What is oblique imagery?
Oblique imagery refers to aerial photography taken at an angle between 40 to 45 degrees. One of the most popular oblique camera systems available in the market today consists of a mechanical rig with five cameras positioned in a cross configuration—four cameras tilted at a 45-degree angle and one camera pointing directly downward (nadir).
Unlike traditional orthographic imagery, oblique imagery provides a more realistic representation of the subject by offering four additional perspectives. This gives users access to a wealth of additional information, including 3D details of ground objects from different viewpoints. Oblique images not only capture the top surfaces but also the sides of structures, such as building facades. Advanced positioning and modeling techniques enable the creation of detailed 3D city models.
What's the benefit of using oblique imagery?
First and foremost, oblique imagery allows users to observe ground features from multiple angles, providing a more realistic depiction of their actual conditions compared to traditional orthophotos.
Secondly, with the support of specialized software, users can directly measure attributes like height, length, area, angle, slope, and more based on the resulting images. This broadens the applications of oblique photography technology significantly.
Thirdly, oblique photography can quickly capture multi-angle and wide-range images of urban areas, overcoming the limitations of conventional imagery. By tilting the camera, oblique photography can gather more information about ground features, particularly building facades. With the aid of parallel computing, 3D models based on oblique images can be constructed swiftly, effectively reducing the cost of urban 3D modeling.
Finally, the image data obtained through oblique aerial photography is much smaller than that of 3D GIS, and its data format can be easily shared over networks.
How accurate is oblique photography?
Georeferencing involves associating identifiable points in an image with known coordinates within a predefined reference system (such as UTM or Gauss-Braga).
All measurements come with geometric and accidental errors. GIS software calculates these errors and applies necessary corrections to the image. This involves applying a deformation criterion tailored to the reference system used by the map.
Accidental errors, which depend on drone movement and terrain morphology, are also minimized. These can be corrected using ground control points.
In the case of oblique photographs, a variable number of Ground Control Points (GCPs) are used on the ground. These points, with known coordinates, are placed either homogeneously across the area or concentrated in areas of maximum interest.
The difference between the collected data values and the actual positions of the points is referred to as residual. The precision of each GCP is measured through a residual value (for x and y).
What are oblique photos used for?
Oblique imagery finds application across a wide range of purposes. Some of the primary uses include:
- Smart City: The information gathered via oblique cameras can be used to rapidly build 3D models, providing urban planners with consistent and reliable data to visualize projects and assess the short-term and long-term impacts of smart city initiatives.
- Open-Pit Mine Management: High-resolution 3D realistic mesh models serve as excellent starting points for engineering operations, construction, project planning, and communication activities. Utilizing an accurate and fully textured 3D model of the site as a simulation planning sandbox allows for redesigning future developments virtually. Communicating these changes to stakeholders becomes easier with the 3D model.
- Planning and Design: Oblique photography can also be used for planning and design purposes, such as designing bridges. Combining BIM technology with oblique aerial photography enables the creation of 3D models of bridge structures. Additionally, using BIM modeling techniques, designers can demonstrate real-time dynamic adjustments and 3D dynamic presentations of bridge designs.
In addition, oblique photography technology is widely applied in land surveys, rural cadastral mapping, river and lake management, property rights, engineering measurement, construction, agriculture and forestry, transportation planning, BIM design, and GIS information systems.
Real-world use cases of oblique imagery
Equipped with oblique cameras of varying tilt angles, our JOUAV VTOL drones have assisted numerous customers from diverse industries in obtaining precise spatial information about ground targets and generating large-scale 3D models swiftly.
Water Flow Study in the Taklimakan Desert
The Taklimakan Desert, located in southwestern Xinjiang in northwest China, is the largest desert in China and the second-largest in the world. To study seasonal river formation in the desert, the Xinjiang Institute of Geology used CW-15 with a CA503R five-lens oblique camera to estimate the volume of sediment scoured by rivers.
This 3D model covers 2.78km² and was captured in a single flight.
Lily Shee, Superintendent of Technical Services at Xinjiang Institute of Geology, mentioned that a ground survey in this area would take more than three days, but with CW-15, they completed the task in just 70 minutes, saving thousands of dollars.
"Using professional 3D design software, we can easily create a 3D model based on oblique images. The 3D model directly displays the river image, which can be used to calculate the river sediment volume more accurately and infer the impacts of water flow direction and intensity on the dunes," said Lily.
Weinan 3D City Modeling Project
The Weinan Municipal Government used CW-15 VTOL and PH-7E multirotor drones equipped with CA-503R cameras to capture high-resolution oblique imagery, generating a comprehensive 3D model with accuracies as low as 5 cm for a 700 sq. km area. The 3D model supported the Weinan Snowlight Project by improving construction efficiency, project monitoring, and maintenance management.
"We were blown away by the level of detail and accuracy provided by CW-15 and PH-7E's 3D modeling drones. The ability to capture data from such a high altitude and still have a comprehensive and realistic representation of our site was truly impressive. This technology has completely transformed our approach to urban construction planning, and we look forward to utilizing it for future projects," said John Guo, Project Manager from the Weinan Municipal Government.
- Read more about 3D fine modeling in Weinan
3D model details of the Weinan Gymnasium
Mapping One of the Chemical Plants in China
Wanhua Chemical Plant, one of the largest chemical plants in China, used CW-25E equipped with a CA503 oblique camera to digitally model the chemical plant. This allowed them to conveniently and intelligently manage traffic, volume ratios, sewage management, black water, greenery, and garbage in the area.
"It takes one to two years for manual modeling to complete small and medium-sized city modeling work. But now, with oblique aerial photography, we can complete the task in only three to five months, which significantly improves the efficiency of 3D model data collection," said Hermann Liu, Manager at Wanhua Chemical Plant.
Conclusion
Oblique imagery recreates the environment from the perspective of a human observer, unlike traditional drone surveying, which captures images directly from above. Due to the greater angle of inclination compared to nadir images, the visualization of vertical structures is enhanced. As a result, users can immerse themselves in images that allow for quick and intuitive recognition of objects present in them.
JOUAV drones for 3D mapping can be equipped with five-lens oblique cameras to capture high-resolution images that meet surveying accuracy requirements in various industries. Currently, JOUAV drones equipped with an oblique photography system are widely used in urban management, land resource management, smart city development, emergency response, land security, geological control, power line inspection, and other fields.
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