Drone photogrammetry integrates the efficient measurement methods of drone technology and photogrammetry, and has irreplaceable value. Here, we will talk about the imaging of NADIR images, orthophotography, and oblique images and what are the differences between them?
What are NADIR images, orthophotography, and oblique photography?
NADIR images, orthophotography, and oblique photography are three common aerial photography methods, which differ in shooting angle, purpose, and the type of images generated. The most fundamental difference is that it is all about perspective.
- NADIR images refer to images taken vertically from directly below the aircraft or drone. The camera lens is at a 90-degree angle to the ground, and the ground area photographed is the closest to the actual distance and shape.
- Orthophotography is an aerial image that has been geometrically corrected to eliminate distortion caused by terrain and camera tilt. Orthophotos are map-level accurate images, and each pixel accurately represents its geographic coordinates.
- Oblique photography refers to images of ground objects taken from a certain angle (non-vertical) perspective. Camera lenses are usually taken at a certain tilt angle, which can capture multiple sides of an object and provide richer three-dimensional information.
In general, the basic principle of photogrammetry is to collect information from photos taken from different angles of the same object and its environment. Then, the software uses this information and correctly aligns the photos to calculate the location of relevant points in 3D space and the distance between these points. Finally, it uses the calculations to generate a polygonal mesh for creating a 3D model.
Should I use NADIR imagery, orthophotography, or oblique photography?
In drone photogrammetry, all three types of imagery can be used to generate a three-dimensional view with the help of software.
Characteristics and uses of NADIR images
In NADIR imagery, the objects in the image are not tilted or distorted, and appear as a bird's-eye view. The ground details in the center are very accurate, but as they move away from the lens, there may be slight distortions at the edges.
Due to its vertical viewing angle, NADIR imagery is ideal for capturing a large range of ground details and generating geometrically accurate two-dimensional images. NADIR imagery can be used in land surveying, urban planning, agricultural monitoring and other fields.
Characteristics and uses of Orthophotography
In orthophotography, the image distortion of terrain undulations and buildings is corrected, and the image has a uniform scale. All objects and buildings are consistent with their actual geographic locations.
Orthophotos can accurately measure distances, angles and areas, and are extremely accurate georeferenced images. They are also widely used in mapping, surveying, geographic information systems (GIS) and other fields.
Characteristics and uses of Oblique Photography
In oblique photography, objects in the image are significantly tilted, and buildings and terrain appear three-dimensional. More facades and details of objects can be seen at different angles.
Oblique photography can provide rich visual information and is suitable for applications that require three-dimensional stereo analysis, especially in modeling and analysis. It is more intuitive than NADIR images or orthophotos. It is also used in emergency rescue, urban planning, cultural heritage protection and other occasions.
Which one is best?
UAV drone photogrammetry brings us more details from aerial images. NADIR images are easy to obtain, orthophotos help generate accurate maps, and oblique photography provides three-dimensional perspective and stereoscopic sense.
Each of these photography methods has its own advantages, and the specific method to choose depends on the actual application needs and mission objectives.
