What is Orthophoto and True Orthophoto ?
What is Orthophoto (Orthoimagery)? Orthophoto, also known as orthoimagery, is a high-resolution aerial photograph that has been geometrically corrected to ensure uniform scale. Unlike regular aerial photographs, orthophotos are free…
What is Orthophoto (Orthoimagery)?
Orthophoto, also known as orthoimagery, is a high-resolution aerial photograph that has been geometrically corrected to ensure uniform scale. Unlike regular aerial photographs, orthophotos are free from distortions caused by camera tilt or terrain relief, making them accurate for surveying and mapping. Orthophotography is widely used in various applications, including urban planning, forestry, and disaster management. Orthophoto, obtained from aerial (airplane-drone) or satellite platforms, corrects distortions from central projection, camera tilts, and terrain elevation differences, ensuring accurate map-like geometric properties.
What is Orthophoto Map?
Orthophoto map is a map created from orthophotos. It combines the visual detail of a photograph with the geometric accuracy of a map. These maps are often used for detailed analysis and interpretation, providing a real-world view that is accurately scaled and can be used for surveying. Geometric corrections are applied to images to achieve a singular scale at ground level, allowing precise surveying.
What are Orthophoto Maps Used For?
Orthophoto maps have a wide range of applications:
- Urban Planning: Helping city planners design infrastructure and manage land use.
- Agriculture: Assisting farmers in crop monitoring and management.
- Environmental Monitoring: Tracking changes in natural landscapes.
- Disaster Management: Providing detailed imagery for assessing damage and planning recovery efforts.
- Forestry: Managing forest resources and monitoring deforestation.
- Geographic Information Systems (GIS): Used for various mapping, measurement, and analysis tasks.
Is an Orthophoto Map a Raster or Vector?
An orthophoto map is a raster image. Raster images are made up of pixels, each representing a specific area on the ground. This allows for the detailed photographic representation of the terrain, which is essential for accurate analysis and interpretation in various applications.
Difference Between Orthophoto and Aerial Photo
While both orthophotos and aerial photos are captured from the air, the key difference lies in their geometric properties:
- Aerial Photo: Unprocessed photograph taken from an aircraft. It may have distortions due to camera tilt and terrain variations.
- Orthophoto: Aerial photo that has been corrected for geometric distortions, ensuring uniform scale and accuracy for measurements and mapping. High objects like buildings, bridges, and vegetation may appear “tilted” due to central projection.
True Orthophoto vs Orthophoto
A true orthophoto goes a step further by correcting for displacement caused by the height of objects. Both orthophoto and true orthophoto are ortho-rectified images, but they differ in the level of geometric correction:
- Orthophoto: Corrected for terrain-related distortions using a Digital Terrain Model (DTM) for uniform scale.
- True Orthophoto: Further corrected using a Digital Surface Model (DSM) to adjust for displacement of objects with different elevations like buildings, bridges, and vegetation. This additional step ensures that all objects appear as if viewed from directly above, without any leaning.
True orthophotos are particularly useful in urban and industrial areas where the precise location of buildings and narrow spaces between objects are critical. They provide an undistorted, geometrically accurate view of the Earth’s surface, ideal for accurate measurements and analyses in GIS applications.
How to Create an Orthophoto
Creating an orthophoto involves several steps:
- Data Collection: Capturing high-resolution aerial images.
- Georeferencing: Assigning geographic coordinates to the images.
- Orthorectification: Correcting distortions to ensure uniform scale.
- Mosaicking: Combining individual orthophotos to create a seamless map.
- Post-processing: Enhancing image quality and accuracy.
For true orthophotos, additional steps include creating or using a Digital Surface Model (DSM) to correct for the displacement of high objects.
Does creating a true orthophoto take more time and cost?
Creating high-precision true orthophotos requires generating Digital Surface Models (DSMs) with 3D vector building lines. This process, done via stereo or point cloud drawing or using accurate LIDAR data, involves higher flight overlap rates, more image captures, and extensive processing time, impacting total production time and cost.
When is it necessary to produce a true orthophoto?
True orthophotos are beneficial in urban or industrial areas where the exact positions of buildings or objects and the narrow spaces between them are crucial. They are also valuable for overlaying GIS data or working with 3D data, ensuring data alignment and accuracy.
Are there alternatives to true orthophotos?
Raster images created from LIDAR data can be used. However, high-resolution models require a dense point cloud. For example, a point density of 100 points/m² allows for raster images with a 10 cm ground sampling distance (GSD).
What is an orthomosaic?
Orthomosaic is a seamless, continuous orthophoto created by geometrically and radiometrically (color) merging overlapping images covering a project area.
What is an orthophoto map?
Orthophoto map is a product that divides orthophotos according to national map sheets, featuring map grids, contour lines, place names, and other map marginal information in digital or printed format.
