How accurate are drone surveys?

Drone surveys are very efficient in gathering large areas of topographical information quickly.  However as drone surveys can only measure what it can see, this type of survey can yield errors if for example vegetation is long and if trees and buildings are in the way.  Errors seen in certain data sets have been up to 300mm or more due to things like long grass.  There are different accuracies between photogrammetry and lidar methodologies also.

However if high accuracy is what you need then hand held GPS rovers or total stations will get your accuracy down to 5-20mm.  This will of course increase the time and cost to produce the data.