Introduction
Flying a roof while taking off from the ground may present challenges as the overlap of flight is calculated at ground level, even if you have factored in the height of the building for the GSD your overlap values will be much lower than the set values.
Depending on the equipment you are using there are a few options. The first and much simpler option if you are using the DJI Pilot 2 app, is using the Target surface to takeoff point field. The second option and more complicated involved calculating the flight at ground level and then recalculate the overlaps once the height of the roof has been added.
We will explain both options in more detail below.
Target surface to takeoff point
The Target surface to takeoff point allows the user to factor in objects that are higher or lower than the takeoff point into the planning of the flight.
For flying rooftops this becomes very useful to ensure that the correct overlaps are used. Planning in the height of the rooftop in this field, the pilot application will recalculate the GSD and overlaps based on this new height. Therefore you will not see a change in the GSD or Overlaps from the original planning, but you will see the height of the flight has been increased by the height input into the target surface to takeoff point field.
If you don't know the height of the roof you are flying, you can use your drone to estimate the height of the roof by flying next to it and using that height information to input into the target surface to takeoff point field.
If there are variations in the roof height and they are within a few meters you can use this methodology and take the highest point to use as your input. If the difference in heights are greater than 5m you should carry out separate flights using different inputs for the surface height.
How to set up your flight without the DJI Pilot 2 app
1 - Estimate the height of the roof.
For tilted roofs, note the height of the highest part
2 - Prepare your flight without taking the building into account.
For the height, do not use the total height for the flight. Start by introducing the height as if there was no building. For example, if you want to fly 34m above the roof, start planning the mission at 34m above the ground.
For the overlap, use the values defined as per the data product requirements.
Note down the estimate of the number of photos.
3 - Modify the mission to take the building height into account
Add the height of the roof to the height of the flight. If the roof is 10m high, add this to the 34m to get to 44m in total.
You will see that the number of photos will have gone down significantly at this stage.
You must increase both overlap values (front and side) until the number of photos reaches the previous amount calculated in point 2.
Do not forget to ensure the front and side overlaps have the same value. Typically, for a roof of 10m, the overlap will have to go from 80% to approximately 86% front and side.