Use of drones in urban rectification

The use of images obtained from Unmanned Aerial Vehicles (UAVs), including drones, i

becoming increasingly common in various areas such as photography and video recording.

The use of images in Surveying has been around since flights in manned aircraft, where a structure for the photographic camera was fixed to the aircraft and the operator took shots at intervals, considering various factors such as the aircraft's altitude, speed, photo scale, among others. The entire evolution could fit into a long and beautiful article, but the purpose of this one is to illustrate how such a tool can be applied in urban surveys for rectification purposes.

The case involved a completely occupied and walled plot. The exact occupation of its vertices with GNSS receivers is unfeasible, as there are buildings all around, and there would hardly be enough signal to ensure the collection of observations necessary to calculate its coordinates accurately in a short or medium time frame. With conventional topography, using a Total Station, the vertices at the front of the property would be easily surveyed, but there would be much difficulty for those at the back. Even if you survey the points at the front of the property and pull a tape measure to the ones at the back, this brings inaccuracy, as there is no guarantee of the orthogonality of the terrain, not to mention irregular polygon cases. Therefore, a survey using a drone was tested.

First, planning was carried out so that the weather conditions were favorable close to the survey time. The flight planning, which lasted just over 2 minutes at an altitude of 60 meters above ground level, was carried out with specific software, always taking care to avoid collisions with surrounding buildings. Finally, something as important as the previous items was the choice of the take-off and landing location for the drone. On the street, in front of the numerous tall buildings, there was no safety regarding radio and GPS signals. Therefore, a nearby open-air parking lot embedded in a block was sought so that the drone could have security throughout its trajectory. With that done, the flight was carried out safely. The image quality is high, with a resolution below 2cm per pixel. The material processing was done with appropriate software, and, as important as the flight, was the collection with GNSS receivers of three points for control and adjustment in the survey processing, carried out on the block.

As a result, it was possible to have an orthophoto with a positional error of about 1cm, ensuring to the client not only the accuracy of their survey but also the quality of the product. Receiving a plan with a perimeter overlaid on a high-resolution image makes a difference. The entire boundary was easily identified in the image. The vertices of the perimeter were easily determined, and thus the plan and descriptive memorial were prepared with georeferenced coordinates, which is not yet required in the registry offices for urban properties.

However, it is also clear that there are restrictions. If the object of the work were the neighboring property, one of the boundary points would be covered by the canopy of a tree. In this case, the determination of this vertex could be done by intersection. But there may be other situations that prevent the exact definition of one or more vertices of the property. Additionally, there are flight-restricted areas, especially near airports. That is, in addition to the checklist for the flight, another must be made for the product that will be obtained after the flight and whether it will meet the demand.

Finally, it is necessary to take into account the cost of field and office work compared to other methodologies. Obviously, in cases of REURB, the subject of the next article, the advantages are greater because entire blocks are surveyed for the same flight, but I conclude that if all conditions are favorable, it is a viable procedure that brings both technical quality and legal security, as neighboring properties are also surveyed and identified.