The following Facebook posting comes from 'Your Irish Roots':

If your family held land in Ireland in the 1600s, there is a chance its name sits on a map made between 1656 and 1658. That was the Down Survey, carried out under the direction of Sir William Petty, and Trinity College Dublin's project describes it as "the first ever detailed land survey on a national scale anywhere in the world."

Its purpose was hard: to measure the land being forfeited by the Catholic Irish so it could be handed to Merchant Adventurers and English soldiers. The name comes from the way the work was done, with a chain "laid down and a scale made" across field after field.

Today the surviving maps and records have been brought back together online at downsurvey.ie.

You can browse county, barony, parish and townland, and search landowners by name as they stood in 1641 and in 1670. For a family historian, that can mean seeing who held your ancestors' townland before and after one of the biggest changes in Irish landholding.

The landowner search lets you compare names from 1641 with names from 1670 for the same townland.