Crown lifting is the removal of the lowest branches, and pruning/reducing of lower branches for future removal, normally to a uniform height from ground level.
Why have it done?
Common reasons for crown lifting are to bring more light and provide clearance for pedestrians, vehicles, buildings or lines of sight. Minimum heights are usually dictated by local laws. In the U.K. common practice dictates clearance should be 5.2 metres (approx 17ft) for vehicles and 2.5metres (approx 8ft) for pedestrians