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Exploring the potential of videogrammetry

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Traditionally, surveying relied heavily on manual measurements and ground-based techniques, which were time-consuming, labour-intensive and often limited in scope. With the somewhat recent emergence of photogrammetry, surveying professionals gained access to powerful tools that streamline processes, improve accuracy and unlock new possibilities in data collection and analysis.

How videogrammetry works
Videogrammetry is effectively an extension of photogrammetry; the mechanics are grounded in similar principles. Photogrammetry involves deriving accurate three-dimensional information from two-dimensional images by analysing their geometric properties and spatial relationships. The core of videogrammetry is to separate the video footage into images with regard to sufficient overlap and image quality. The subsequent workflow is almost the same as in photogrammetry. The quality of the output depends heavily on image resolution, frames per second (FPS) and stabilization.

Integration with hardware and software
Central to videogrammetry is the camera system used to capture video footage along with any supporting GPS devices used. Ground-based videogrammetry may utilize smartphone cameras or handheld digital cameras with video capabilities.

While many smartphones and digital cameras have built-in GPS capabilities that geotag photographs with location data, their accuracy is at best 2.5m which is not always optimal for professional surveying. For this reason, when using a smartphone camera, many surveyors opt for an external real-time kinematic (RTK) antenna with 2cm accuracy. Attaching it to their smartphones enables them to receive correction data from a local NTRIP provider. This is a far more convenient option than placing and measuring ground control points (GCPs) on the terrain in combination with a GNSS device. Some of the well-known smartphone RTK products on the market today include REDCatch’s SmartphoneRTK, ArduSimple’s ZED-F9P RTK receiver, and Pix4D’s ViDoc RTK.

It is important to note that while most smartphones can geotag photographs, not all can geotag video footage. Moreover, those smartphones that can geotag video can only geotag the first frame of the video. For this reason, users may require additional apps (e.g. PIX4Dcatch: 3D scanner) that can embed location data into the video’s metadata so that it can be used for surveying and mapping purposes.

While non-geotagged videos can still be used for 3D model creation in various software solutions, it is recommended to opt for an RTK receiver as a minimum prerequisite for professional applications. At 3Dsurvey, utilization of Google’s Pixel 7a smartphone paired with REDcatch’s external RTK is the preferred option. Later this year, 3Dsurvey is set to release a ScanApp that it has developed to embed RTK correction data into video file metadata, enabling automatic georeferencing for videogrammetry projects.

Examples of videogrammetry project approaches
Non-geotagged (using any smartphone or camera)
This can be ideal for the 3D documentation of cultural heritage projects. However, this approach lacks the spatial accuracy necessary for tasks such as outdoor mapping or infrastructure monitoring, which demand precise georeferencing.

Accurately geotagged using external RTK (using a smartphone)
Accurate geotagging using a smartphone equipped with an external RTK GNSS receiver ensures that the resulting 3D models maintain high spatial fidelity. Therefore, this approach is suitable for applications such as land surveying and small-scale construction monitoring projects where precise positioning is crucial. Examples include mapping manholes, pipes, low-level material piles, dig sites and cables for telecommunication or electricity.

Within a larger photogrammetry/Lidar project
For situations demanding the highest level of accuracy, videogrammetry can fill a gap or add another perspective to the aerial dataset obtained using other technologies, such as ground-level videogrammetry in combination with aerial Lidar (which lacks oblique views). Videogrammetry can also prove invaluable on site while drone mapping, such as when trees obstruct the flight path or if the project requires capturing details facing upwards. Similar to drone workflows, strategically placed and precisely measured GCPs can significantly improve the overall precision of the generated 3D model. Since videogrammetry usually involves capturing data from low angles, consider using AprilTag targets for superior oblique detection.

Challenges and considerations
Videogrammetry offers immense potential for various applications, yet its implementation comes with a set of challenges. Filming excessive footage can result in software inaccuracies, leading to duplicate surfaces in the 3D model. Therefore, it is important to carefully consider the path taken when filming. Some areas may be challenging to film, but if those areas are not captured in the video, they cannot be included when reconstructing the 3D model. 

Filming while navigating through obstacles, especially on construction sites, requires caution and precision on the part of the user. This sometimes gets in the way of creating a perfect video. Weather conditions such as puddles and sunlight can affect data accuracy by creating reflective surfaces and casting shadows, respectively. Filming areas obstructed by roofs, trees or walls can degrade the RTK signal, leading to inaccuracies in the final model.

