There are several theories about what the proper flight line distance should be based on the IMU drift. IMU’s require movement to find themselves and continuing on a straight flight path causes them to drift and loss accuracy. I have seen flight lines as long as 150km and the solution at the end of the line is marginal at best. It seems that the Positional System manufactures (Applanix, Leica and others around the world) have improved their systems but we are still seeing issues when the lines are too long. In addition, if the system is flown long distances from the airport to the project site and collection begins immediately without turns the IMU doesn’t find itself usually until the second flight line collected. S – Turns should be flown prior to project collection when the airport is more than 20 minutes away. A good rule of thumb and it has been this way for sometime now is 20 minutes or 40 miles for higher accuracy jobs and 24 minutes for the 15 cm vertical and 30 cm horizontal jobs. I know this is pushed and sometimes the accuracies work out so I was looking for input from others, that have an opinion on the topic. The nice think about helicopter systems is these guys don’t have this problem typically, If you are one of these providers I would also be interested in hearing what you have to say about this topic.