Overview

At present, one of the most commonly used technique for topographic surveys is aerial photogram-metry. This technique uses aerial images to determine the geometric properties of objects andspatial situations. Therefore, three-dimensional information is obtained from two-dimensional im-ages.

It is important to keep in mind that drones are only the platforms where the sensors that collectthe information are installed (in this case cameras to capture images), and that depending on thetype of work do not have to be the best option.

If it is determined that the use of a UAS platform is the best option, it must be taken into ac-count that there is a great variety of UASs in the market, and it is necessary to choose the mostappropriate system depending on the needs of the project.

Due to the great advance in drone technology, have emerged some UAS systems for photogram-metry that offer a very simple workflow of one-button survey. However, it is necessary to take intoaccount the importance of having knowledge about photogrammetry in order to use this technique,since leaving all procedures and decision making in the hands of a system can be risky.

Low cost systems have the advantage that the initial investment to acquire these equipments islower compared to professional systems. However, they are usually closed systems that do notallow modifications to adapt them to the different types of jobs that may arise. In this case, thebiggest limitation of this type of equipment is that the camera is integrated into the system, so youcould not use another camera that can be better adapted to the needs.

Objectives

• The aim of this article is to compare different UAS systems for use in aerial photogram-metry. As mentioned, there is a wide variety of equipments available in the market. In thiscase we have tried to compare low cost systems that offer complete and easy solutionswith professional systems specifically designed for aerial photogrammetry.

• The ultimate goal is to compare the performance that different equipment offers us in aphotogrammetric flight.

1 Materials and Methods

1.1 Materials

To perform the tests, two multirotor of different characteristics have been used. In this case thePhantom 4 model was chosen as an example of a UAS not specifically designed for aerial pho-togrammetry and the LOOKOUT VTOL model as an example of a UAS specifically designed forphotogrammetry.

• Phantom 4

As an example of a UAS that offers an integrated and lower cost solution Phantom 4 of DJI.

Figure 1: DJI Phantom 4

• LOOKOUT VTOL

As an example of a professional UAS designed specifically for aerial photogrammetry, the LOOK-OUT VTOL product of Throttle Aerospace Systems has been used.

The main difference between the two systems is that the LOOKOUT VTOL model allows theintegration of different sensors, while the Phantom 4 model has its own integrated RGB camera,so it is not possible to integrate any other sensor.

Figure 2: Throttle Aerospace Systems LOOKOUT VTOL

Table 1: The following table summarizes the main features of both devices.

Parameters Phantom 4 Lookout VTOL

Endurance (min) 20 40

Payload in (g) 250 2000

Physical size in (mm) 350×350 500×500×175

Range in (km) 2 8

As you can see, endurance and payload capacity are major difference between both systems,more endurance helps to airborne the sensor.

Table 2: Comparison table of the sensors.

Parameters Phantom 4 Sensor Lookout VTOL Sensor

Sensor Size (mm) 6.20×4.65 23.5×15.6

Resolution (píxel) 4000×3000 (12.4 Mp) 6000×4000 (24.3 Mp)

Pixel size (um) 1.55 4.2

Focal length (mm) 3,6125 16

ISO sensitivity 100 - 1.6000 100 - 16.000

Shutter speed (s) 8 - 1/8.000 30 - 1/4000

1.2 Methods

To make the comparison between both systems, two missions have been planned, one with eachof the equipment and with the sensors mentioned above.

One of the most important factors when planning a photogrammetric flight is the GSD (GroundSample Distance), since it is the spatial resolution (the detail) of the images that will be obtainedfrom the aerial flights, that is to say, the distance that corresponds to each pixel in the terrain. TheGSD need to a job depends on the needs of this.

As the objective of these tests has been to compare systems with different characteristics, flightheights have been adapted to the terrain (H) to obtain a similar GSD with both systems and tomake comparable comparisons.

Table 3: Comparison table of the relationship between flight height and GSD obtained withthe different systems.

To obtain a similar GSD with both systems, the flight of the Phantom 4 has been planned at aheight of 60 meters (GSD: 2.57 cm) and the LOOKOUT VTOL flight at a height of 100 meters(GSD: 2.65 cm).

On the other hand, the missions have been planned with the same speed of the aircraft andobtaining the same overlaps between images, so that both flights were comparable.

Height (m) GSD of Phantom 4 Sensor (cm) GSD of Lookout VTOL (cm)

60 2.57 1.59

80 3.43 2.12

120 5.15 3.18

In this case the values used for these parameters were as follows:

• Aircraft speed: 6 m/s

• Longitudinal overlap: 70%

• Transversal overlap: 70%

In both cases, planning has been done trying to cover the largest possible area on each flight.For this, the limitation of the flight time of each aircraft has been taken into account. The followingtable shows the most determinant parameters used in each of the flights.

Table 4: Comparison table of plannings

Parameters Phantom 4 Lookout VTOL

Overlaps 70% / 70% 70% / 70%

Flight height (m) 60 100

GSD (cm) 2.57 2.65

Flight time (min) 25 40

2 Results

Using the mentioned flight parameters, the missions were planned and the following results wereobtained related to the performance of the two systems:

Table 5: Comparison table of results.

Parameters Phantom 4 Sensor Lookout VTOL with Sony Alpha 6000

No of images 260 (Aprox) 450 (Aprox)

Output Image Size (Mb) 12 Mb 10 Mb

Dimensions of each photography (m) 103 × 77.2 158.75 × 105.8

Area of each photography (Acre) 1.96 3.45

Total covered area (Acre) 42 166

As you can see in this table, the results obtained in terms of performance are much better usingthe professional LOOKOUT VTOL system.

In a single flight it is possible to cover a three times greater area using the LOOKOUT VTOL incomparison with the Phantom 4. Therefore, it would be necessary to make three flights with thePhantom 4 to cover the same area that LOOKOUT VTOL (with the Sony Alpha 6000 camera) isable to cover in a single flight, considering that in both cases we want to obtain the same spatialresolution (GSD). This would mean an increase of time, since to realize three flights with thePhantom 4 would be necessary 75 minutes, against the 45 of the LOOKOUT VTOL. And thatwithout taking into account that between different flights it is necessary to land, take off, changebattery, check the new flight plan.

On the other hand, the volume of photographs obtained to cover the same area would be higherwith the Phantom 4, since in three flights it would be necessary to make 1027 (12.5 Gb data)photographs in front of the 450(4.5 Gb data) that would be necessary with the LOOKOUT VTOLto cover the same area with the same resolution.

3 Conclusions

As mentioned, there are significant differences between the two systems in terms of performance.In this aspect, LOOKOUT VTOL wins. The saving of flight time (field work) and image processing(office work), saves time when doing a project, which translates into a saving of money.

In addition, LOOKOUT VTOL allows the integration of different sensors, offering the possibility ofusing other sensors that can further increase this difference in performance. Not to mention that itis possible to integrate other cameras like multispectral or thermal.

Therefore, low-cost drones can be an economical option for small jobs less than 1sqkm projects,but for larger jobs or in which different sensors are need to adapt the equipment to differentprojects, it is necessary to go to a professional system.