Orthomosaics Explained
Orthomosaic Maps & Orthophotos Explained
Before we jump into the nitty gritty, do you want to just build one yourself? Download some sample data here, and build an ortho in three clicks!
What is an orthomosaic?
If you’re new to GIS or drones you may not be totally familiar with the term ‘orthomosaic’ just yet. But as you dive deeper into this world, it is unlikely that you will be able to avoid it. You are on the fast track to learning what orthomosaics (aka orthos) are and how beneficial they can be. Before we get too far ahead of ourselves, let’s chat about photogrammetry. Photogrammetry is the foundation of all orthomosaics and although it sounds like a big confusing word, it is ultimately the science of making measurements from photographs.
Meaning, “light drawn to measure” the early beginnings of photogrammetry are tied to the innovation and curiosity of a few pioneering individuals who explored how photography could be used to measure and interpret the physical world. Aimé Laussedat is often credited with being the pioneer of photogrammetry, recognizing the potential of photography for topographic mapping long before the widespread use of aerial imagery. Realizing the value of a higher perspective, Laussedat is most recognized for his innovative use of photographs taken from elevated viewpoints, such as high windows and other accessible high points, which allowed him to experiment with creating topographical maps. He utilized glass plate cameras and early photographic equipment to take these photographs, which he then used to project onto a drawing surface to extrapolate measurements.
His work primarily revolved around understanding how to use these early photographs for precise measurements and mapping, setting the groundwork for the future development of photogrammetry.
Modern photogrammetry combines traditional techniques with advanced technologies like GPS and inertial measurement units (IMUs) to enhance accuracy and detail. These technological advancements have democratized access to high-quality mapping, making it not just a tool for professionals but also accessible to enthusiasts and researchers in diverse fields.
Orthomosaics are a direct application of photogrammetric principles. An orthomosaic is essentially a detailed, large-scale map created from numerous overlapping photographs that have been corrected for topographical relief, lens distortion, and camera tilt. These images are stitched together to form a single, coherent image. This seamless mosaic is georeferenced to real-world coordinates, which means every point on the image can be associated with a precise location on the earth's surface. The process of creating orthomosaics is meticulous, requiring the careful alignment of each image to ensure that scale is uniform across the map.
The term orthomosaic breaks down into sections, ortho and mosaic.
Ortho: comes from the Greek term orthos meaning straight or right.
Mosaic: the process of producing a picture or pattern by arranging together small
colored pieces of hard material, such as stone, tiles, or glass.
Think about when you take a panoramic image on your phone; this process is similar to mosaicking, where your phone captures multiple overlapping images to create one larger image. However, you might have seen panoramic photos where people appear squashed or stretched, or the landscape seems distorted. While smartphones attempt to correct some of these distortions, they often can't completely eliminate them. These distortions can include radial displacement, which results from lens distortions and the angle of capture. In professional aerial imaging, such distortions are more pronounced and can be influenced by factors like the Earth's curvature. Although radial displacement can be useful in specialized applications like estimating objects' relative heights and terrain slopes, it is generally undesirable when creating accurate maps.
Radial displacement or distortion is something you want to avoid when creating maps due to its impact on accuracy and reliability. This distortion, caused by lens imperfections and viewing angles, leads to inaccuracies in object positioning and inconsistencies in scale, which can compromise navigational reliability and precision accuracy required for a variety of projects. Maps require highly precise geographic representations to ensure reliable data. Tying this back to orthomosaics, the “ortho'' part of the word refers to the geometric correction applied to the images to adjust for topographic relief, lens distortion, and camera tilt to create a uniform scale. This is why orthomosaics are extremely popular in mapping projects, they are scaled and provide a precise and comprehensive view of the landscape that supports measurement and analysis tasks. The process of correcting distortion in orthoimages is called orthorectification.
The relationship between photogrammetry and orthomosaics is integral. Photogrammetry provides the methodological foundation needed to produce orthomosaics. By capturing overlapping images from different perspectives, photogrammetry allows for the construction of a comprehensive and detailed view of an area, which is crucial for the accurate stitching required in orthomosaic production. These mosaics are especially valuable in applications where precise, up-to-date, and easily interpretable visual representations of large areas are necessary, such as in land management, agriculture, urban planning, and environmental monitoring. With advancements in drone technology and software algorithms, the integration of photogrammetry with orthomosaic creation has become increasingly sophisticated, offering unprecedented accuracy and efficiency in aerial mapping.
TLDR: orthomosaics are detailed, accurate photo representations of areas, created by stitching together several overlapping aerial photographs that have been corrected for geometric distortions, camera tilt, and topographic relief. The process ensures that the scale of the mosaic is uniform across the entire image, allowing it to serve as a true-to-scale map.
TLDR in layman's terms: a bunch of photos, with sufficient overlap, are stitched together into one, big photo. A bunch of corrections are done to the photos make sure that the photo is as true-to-scale as possible.
How does it work?
Orthomosaics are made by stitching together hundreds of overlapping images, typically captured with a drone. Overlapping images is a crucial component of creating successful orthomosaics for a number of reasons:
Image Alignment and Stitching
Side and forward overlap between images provides multiple reference points that are common across adjoining photos. These points are known as tie points and are analyzed by software to determine how images should align relative to each other. Effective alignment is essential for creating a seamless image without visible seams or misalignments.
Error Minimization and Redundancy
Another reason why overlap is important is because it helps reduce redundancy and errors. For example, if one image is partially obscured by a shadow from a cloud or an object like a passing bird, the overlapping images can provide alternative data for that obscured area. This redundancy ensures that the final orthomosaic maintains consistent quality even if some individual images have issues.