Tips for accurate data capture
The quality of the output depends on a number of factors, including how the data is captured. The following basic principles can help users to obtain the necessary coordinates when filming so that the 3D model will be as realistic as possible:

Move slowly and steadily: To obtain sharp images, maintain slow and smooth movements. This is especially crucial in poor light conditions when the shutter speed is low and video frames are more susceptible to blur.
Rotate slowly and move while turning: Just like when towing a trailer, it is necessary to move back and forth rather than trying to turn on the spot.
Don’t ‘wall paint’ when scanning vertical surfaces: Standing in one place while tilting the device up and down will generate a lot of images, but they will all have the same coordinates. Instead, move in a lateral direction while recording at different heights.
Film in connected/closed loops: Try to ensure that the filming ends precisely back at the starting point.

Advantages of videogrammetry
Videogrammetry offers significant advantages in surveying, particularly when smartphones are leveraged as data capture devices. The portability and convenience of smartphones enables swift, efficient and accessible data collection in a wide range of situations, making it possible to document areas that are either small, rapidly changing, or require close-up details in hard-to-reach places. Moreover, unlike traditional methods requiring specialized equipment and expertise, smartphone videogrammetry empowers more professionals to capture and reconstruct 3D data. The standout feature of videogrammetry is that it eliminates overlap concerns, since the video is continuously shot at approximately 30FPS. This accessibility paves the way for even broader surveying applications.

Integrating videogrammetry into a data collection toolkit promises to accelerate project timelines, streamline workflows and above all improve responsiveness, since a smartphone is always on site. This makes videogrammetry a cost-effective solution for surveying tasks.

Videogrammetry in practice
These three case studies illustrate the practical application of videogrammetry in various situations, ranging from a simple scenario to a mid-sized construction site. A Pixel smartphone with RTK and the 3Dsurvey ScanApp were used in all cases.

1. Pile volume calculation
 Photogrammetry is commonly used to calculate the volume of material piles, but the pre-flight preparation and planning can be time-consuming. Moreover, drones are bulky and less portable than a smartphone. Therefore, videogrammetry – with a handheld RTK antenna connected to a smart phone – can offer a much faster and simpler alternative. In this project to capture a small pile of material, the smartphone was simply held up high, tilted down and encircled the heap. During a five-minute site visit, a 75-second video was recorded, from which 152 frames were extracted. The processing time amounted to 30 minutes.


2. Preserving cultural heritage
Archaeological excavation sites and statues that require 3D documentation are often located in crowded urban areas which may be subject to strict drone regulations. Some culturally significant items may be located indoors, such as in museums, where photogrammetry is challenging. Moreover, using ground surveying equipment like laser scanners requires highly technical knowledge. This can be an expensive option and therefore unsuitable for such projects. In a project to capture a complete and accurate 3D scan of a dragon statue, a total of 20 minutes were spent on site. 226 frames were extracted from 113 seconds of video. The subsequent processing time was one hour.



3. Underground infrastructure project
Videogrammetry can be successfully used in the context of underground construction and engineering projects, such as when laying pipes into a trench. Compared with documenting the site with traditional equipment, using a smartphone equipped with RTK technology makes the process of documenting remarkably efficient. Just as with drone photogrammetry, multiple mappings can be performed to track progress. As another advantage, videogrammetry makes it possible to get really close and record details that may be hidden from the top-down aerial view. In support of a construction and engineering project for the installation of underground fuel tanks, videogrammetry was used to document and extract the exact measurements, monitor the width and the depth, calculate the volume and extract profile lines. A 15-minute site visit was sufficient to record 87 seconds of video, leading to 175 extracted frames. The processing time amounted to 45 minutes.    

While it should be pointed out that it is not a replacement for established surveying techniques like photogrammetry and laser scans, videogrammetry has emerged as a valuable addition to the surveyor’s toolkit. Overall, this technology offers professionals significant gains in convenience, efficiency and flexibility, because surveyors can capture data using just a smartphone. This allows for faster and more accessible data capture across versatile situations, and provides cost-effective solutions adaptable to specific needs thanks to integrating seamlessly with existing surveying workflows.

While filming with a smartphone currently has its limitations that present challenges, continuous advancements in hardware, software and best practices are steadily improving the accuracy and reliability of data collection. This ongoing evolution means that videogrammetry has the potential to contribute to better-informed decision-making and become an indispensable tool for the modern construction professional.

source : gim-international

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