Even though the vast majority of people associate the use of drones with the audiovisual sector, it is true that more and more people are becoming aware of their many other uses. One of the most relevant is the use in topography and cartography, performing the technique called photogrammetry to make 3D surveys, volume calculations, etc.
Next, we will talk about the advantages and disadvantages of photogrammetry with drones, explaining it in a simple and understandable way.
The drones are equipped with cameras and video cameras. These images are useful to obtain models that give a series of very helpful data in many fields. Let’s see a practical example:
Let’s imagine a coal mine, in which a series of extracts are piled up when the material is extracted. Currently, a team of surveyors on foot and manually perform a series of mathematical calculations to estimate the volume of each pile. The accuracy of these calculations has significant mistakes that are generally assumed to be normal. With the drones and by pre-marking a series of control points, incredibly accurate calculations of the exact volume can be obtained. In addition, one drone can do in one morning the work that several technicians would do for a week or more. In some cases in the mining sector, the hazardous nature of some work with toxic materials is also added, which allows the drones to considerably reduce occupational risks.
The process involves flying and capturing photographs in the first place. These images are subsequently processed by specific programs to obtain the 3D and ortho-mosaic models, such as Pix4D or Agisoft PhotoScan. With this type of program it is possible to obtain an accuracy of up to 1 centimeter in planimetry thanks to the inclusion, as we have already explained in the example, of support points taken by topographic GNSS. The ortho-mosaics obtained can reach up to 1 centimeter GSD (Ground Sample Distance), in other words, one pixel of the generated image represents a square of 1 centimeter side in reality. Ideal precision.
On the other hand, through GIS programs (Geographic Information Systems) it is possible to process the results in order to measure coordinates, distances, areas, and even volumes. In addition, it is possible to generate level curves, make profiles, and prepare DEM (Digital Elevation Model).
The several ADVANTAGES are remarkable. Some of them are as follows:
- The safety of the surveyor, this is the most remarkable advantage. Previously, the surveyor needed to be able to take stockpile points of material, so he/she had to climb up to the highest point of this, go along a steep slope in order to represent it, etc. with the possibility of slipping or with the unfortunate result of sinking into the stockpile.
- The efficiency. With the flight of the drone, millions of coloured points can be obtained; whereas before the surveyor had to observe point by point, obtaining only a few coordinates in which it would be difficult to obtain 500 points per day. In this way, the surface is better represented and a more realistic DEM can be obtained.
- The visual value. The possibility of having a graphic document with a history in which the progress can be appreciated as a possible guarantee for future claims or simply for a visual analysis was created. In this way, it is possible to visually compare the amount of material that has changed in a warehouse.
- The deadlines. Processing time is reduced and therefore labor costs are lowered. Years ago, photogrammetric flights by aircraft specifically suited to this work gave deadlines of approximately 1 month. By flying a drone and taking support points, this time is reduced to days.
The disadvantages, although few, have to be taken into account. Some examples include:
- Flying distance. It is limited by the law in force in Spain. Currently this distance is specified at 500m from the takeoff point for a BVLOS flight for a drone that is between 2 and 25kg. In the case of a drone whose take-off weight is less than 2kg this distance would be given by the range of the control station’s radio broadcast.
- The maximum height. As in the previous point, the maximum height is limited in Spain to 120 metres.
- The battery, this is the most important disadvantage. Batteries usually give a limited flight time. It is true that this is the component that is currently being researched the most in order to give the drone more flight time. Although the above makes the most efficient way of working difficult, it is easily solved by including a pair of spare batteries in order to be able to make several flights, since even having a long battery life we would still have the difficulty of distance and height restrictions. Nevertheless, what seems to be a disadvantage today is usually the easiest thing to do, given that most locations requiring this type of works are located away from urban centres.
The study areas are extensive, from quarries, mines, landfills, monitoring of works, archaeology, cartography, photogrammetric restitution…