Improved Georeferencing Accuracy
Finally, overlap helps in accurately placing the stitched images on the map with correct geographical coordinates, a process known as georeferencing. Multiple overlaps provide numerous ground control points, which can be used to anchor the orthomosaic accurately in geographical space. This is vital for applications where precise geographic placement is necessary, which is likely to be the case in most mapping projects.
As the images are captured, each is geotagged with GPS coordinates, which are crucial for accurately positioning them on a digital map. Once uploaded, these georeferenced images undergo a detailed processing phase in specialized software designed for photogrammetry. This software not only aligns and stitches the images based on common points, but also performs critical adjustments – orthorectification. Orthorectification corrects for any distortions caused by camera tilt, lens imperfections, and variations in the Earth's surface, ensuring the scale remains uniform across the image. The software then uses tie points—distinctive features identified in overlapping sections of images—to seamlessly merge them into a single, coherent orthomosaic map. This comprehensive map is not only spatially accurate but also provides a high-resolution representation of the surveyed area, making it an invaluable tool for a wide range of applications.
Technical Requirements: What you need to know.
Creating high-quality orthomosaics requires careful consideration of various technical specifications and requirements across the equipment and software used. Here’s a detailed look at the critical components needed for effective orthomosaic generation:
Camera Resolution and Quality
The quality of the camera attached to the drone or aerial platform is paramount. Higher resolution cameras capture more detailed images, which are crucial for producing high-definition orthomosaics. A camera with a large sensor and high megapixel count (typically 20 megapixels or higher) is ideal. This allows for capturing fine details necessary for applications like precision agriculture, surveying, and urban planning. The lens quality also plays a significant role; a sharp, distortion-free lens ensures that the images are clear and accurate, minimizing the need for extensive post-processing corrections.
Drone Specifications
The choice of drone impacts the quality of the orthomosaic. Key specifications to consider include:
Stability and Flight Precision: Drones with stable flight capabilities and precise navigation (often achieved through advanced GPS and IMU systems) are crucial for ensuring consistent image quality and accurate geotagging.
Flight Time and Battery Life: Longer flight times allow for covering larger areas in a single session, reducing the variation in lighting and environmental conditions, which can affect the consistency of the images captured.
Payload Capacity: Some high-resolution cameras can be heavy, so the drone must have adequate payload capacity to carry them without compromising flight performance.
Automated Flight Planning Software: Drones that integrate with or include automated flight planning software make it easier to set up flight paths that ensure optimal overlap and coverage, essential for creating comprehensive and accurate orthomosaics.
Check out our Guide to Buying Drones for Mapping for more detailed information!
Photogrammetry Software Capabilities
The software used for processing the images into an orthomosaic must be capable of handling large datasets efficiently and accurately. Essential features of photogrammetry software include:
Robust Image Stitching Algorithms: Software should have advanced algorithms capable of accurately stitching together hundreds of high-resolution images, taking into account variations in angles, lighting, and perspectives.
Error Correction Tools: Good software provides tools to manually or automatically correct common issues like misalignment, blurring, or discrepancies in overlap.
GIS Integration: Modern photogrammetry software should be able to integrate with GIS (Geographic Information Systems) for further analysis and application of the data.
Scalability and Performance: As projects vary in size, the software must be scalable, and capable of processing small to very large datasets without a significant loss in performance or speed.
Nova is designed to streamline the process of transforming raw aerial images into precise, actionable orthomosaics, automating the complex processes so you don't need a deep understanding of GIS. With Nova, everything from image stitching to error correction and scalability is handled automatically by the software. This means you can quickly produce high-quality maps without getting bogged down by the technical details. Nova’s intuitive interface ensures that the process is fast and straightforward, making it accessible even for those new to drone mapping. Simply fly your drone, following these Best Practices, upload your images, and Nova does the rest, efficiently processing large datasets to deliver precise, actionable results with minimal input.
Computer Hardware Requirements
Processing large sets of high-resolution images for orthomosaics is resource-intensive. A powerful computer system with a high-speed multi-core processor, substantial RAM (32 GB or more), and high-end graphics processing capabilities are recommended. Adequate storage is also crucial, as photogrammetry files can be very large, especially when dealing with high-resolution images over large areas. Using a platform like Nova, hardware requirements become obsolete as data is processed in the cloud, allowing you to use something as small as your phone to produce high quality orthos.
Applications
Agriculture: Crop Monitoring and Management
In agriculture, orthomosaics are extensively used for crop monitoring and precision farming practices. By analyzing these detailed images, farmers can observe crop health, detect pest and disease infestations, and monitor drought impacts with remarkable accuracy. For instance, a case study in California’s Central Valley demonstrated how orthomosaics derived from drone imagery enabled vineyard managers to identify variations in vine vigor. This insight allowed for targeted interventions, such as adjusting irrigation and nutrient applications, leading to a more uniform and productive crop. Additionally, orthomosaics can assist in planning farm infrastructure, like the layout of irrigation systems and access roads, optimizing both the use of space and resources.
Urban Planning and Civil Engineering
Orthomosaics play a critical role in urban planning and civil engineering by providing up-to-date, accurate representations of cities and construction sites. Planners use these maps for everything from assessing current infrastructure to designing future developments.
Environmental Monitoring and Conservation
Environmental scientists use orthomosaics to monitor changes in natural habitats, track deforestation rates, or manage wildlife reserves. By comparing orthomosaics taken over a range of time, environmental scientists can observe and track changes or progression in critical environmental health factors such as erosion, or recovery of vegetation.
Disaster Management and Response
In disaster management, speed and accuracy of information are crucial. Orthomosaics are invaluable for assessing damage after events such as avalanches, floods, or wildfires. Providing rapid damage assessment, the ability to plan and coordinate response efforts quickly, and monitor the situation as it evolves, orthomosaics provide a crucial insight to enhance the effectiveness of disaster management.
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