What is digital photogrammetry and how do drones be used?

The development of the drone economy is not only related to drones, but especially to expanding the possibilities of their use using several types of sensors. The most common drone equipment is a camera, which can have different resolutions and features. In addition to nice holiday videos, nature shots or commercial videos and commercials, the UAV (Unmanned Aerial Vehicle) camera is becoming an important sensor for collecting data on the earth’s surface. Drone orthophotos are widely used in geodesy and mapping of smaller areas, construction, forestry, agriculture and environmental protection. In this article, we will look at how 3D models of the earth’s surface and buildings are created from two-dimensional photographs.

What is digital photogrammetry?

Photogrammetry is a scientific method of capturing various geometric shapes using images from cameras and camcorders. The captured images are then processed in specialized software into orthophotomaps and 2D / 3D visualizations.

Near-field photogrammetry captures an object from different angles, with accuracy and geometric resolution depending on the shooting distance, the size of the lens focal length, as well as the image configuration and data processing procedure. Ground photogrammetry is often used in construction to produce 2D / 3D documentation of buildings, video visualization of buildings, but also in the automotive industry (measurement of deformations in crash tests, inspection of parts from suppliers and others).

Aircraft, helicopters and unmanned aerial vehicles are used in aeronautical digital photogrammetry to obtain geographic data on larger areas. Large-format and medium-format digital cameras placed in an airplane or helicopter bring quality outputs, but at high procurement costs. The great revolution in this direction was the involvement of drones, which brought greater efficiency and availability.

Drones as an effective tool for collecting geographic data

The ability to relatively easily maneuver the drone at heights not only accelerated the process of obtaining orthophotos, but also significantly reduced costs. Aviation UAV photogrammetry is experiencing a great boom, as we can observe rapid technological developments and increasing competition in this industry, which leads to even greater availability of this solution. Fixed-wing aircraft drones as well as quadro-, hexa- and octocopters are used to collect images.

The advantage of aircraft platforms is the speed of data collection, an order of magnitude greater endurance, while with propeller drones it is possible to obtain more detailed images and oblique to horizontal images. Despite the gradual improvement in the quality of UAV cameras, the quality of orthophotos is lower than that of heavy large-format cameras, which are usually carried by an airplane or helicopter. Therefore, drone shots are mainly used for:

• mapping of smaller areas
• creation of 3D models of objects
• creation of geodetic data
• volume measurements of wood in forestry
• multispectral analyzes examining stand vitality
• fire risk investigation
• analyzes of damage after natural disasters
• preparation of documentation for water management
• environmental research

How does photogrammetry work?

The whole process of obtaining a three-dimensional view from 2D photographs takes place in two phases: data collection and processing. The site or object is first captured from different angles and perspectives. The acquired images are then processed by software, which draws common points in relation to the position based on the translation of individual images. The system of such points is called pointcloud and is the basis for the process of generating a 3D model of an object / territory. The accuracy of the results depends on the density of such points on the monitored area.

Thanks to the GPS modules built into the drones, each image contains metadata about altitude, latitude and longitude, and the type of sensor and optics. This data is also evaluated when creating a 3D model in specialized software such as DroneDeploy or Pix4D. To achieve good results, it is important to set how the images overlap, that is, what percentage of the first image is covered by the next image.

When obtaining data for a 3D model, it is also important to set up the automatic paths that the drone will perform in the given location or around the specified object.

Concrete example:

For example, we want to get a 3D model of a building. The drone first takes the first shots in a circular circle with the camera turned vertically at the highest height above the object. The second set of shots is performed at medium height with a smaller radius of the circle at an angle of 45 degrees. The drone gets the last shots at the lowest height at an angle of 70 degrees. The frequency of shots, flight paths, as well as other parameters are set by the drone operator in the application designed for it.

Data processing

As soon as the drone obtains the necessary images from different angles, they need to be processed. There are several software on the market, but in principle they all work with a certain algorithm, which evaluates the geographical data from individual points of the photographs and processes them into three-dimensional visualization. This operation is not so time consuming, because the operator just sets the calculation parameters and the software does the rest.

Photogrammetry versus LIDAR

In addition to photogrammetry, LIDAR technology is also used to make maps and 3D models. Unlike radar, which uses radio waves to detect objects, LIDAR emits infrared light and creates a point terrain model based on reflections.

The lidar sensor can also be mounted on an unmanned vehicle. It is used, for example, to study lands with dense vegetation, where light impulses can still penetrate between branches and leaves. In this case, LIDAR can provide better vertical accuracy than photogrammetry. This provides sufficient data accuracy only when scanning sparse vegetation. LIDAR is therefore the ideal solution for the forest industry and nature protection. However, in terms of broader research needs in construction, mining and quarrying, for example, photogrammetry is a more affordable solution with great accuracy and ease of use.

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Source: Nextech by www.nextech.sk.

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