area figures for each of these categories. Coverage of other subjects was uneven. In the parish maps, dwelling houses with the owners' names are entered in each townland.