What is Fly Levels in Surveying?

Thank you for your explanation

Fly levelling it is a very approximate form of levelling in which sights are taken as largeas possible. In this method a line of levels is run to determine approximately reduced levelsof the points carried out with more rapidly and less precision. It is a levelling that is done toconnect benchmark to the starting point of the survey line. In this levelling only back sightand fore sight readings are taken and auto level is moved strictly on the line joining benchmark and starting point of survey line