Orthophotography was once seen as a technology in search of an application, and the events that happened to change that view will be discussed in my essay. Definition of orthophotograph, orthophotos, orthophotomap, aerial photography and the origin of orthophotography were discussed to understand the meaning of orthophotography, its concepts in photogrammetry. I also went ahead to talk about the traditionally orthophotography, second generation orthophotos, digital orthophoto, true orthophoto and method of generating true orthophoto which changed the view of orthophotograph being seen as a technology in search of an application.
The orthophotograph is a vertical aerial photograph from which the image displacements, due to camera tilt and ground relief variation, have been removed. Within certain limits, normally equivalent to photogrammetric plotting, the orthophotograph is a scale correct, orthogonal projection of the terrain photographed.
Orthophotography is the method of aerial photographs that have been rectified to produce an accurate image of the earth by removing tilt and relief displacements which occurred when the photo was taken. It is also an aerial photograph that has the distortion due to tilt, curvature, and ground relief corrected.
Orthophotographs is readily used for measurement and spatial analyses due to the fact that they maintain a regular scale across the image. Orthophotgraphs are digital images that are created by making geometric corrections to scanned aerial photographs. Orthophotographs is a photographic copy, prepared from a photograph formed by a perspective projection, in which the displacements due to tilt and relief have been removed.
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Orthophotomap is one in which the line-work and symbols of a conventional map are replaced by the orthophotographic image. The outcome map will have it positional accuracy of a conventional line-map, alone with the sequence content of the existing air photo. A major important advantage of the orthophotomap as the photogram metrically produced line-map has only 10% of the information content of the existing air photos.
Aerial photography is snapping of photographs from the air, copying the visible electromagnetic spectrum, as maps of geographic areas. Remote sensing involves photographs snapped from the air and from beyond the atmosphere of areas on the earth surface and other celestial bodies, which entails many segments of the electromagnetic spectrum together with visible light, ultraviolet, infrared, and radar illumination while, Digital orthophotography digitally rectifies the pixels of digitized aerial photographs into a continuous map, usually registered to a layer of a Geographic Information System.
Orthophotography original came into exploit in the 1960’s, It was time consuming and expensive to produce. At the early 1970’s, technological advancement brought this data source into affordable commercial applications and its use has continued to increase. The first orthophotography was produced by computer driven optical methods and tools. The orthorectification process ties each pixel in a digital image to its true earth location.
Orthophotography combines the image description of an aerial photograph with the geometric qualities of a map. Unlike a typical aerial photograph, distortions due to relief displacement, camera lens, and aircraft attitude which have been separated so that all earth features are exposed in their correct land positions. It display a true image map potential and permits direct measurement of areas, angles, distances, and the detailed portions of ground features that are usually omitted or generalized on traditional maps.
The digital orthophoto method reproduces a vertical aerial photograph into a comparable of a traditional map. Which in-turn retains the advantages of a photograph-visually displaying actual land features, cultural, and the built environment, relatively than identifying those features using lines and symbols.
Digital orthophotograph has become a commercial production reality due to the primarily increased availability of more powerful computers at reasonable prices.
There are four crucial operations or corrections that must be applied to standard vertical aerial photograph in producing an orthophoto:-
Standardization of scale across the image.
Removing the relief displacement to position the terrain in its true location.
The radiometric or tonal adjustments to allow the image to blend with neighbouring images.
The assignment of X & Y coordinates values to the image.
Orthophotos have a variety of uses, and once in digital format, they can be viewed and printed at various scales, which are extremely valuable in the development of land information systems and land use planning issues such as zoning, transportation, and agriculture.
If the terrain photographed was approximately flat, within + 25 meters on 1:50,000 scale photography, the resulting rectified photo could be considered an orthophoto. Several civil mapping agencies make use of the above mentioned method in producing photomaps, e.g. The Ordnance Survey in Great Britain at aftermath of the Second World War, military mapping agencies used the approach extensively, mostly as a rapid solution to a mapping constraint prior to the production of conventional line maps. The U.S. Army unrelenting makes extensive use of what it called ‘map substitute products’ until orthophotography was introduced.
Orthophotography was seen as a technology in search of an application at the earlier stage of it invention, the U.S. were more involved in map substitute products until the arrival of orthophotography, U.S. Army tried out different techniques of recreating, designing to improve the interpretability of a photomap. Which includes pictoline and pictomapping processes to produce a better image.
Traditional orthophotos are correct at the terrain level. And do not generally take consideration of the surface model in the superimposition and rectification of vector data. In a big city or urban area traditional orthophotos was unsatisfactory, when it involves height differences. Traditional orthophoto removing displacements was done by bring each elements needed to correct scale and then projecting it to its actual plan position. These methods were formally called differential rectification.
Russell K. Bean was the first to produce a practical instrument to bring about the process, he also introduced the term Orthophotography. Scheimpflug is usually credited with proposing the use of orthophotographs and orthophotomaps
In producing orthophotograph, several instruments were fitted with a device called an orograph. This instrument produced a sequence of lines equivalent to the individual scan lines on the orthophotograph. The lines by the orograph varied in depth as the height of scanning changed.
