One of the first scientific methods of remote sensing was photogrammetry.
Three separate Greek words—”photos” meaning light, “gramma” meaning to draw, and “metron” meaning to measure—combine to form the word photogrammetry. The original meaning of the root words was “measuring graphically by means of light.”
Establishing the geometric link between an object and an image rigorously and using the image to infer information about the object are the core objectives of photogrammetry.
The technical ability to measure any item via photographs is known to the general public as photogrammetry.
Why Use Digital Photogrammetry?
Analogue, analytical, and digital (softcopy) photogrammetry have all been developed since the invention of computing and image technologies.
Digital photogrammetry differs from its analogue and analytical predecessors primarily in that it works directly with digital imagery instead of (analogue) pictures.
All forms of imagery, whether passive (such as optical sensing) or active (such as radar imaging), and obtained from any platform (such as airborne, satellite, close range, etc.), are processed in digital photogrammetry.
Opportunities for automating DEM/DTM and integrating pictures obtained from many platforms and sensors are presented by the distinct benefits of digital photogrammetry in terms of accuracy and precision.
Methods used in photogrammetry
A) Based on the lens’s configuration:
- a. Far range photogrammetry (with camera distance setting to indefinite).
- b. Close range photogrammetry (with camera distance settings to finite values).
B) Based on the surveying type:
- a. Terrestrial or ground photogrammetry.
- b. Aerial photogrammetry.
A. Terrestrial or Ground Photogrammetry: In terrestrial photogrammetry, employees capture pictures from various locations on the earth’s surface for measuring reasons, or maps are created using terrestrial (or ground) photos.
Plane table surveying is thought to have been further developed by terrestrial photography surveying.
B. Aerial Photogrammetry:
in aerial photogrammetry, maps are created using photographs taken from the air, typically from aircraft or drones. These images are processed and analyzed to extract accurate spatial information about the Earth’s surface. The process involves stereo-photogrammetry, where overlapping aerial photos are used to create 3D models and orthophotos, which are geometrically corrected to ensure accurate scale and measurements.
Aerial photogrammetry is widely used in topographic mapping, land surveying, urban planning, environmental monitoring, and infrastructure development. It offers high-resolution imagery, cost-effective data collection, and detailed terrain analysis compared to traditional ground-based surveying methods.
There are two main areas of expertise in aerial photogrammetry:
A, Metrical photogrammetry: Aerial photogrammetry is of significant importance to surveyors as it provides an efficient and precise method for mapping and analyzing the Earth’s surface. By capturing overlapping aerial photographs and processing them through specialized software, surveyors can extract valuable spatial data for various applications. One of the key uses of photogrammetry in surveying is the determination of distances and elevations, which are essential for infrastructure development, land parceling, and engineering projects. The ability to measure height differences allows for the creation of Digital Elevation Models (DEMs) and Digital Terrain Models (DTMs), which are widely used in flood risk assessment, construction site planning, and geological studies. Unlike traditional surveying techniques, aerial photogrammetry significantly reduces the time and labor required to survey large and inaccessible areas while maintaining high accuracy.
Beyond distance and elevation measurements, aerial photogrammetry is extensively applied in calculating areas and volumes, making it indispensable in industries such as agriculture, mining, and urban planning. For instance, surveyors can accurately estimate land areas for zoning regulations, property assessment, and deforestation monitoring. In mining and construction, photogrammetric techniques help determine material volumes, such as stockpiles and excavation sites, ensuring precise resource management. Additionally, the generation of cross-sections and topographic maps from aerial imagery allows engineers and planners to visualize terrain features and design roadways, drainage systems, and other infrastructure projects efficiently. With advancements in drone technology and high-resolution imaging, aerial photogrammetry continues to revolutionize modern surveying, offering a cost-effective and reliable approach to spatial data collection and analysis.
B) Interpretive photogrammetry is the process of analyzing aerial or satellite photographs to determine the meaning and characteristics of objects captured in the images. This technique is widely used in various fields, including geology, environmental monitoring, urban planning, and military reconnaissance. By examining photographic data, experts can classify terrain features, identify land-use patterns, and detect changes in the environment over time. Unlike metric photogrammetry, which focuses on precise measurements, interpretive photogrammetry is more concerned with qualitative analysis, helping surveyors and researchers understand the context and significance of what is being observed in the imagery.
