From Planning to Execution: Ensuring High-Quality Imagery with the Right Sensors
Reality Mapping is a powerful technique for creating high-fidelity 2D and 3D maps from aerial imagery. Whether you're involved in urban planning, environmental monitoring, or infrastructure management, optimizing your Reality Mapping workflow is crucial. This guide provides valuable tips on key considerations for collecting input data, regardless of the acquisition platform you use. Making informed decisions at this stage can significantly enhance image quality, which will be reflected in the final products.
Reality Mapping Applications
When talking about Reality Mapping, two primary applications can be distinguished:
- Ortho Production: producing high quality True Ortho imagery.
- 3D Production: utilizing stereo techniques to generate 3D data such as DSMs, Meshes, and Point Clouds.
Meeting the specific requirements of each application necessitates a deliberate choice of camera setup and acquisition parameters. This selection is crucial for achieving the desired output quality in a cost-effective manner.
Flight Planning
A well-prepared flight plan is essential for both the cost and success of a Reality Mapping project and the two should be balanced against one another. In the pursuit of efficient data collection, one might be inclined to fly at high speeds with minimal lateral overlap. However, this bears a cost on the quality of the data needed for 2D and 3D products. Moreover, various factors can further limit data collection. Hence, it is essential to consider the following aspects already during the planning phase:
- Airspace regulations and restrictions: Project areas may have regulations that restrict operations, such as no-fly zones or altitude limitations.
- Operational limits of sensor platforms: Speed, altitude, and turn angle limitations depend on the chosen survey platform. Whether selecting a drone or a plane, and even the type of aircraft, can impact your operational envelope. Choosing the appropriate platform can enhance acquisition efficiency and data quality.
- Camera system selection: The choice of camera system plays a crucial role in structuring a flight plan, as shown in the next sections. Companies that have multiple sensor systems at hand, can achieve considerable time savings by selecting the best sensor for the job.
- Data quality considerations: Maintaining sufficient image overlap, both forward and sideward, is crucial for producing high-quality data products. Utilizing professional-grade GNSS/IMU systems is also recommended, as they ensure accurate initial orientation of the collected data. Additionally, operating ground reference stations during data acquisition and surveying ground control points can significantly improve data positioning during processing.
In addition to flight planning considerations, paying attention to weather conditions during data acquisition is of utmost importance. Factors like wind, cloud cover, sun angle, and haze can directly influence image quality, which, in turn, affects the geometric quality of the final products.
A quick verification of the data coverage while still being in the air can help to eliminate the need for an additional flight to re-fly gaps in the data.
Ortho Production
Ortho Productions are most effective when the input imagery offers a near-orthographic view of each ground point. This can be attained through the use of long focal lengths (such as in efficient high-altitude content programs) which typically minimize building lean effects. Alternatively, highly overlapping stereo imagery can also achieve this effect by leveraging the most center parts of each image.
The schematic below illustrates two distinct camera setups: green represents a telephoto lens, while blue signifies a wide-angle lens. A shorter focal length (wide-angle) may introduce more occlusions and leaning compared to a telephoto setup.
3D Production
3D Production uses the principles of stereo Photogrammetry. Therefore, each pixel requires multiple views forming stereo models with a baseline. An intersection angle (baseline) is required to achieve depth information at high quality. Image similarity is at the same time needed to perform image matching. The following schematic drawing shows the same camera setup as above. On the one hand, a longer focal length (tele) enables to fly at higher altitude to achieve the same GSD, which makes the base-to-height-ratio less favorable (smaller intersection angle) and introduces a larger depth error. On the other hand, image similarity is better represented by tele setups and less optimal for wide-angle setups.
Taking into account both image similarity and intersection angle, as depicted in the schematic drawing above, a setup that strikes a balance between telephoto and wide-angle configurations is deemed most optimal for 3D Productions. This balanced setup is crucial for achieving high-quality results in stereo Photogrammetry.
The Right Tool for the Job
The selection of focal lengths according to the target GSD – or vice versa the selection of an operation GSD range for a given focal length, is key for good output quality for both Ortho and 3D.
Scenario analysis
For an identical ground resolution (e.g. 5cm GSD) and image overlap (e.g. 80/60) of a traditional 5-head camera system, the impact of focal lengths can be approximately as follows:
| Scenario | Focal length | Typical opening angles | Stereo Capability | Occlusions Nadir | Occlusions Oblique | Atmospheric impact on radiometry |
| Wide angle | Short | 80° … 100° | optimal | strong | reduced | reduced |
| Balanced | Normal | 45° … 65° | optimal | average | average | average |
| Tele | Long | < 35° | non-optimal | reduced | very strong | very strong |
The table illustrates some key insights:
- The choice of focal length significantly impacts output quality by influencing the amount of observable occlusions as well as the effect atmospheric conditions have on the image radiometry.
- Ortho collection performs well at high altitudes, even with limited stereo capabilities.
- For 3D output products like Mesh and DSM, stereo capability must be considered along with occlusion levels to achieve sharp and clean results.
Adjusting to operational restrictions
When flight restrictions necessitate the use of long focal lengths, a decrease in quality may occur due to glancing intersections (resulting in a poor base-to-height ratio). In such scenarios, achieving the same quality level as a balanced setup may require a higher resolution Ground Sample Distance (GSD) by flying at the lowest allowable altitude.
Conversely, when restrictions mandate the use of short focal lengths, operating at higher overlap (e.g., 80/80) can compensate for occlusions in the Nadir frame caused by perspective distortion. Flying at lower altitudes allows for capturing data under overcast conditions (below clouds), which is advantageous for Photogrammetry due to homogeneous color and minimal shadow loss.
Summary
Efficiently planning data acquisition for Reality Capture involves striking a balance between minimizing flight efforts and maintaining high data quality standards.
For projects focused solely on Ortho imagery without the need for 3D information, employing balanced or Tele setups is advisable to maximize productivity. It's important to consider that atmospheric haze can introduce qualitative differences in the output.
When aiming to generate 3D products, it is recommended to utilize a balanced system with standard opening angles that offer good stereo capabilities within the operational Ground Sample Distance (GSD) range. This approach ensures optimal output quality for both Ortho and 3D products.