Since aerial photography and computer methods of data processing came into geodetic research, the first step towards a new era in this sought-after area of human activity has been made. The second step can be confidently considered the introduction of unmanned aerial vehicles equipped with compact data collection equipment. Together with new software applications, mobile devices (smartphones and tablets), as well as cloud services for working with data, unmanned technologies have made, if not a revolution in geodesy, then they have certainly proven their reliability, cost-effectiveness and the highest quality of data.
Traditional methods of geodetic research and their shortcomings
Ground methods of geodetic surveys are complex and rather costly in terms of time and human resources. As specialists know, for some time now the “gentleman’s set” of surveyors has become total stations, GPS receivers and ground-based laser scanners, which allow obtaining high-resolution spatial data for the topography of the earth’s surface.
But with the continuous progress in drone technology over the past few years, drones have become a prominent tool for surveying and mapping. To perform mapping tasks, drones are equipped with high-precision digital cameras, geo-positioning modules, and complement hardware and software to improve accuracy and reliability. UAVs equipped in this way can perform the following tasks:
- to create digital 3D terrain models,
- on creating photographic schemes and photoplans by means of aerial photography
- on the creation of orthophotomaps and topographic plans.
To create accurate real-life 3D models from 2D images, drone mapping uses a technique called photogrammetry. By merging and processing multiple geo-referenced aerial photographs, photogrammetric techniques are used to generate output data: 3D point clouds, raster digital elevation models, and orthomosaics.
Benefits of using UAV for photogrammetry
Experts who already use drones for geodetic work were able to appreciate the many advantages of the new method. This is also evidenced by the fact that in addition to standard drone models equipped with specialized full-time cameras and other accessories for high-precision shooting, there are even home-made models based on DJI drones. Some decide to make their own changes to the design, supplementing the capabilities of reliable and powerful models of the Chinese UAV market leader with their own functionality.
So what are the advantages of drones in geodetic research most often emphasized by experts? Here are the most important ones:
- availability of precise positioning systems;
- flight automation and survey technologies;
- automation of the flight task, including the flight route;
- many possibilities for installing additional equipment (especially on the Matrice series models);
- the ability to work in adverse weather conditions (mainly for the DJI Matrice 200 series);
- the ability to work at low temperatures (also for the DJI Matrice 200 series);
- powerful power plant and intelligent power management system;
- flight duration sufficient to perform tasks;
- availability of intelligent piloting systems;
- compactness and rapid deployment.
One of the most important benefits of drone mapping can be considered significant time savings. In a much shorter period of time surveyors using unmanned technology perform a much larger amount of work than their counterparts working through traditional methods. That is, the amount of filming that was traditionally carried out for a week, or even more, the team, “armed” with a drone, will do in 1 day. And reducing the time of work while maintaining high quality also means significant budget savings.
The speed and flexibility of deploying drones, along with supporting equipment, allows them to be used in a wide variety of places. This is extremely important, since it eliminates the need to send a survey team to dangerous areas: areas with difficult terrain, areas of heavy traffic, areas of unstable soil and steep embankments. Even cell towers and treetops are no longer an insurmountable obstacle if the work is done with the help of UAVs.
The versatility and flexibility of drones and their obvious advantages in the field of photogrammetry began to turn, in a number of reviews, these devices into almost the only means of solving the problems of geodetic research. Indeed, many works talk about the high accuracy of drones. But is it really so? One thing is theory. And what does the practice of using UAVs for geodesy tell us? Is it reasonable to consider drones as an effective tool for photogrammetry?
Although real work on photogrammetry has already been carried out for more than a year, some results of scientific research and testing will not be superfluous for a positive answer to the above question.
In December 2019, the Journal of Unmanned Vehicle Systems published a report on the use of the latest unmanned systems and equipment for them. Testing was carried out to test the mentioned technologies in the issue of mapping data processing and the possibility of their further use for geodesy. Testing has proven that the UAV, together with specialized equipment and software modules, can perform high-quality surveying and data processing for the needs of cartography. Depending on the situation, these methods can either complement traditional ground technologies or replace them completely.
In support of their words, the researchers performed a triangulation survey of the church, which was located on a plot of 1.6 hectares. Surveyed 7 times by DJI Inspire 2 drone with high-precision PPK GNSS system and Zenmuse X4S camera. On the site there were objects with different types of surface and different shapes. For example, objects with a smooth surface have always been an easy task for drones (creating models of parking lots, fields, building roofs, etc.). In addition to them, the site also had more complex structures and objects to model: walls, wires in the air and vegetated ground. The researchers used data from pairs of stereo images to fill in missing or distorted features.
Using a professional grade drone with a positioning system meant no ground control points. Despite this, the survey team was able to collect 23 well-distributed control points to calculate the redundancy transformation and provide enough data to meet the research objectives. At the same time, the process of creating images for their subsequent processing took only half a day of work in the field. And the processing process itself was completed in a day and a half. If the same task were performed by a team working on traditional technologies, then the entire process of collecting and processing data would take at least 3 days.
When performing drone mapping, the researchers noted consistency and high accuracy (mean square error was only 2 cm (RMSE)). That is, the technology can be used for many practical tasks that are typical for land topographic surveys. At the same time, the researchers also noted that the shooting accuracy was higher when the drone was at low altitude. The only drawback of this solution was a slightly lower efficiency. Test flights and surveys performed by the researchers showed that with this use of UAVs for geodetic surveys, savings in time and financial costs would be 33% and 58%, respectively.
To be more objective and to illustrate the practicality of drone mapping technology, the researchers also completed a roof survey to install solar panels on it. 3 grocery stores were taken as objects of shooting (more precisely, their roof), and the area of \u200b\u200bthe objects ranged from 1.2 to 2.8 hectares. The aerial photography work was completed in one day. All parts, including gas supply lines, ventilation and heating systems, skylights and electrical panels that were in the roof area, were filmed without the presence of any of the people on the roof. If the work were carried out using traditional ground means, then, according to experts, all the work would take up to two weeks (more precisely, 12 days): from the start of the project to its delivery to the customer. UAVs allowed to reduce 58% in terms of time and 41% in terms of costs, since all work was completed in less than 7 days.
As another example, a 105 ha golf course was taken as the subject of a complete topographic and contour survey using aerial photography. All data collection work was completed in 1 day. On the contrary, if the team used classical methods, then it would be necessary to recruit 3 teams of specialists to complete the work and allocate approximately 30 days for the complete collection of all data.
Of course, drones cannot be (at least not yet) a complete replacement for ground methods, because in some areas with thick vegetation cover, you still have to use conventional methods. But even in such cases, or when the terrain is not too covered with vegetation, drones successfully solve the tasks set, compiling an accurate map of such objects using stereo compilation and selecting an appropriate stereo model. In the example above, the entire mapping job was completed in 15 days, reducing budget costs by 75% and time costs by 50%.
In summary, today drones have become an extremely valuable addition to the toolkit of surveyors and are increasingly complementing or replacing ground-based surveying methods for various needs. A next-generation mapping drone like the DJI Phantom 4 RTK is already enabling surveyors to achieve centimeter-level data accuracy with fewer control points. And with powerful mapping software such as DJI Terra, which enables the use of 2D orthophotos and 3D models with increased absolute accuracy, surveyors can achieve the desired product quality for important projects.