Several key variables play a crucial role in identifying objects in interpretive photogrammetry. Shape is an important factor, as many natural and man-made objects have distinctive forms that make them recognizable, such as roads, buildings, rivers, and agricultural fields. Size provides additional clues, helping differentiate between small structures like houses and larger ones like warehouses or airports. Pattern refers to the spatial arrangement of features—regular patterns often indicate human activity, such as city grids or agricultural fields, while irregular patterns are more typical of natural landscapes. Shadow is also an essential variable, as it provides depth and perspective, allowing interpreters to infer the height and orientation of objects. By considering these elements together, experts can accurately interpret and classify objects in aerial and satellite imagery, making interpretive photogrammetry an invaluable tool for mapping and analysis.
Orthophotos and Digital Orthophotography: An orthophoto is an aerial photograph that has been “ortho-rectified,” or geometrically corrected, to ensure that its scale is consistent.
Orthophotos are images that have been adjusted to account for distortions brought on by the camera tilting during the survey, lens distortions, and relief distortions.
Orthophotos show all the important details of a snapshot, but they allow for exact measurement of areas, angles, and actual distances (Rossi, 2004).
An orthophoto is a geometrically corrected aerial or satellite image that provides an accurate representation of the Earth’s surface. Unlike raw aerial photographs, which may contain distortions due to terrain relief, camera tilt, or lens imperfections, orthophotos are processed to remove these distortions, ensuring uniform scale and precise spatial accuracy. This makes them an essential tool in surveying, cartography, urban planning, and environmental monitoring. Orthophotos retain the rich detail of aerial imagery while maintaining the accuracy of a traditional map, making them highly valuable for applications that require both visual interpretation and precise measurements.
One of the key benefits of orthophotos is their ability to offer consistent scale and correct geometry, allowing users to measure distances, areas, and elevations with confidence. Unlike uncorrected aerial images, orthophotos align perfectly with geographic coordinate systems, enabling seamless integration with Geographic Information Systems (GIS) and other mapping tools. Additionally, they provide high-resolution detail and rapid coverage, making them useful for monitoring land use changes, planning infrastructure projects, and assessing environmental conditions over large areas. With advancements in remote sensing and drone technology, orthophotos have become an indispensable resource for decision-makers in various fields, offering a reliable and up-to-date view of the Earth’s surface.
Digital Orthophotograph:
A digital orthophoto is an aerial photograph that has been corrected for distortions caused by camera tilt and variations in terrain elevation, ensuring that it has a uniform scale and accurate geographic alignment. Unlike raw aerial photographs, which may exhibit displacement due to perspective and terrain relief, a digital orthophoto undergoes orthorectification, a process that removes these distortions using elevation data and precise georeferencing techniques. This correction allows digital orthophotos to serve as accurate basemaps for various applications, including land surveying, urban planning, and geographic information systems (GIS).
By combining the geometric accuracy of a traditional map with the visual clarity of a photograph, a digital orthophoto offers a powerful tool for spatial analysis. Surveyors, engineers, and environmental scientists rely on these images for precise measurements, land-use planning, disaster assessment, and infrastructure development. Because digital orthophotos maintain real-world spatial relationships, they can be overlaid with GIS data, such as property boundaries, transportation networks, and vegetation indices, for more in-depth analysis. With advancements in remote sensing, drones, and satellite imaging, digital orthophotos have become increasingly detailed, cost-effective, and widely used across multiple industries.
Requirements:
To create a digital orthophoto, several essential components are required, beginning with aerial or terrestrial imagery. High-resolution images are captured using specialized cameras mounted on drones, aircraft, or terrestrial platforms. These images must have sufficient overlap (typically around 60-80%) to allow for stereoscopic viewing and accurate triangulation. The images should also be free from distortions caused by lens aberrations, atmospheric conditions, and perspective errors. Additionally, metadata such as camera calibration parameters, flight altitude, and image orientation are critical to ensuring accurate photogrammetric processing.
Another key component is ground control points (GCPs) and a digital elevation model (DEM). GCPs are precisely surveyed points with known geographic coordinates that help georeference the images and correct positional errors. These points enable the transformation of raw images into spatially accurate orthophotos. Meanwhile, the DEM represents the terrain’s elevation, allowing for the removal of distortions caused by surface relief variations. By integrating the DEM with aerial images and applying orthorectification techniques, a distortion-free, geometrically accurate digital orthophoto can be generated for applications such as mapping, land-use planning, and geographic analysis.
- 1) Photo identifiable ground control points.
- 2) Camera calibration and orientation parameters.
- 3) A digital elevation model (DEM).
- 4) A digital image produced by scanning an aerial photograph with a precision high-resolution scanner.
- 5) Softcopy Photogrammetric Workstations: Processing the imagery to derive image and vector products using Digital Photogrammetric Workstation (DPW).
