Dronewatch helps the province with green inventory in the urban environment
“Is it also possible to map out the landscaping in Leiden-Oost with a drone?” With that demand from the province of South Holland started a search for a way to conduct a mapping flight within the regulations with a drone over urban area, something that is difficult to achieve even within the Specific category. Dronewatch found a solution and carried out the inventory successfully.
Those living in the Leiden area know that there is currently a lot of work on the RijnlandRoute, an infrastructure project of the province. Not only is a new provincial road being built between the A4 and the A44 near Leiden (the N434), work is also being made on better access to Leiden on the east side of the city. As part of this improved opening of Leiden East, the N206 Europaweg and Lammenschansplein will be redesigned. This means that roads will be shifted and widened.
To get a good idea of the green in an urban environment, a tree count often takes place, whereby the position of each tree is determined by means of a GPS receiver on the ground. These coordinates are then entered into the GIS system. This method is time-consuming and the coordinates are not always accurate. This led from the province to the question whether it is also possible to carry out such an inventory with a drone, in addition to the field visit.
Now it is technically no problem to create a detailed orthomosaics (a 2D top view of an area consisting of several strung source photos) from a piece of territory using a drone. Using a mapping drone such as the DJI Phantom 4 RTK and associated apps, running a mapping flight is a breeze. But it will be a different story as soon as it comes to a somewhat larger area of urban area: then the regulations quickly become a bit of a problem.
The problem is that as a drone operator in the Specific category with an operational authorization based on SAIL level II, you can only perform drone flights over controlled territory. That is, there are no uninvolved persons on the ground, and everyone is aware of the drone operation. When it comes to an inspection of a house or block, that is still to be done, but when it comes to a somewhat larger area of urban area — as in this case — that method is not realistic.
This led to the idea of performing the mapping flight with a very lightweight drone, the DJI Mini 2, which weighs less than 250 grams, which means that in Open category A1, it can be flown with it above built-up area and even above uninvolved persons, as long as it is. is not about a crowd. The camera performs relatively well and a first test quickly revealed that it was perfectly possible to collect source photos with these minidrone source photos for the purpose of orthomosaics. Based on that first test, Dronewatch was given the green light for the inventory.
The first challenge was to control the intended drone automatically. That initially proved problematic: DJI initially had no support for the Mini 2 in the SDK, so that external developers could not support this drone in their flight planning apps. And flying a mapping mission manually isn’t actually an option, because you want to keep things like overlap between photos in control.
Fortunately, an update to the SDK came at the end of December. Soon, the DroneLink app provided support for the Mini 2, in this app it is possible to create mapping missions, which are then automatically executed by the drone. Depending on parameters such as desired flight height and speed, overlap between the photos and perimeter of the area, a ready flight plan rolls out.
One of the challenges was not to let the drone fly too fast, in order to prevent distortion by rolling shutter and motion blur. Fortunately, the Mini 2 has a relatively long flight time, and in twenty minutes a nice stretch of territory can be covered. In this way, the necessary source photos could be taken over the course of a few days (in relatively windless weather).
While processing the source recordings into an orthomosaics, another challenge arose. Which turned out: the Mini 2’s camera does not always point exactly 90º down. As a result, (part of) the source photos were taken at an angle. This led to an orthophoto that was difficult to position in the GIS system. Fortunately, with the help of other processing software, this stand error proved to compensate for fine. Although there was a slight deviation in absolute accuracy, it could be largely eliminated by means of visual reference points.
Remarkably, almost no one on the ground had any notice of the drone flying over: at a flight height of 80 meters, the drone is virtually inaudible and only visible if you know where to look. Only during the start and landing did there sometimes inform passers-by what we were doing. Without exception, they found it interesting to hear what the small drone was used for.
Due to the relatively high flight altitude, no problems arose in terms of privacy either. Although there was flown above residential houses and gardens, no one was identified in the picture. As a result, no DPIA (data privacy impact analysis) had to be drawn up. In fact, the delivered orthophoto is nothing but an aerial photo as you can see it in Google Maps, but more up to date.
It is the first time in the Netherlands that a drone mapping of urban area has been carried out in this way, within the operational possibilities of the Open category. According to the client, the delivered orthomosaics gives a good overview of the current landscaping. The tree locations could be accurately determined on the basis of the aerial shots.
As far as Dronewatch is concerned, this case study clearly shows how powerful small drones can be and that even an automated workflow is possible when using a drone such as the DJI Mini 2, so even if a mapping mission cannot be carried out within the operational Preconditions of the Specific category, there are still options.
“Incidentally, the trees were the 1st reason, so that we can properly map our field inventory and therefore gain insight into the omissions. Furthermore, it is nice to have such sharp images of the project area anyway. We can project designs on it and have insight into the details outside immediately,” adds the client.