Photogrammetry Concept
Definition of Photogrammetry
Photogrammetry is made up of three Greek root words: “Photos” meaning light, “Gramma” meaning something drawn or written, and “Metron” meaning to measure. Thus, Photogrammetry is essentially the science and technology of the acquisition and interpretation of photography in surveying and mapping to obtain spatial and spectral information of physical objects and their environment. The term Photogrammetry came into general usage within the United States in about 1934 when the American Society of Photogrammetry was founded, although the term already had been widely used previously in Europe for several decades.
Types of Photogrammetry
- Remote-Sensing Photogrammetry
- Close-range Photogrammetry
Remote-Sensing Photogrammetry
The photographic camera is usually mounted on a Drone/Aircraft/Satellite platform with imagery acquired usually in the nadir position, i.e. pointed vertically towards the ground. Multiple overlapping photos of ground objects are taken as the remote sensing platform flies along a pre-determined trajectory. 60% or more overlap is necessary for ensuring contiguity and to obtain 3D information of the objects through the combination and processing of stereo imagery. 3D information is important for understanding the variation of heights in the terrain as well as objects such as buildings. Photogrammetry processing of stereo pair imagery results in the creation of Digital Elevation Models (DEMs). DEMs can be of two kinds: Digital Terrain Models (DTMs) representing the elevation of the bare soil minus vertical objects such as buildings and trees, and Digital Surface Models (DSMs) representing the elevation of the tallest surfaces (vertical objects are included).
Close-range Photogrammetry
Close-range Photogrammetry is the “static” branch of photogrammetry where photographs are taken from fixed, and usually known, positions relative to the object being measured. Instruments used for acquiring such imagery are also called Photo Theodolites, where a Theodolite is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. Flexibility in configuring the overlaps and fields-of-view allows the creation of highly-detailed 3D representations of the object being measured.
Photogrammetry Products generated by SaraniaSat CANEUS include:
Mosaics – A mosaic is a continuous picture of the terrain and objects within it constructed by assembling, overlapping individual photographs together into a composite image. It is thus a contiguous perspective view of the terrain.
Orthorectification is the process of removing the effects of image perspective (tilt) and relief (terrain) effects for the purpose of creating a planimetrically correct image. The resultant orthorectified image has a constant scale wherein features are represented in their ‘true’ positions.
Orthophoto – An Orthophoto is a digital image of the ground in which distortions caused by terrain relief and camera tilts have been removed. Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map. The Orthophoto can be used as a planimetric map in which only the relative horizontal positions of various objects are presented. A planimetric map is distinguished from a topographic map by the omission of measurable relief. If contour lines are superimposed over the imagery, then the resulting Orthophoto map can be used as a topographic map.
Orthomosaic – A series of contiguous individual Orthophotos are combined or mosaicked to make a single Orthomosaic. Orthomosaics have uniform scaling and can be used as large-scale maps.
Topographic map -Topographic maps represent both planimetric features (horizontal relative positions) of the terrain combined with the shape and elevation of the ground by means of contour lines.
Thematic maps -A thematic map is one that portrays the geographic pattern of a particular subject matter (theme) within a geographic area. Any number of thematic maps can be made of a single region. For example, separate thematic maps can be generated for a particular area depicting transportation networks, drainage patterns, vegetation types and slopes.
Digital Elevation Models (DEMs): As described above we generate both DTMs representing the elevation of the bare soil minus vertical objects such as buildings and trees, and DSMs representing the elevation of the tallest surfaces (vertical objects are included).
Strengths of SaraniaSat CANEUS:
- Rapid and cost-effective collection of Photogrammetric data
- Highly accurate, reliable, quantifiable processing results
- Re-processing acquired data for new information extraction without the need for expensive fieldwork to re-survey the area
- Ability to acquire data in remote, unsafe, or difficult to access locations, thus lowering safety risks
- Data acquisition can be done without disrupting field operations with analysis performed remotely