In 1960s virtually all the major photogrammetric instrument manufacturers produced orthophotoscopes based on previous analogue instruments. The most successful of these instruments was the Topocart B manufactured by Karl Zeiss, Jena. Few instruments were fitted with a device called an orograph. The orograph create a series of lines equivalent to the individual scan lines on the orthophotograph. And the lines produced by the orograph ranges in thickness as the height of scanning changed, which usually changes as the scan went through a contour height.
During 1970s Hobrough introduced an analytical instrument, Gestalt Photo Mapper (GPM) that uses image correlation to create a Digital Elevation Model (DEM). This DEM were used to produce an orthophotograph. Which used a scanning approach, the GPM used an area matching approach to create the DEM.
Orthophoto imagery serves as an important layer in a Geographical information system because it provides inexpensive and accurate based maps. The traditional orthophotograph rectification displaces buildings and bridges, which are both modelled in the digital terrain models.
The prospective of the digital systems to make available orthophotos quickly and cheaply was appreciated by the users of Geographic Information Systems (GIS), who needed up-to-date backdrops for GIS applications.
In Great Britain, the NMAs never had the idea that there would be user demand for orthophotomaps. This led to the OS using only orthophotos as revision tools in conventional line-mapping programs.
The Ordnance Survey was not always so resistant to the use of photomaps, as it had used rectified photos to make interim editions of its large-scale series after the Second World War. However, it was always intended that these photomaps would be replaced once up-to-date line-maps had been produced.
In United States, the growth of Orthophotoscope by Bean in the 1950s meant that the U.S.G.S. was able to improve on an ambitious program of orthophotomapping.
At 1990s, the development of GIS has led to an unprecedented demand for orthophotographs. This requirement was generated by the need of GIS users for up-to-date image backdrops while performing GIS research.
GIS users around the world have found that much of the existing line-mapping was not up-to-date, even where it was available in digital form. By the end of the century, the demand for digital orthophotos from the GIS community led even a traditionally reluctant Ordnance Survey to offer an orthophoto layer as part of its Master map product.
Majority of the GIS users’ worldwide lack the competence to carry out their own conventional photogrammetric mapping, but can carry out orthorectification with the help of digital photogrammetric systems. The introduction of digital photogrammetric cameras opened up the prospect of a fully digital work-flow, from camera to orthophoto.
Most users are utilizing these images for GIS maintenance, community planning, hazard mapping, disaster planning and commonly for any urgent planning purposes within short time, while line maps would not be at hand due to longer time in producing them. If vector information is required, digital orthophotos can still be used to extract this information by a simple head up digitizing, mainly if the accuracy needs are not too elevated. The geometric accuracy of orthophotos depends primarily on the quality of the DTM describing the terrain surface. Therefore, a user of orthophoto data should be aware of the effects inherent to orthophotos such as misplacement of objects that are not modelled by the original map.
Second-generation digital orthophoto imagery is becoming popular throughout the GIS Industries as cost-effective, timely and able to retain accuracy.
Advantages of Second-generation orthophoto
The second generation eliminates cost of:
Manual aerial triangulation method.
DTM/DEM capture and processing.
Ground survey mobilization and GPS capture.
Can fly a project at 30% sidelap and 30% overlap if stereo pairs are not desired.
Shave one to three months of project duration.
Easily incorporate areas that need updating.
Preserves initial investment of mapping previously done.
A digital orthophoto image is an unprocessed digital aerial photo image rectified to an appropriate DEM of the equivalent area. Software merges the digital image with the DEM and aligns the image orthogonally. While Non-digital-orthophoto outcomes are generally considered to be digital image enlargements or semi-rectified digital images.
Merge Scan Imager with DEM
Raw Digital Aerial Photo (Scan Image)
Elevation Model (DEM)
Most general users of digital orthophotos recently have very robust hardware and sophisticated software to view and manipulate orthophoto images. In regards to meet this request, suitable design of an orthophoto is vital.
The following factors should be considered in design:
1. Accepted uses of the orthophotos and smallest number of features to be viewed and considered
2. Accuracy requirements (relative and feature)
3. Predictable equipment with which the orthophotos will be viewed
4. The equipment, data, and processes used to generate the orthophotos.
Design parameters for an orthophoto are by and large attached to the expected absolute accuracy. Appropriate imagery and ground control are the basic elemental data that determines the final orthophoto reliability.
1. Imagery and Ground Control: Suitable variety of imagery scale and ground control as known above is important to the dependability of the final orthophoto.
Some items to consider are as follows:
â€¢ Scale of the imagery
â€¢ Type of imagery required (i.e., black and white, natural colour, and colour infrared)
â€¢ Clarity of the imagery (i.e., cloud cover, vegetation cover, seasonal requirements)
â€¢ Timeliness of the imagery
â€¢ What is the format of the imagery and how effectively can it be introduced into the orthophoto generation process
2. Image Scanning: Imagery for digital orthophotos may possibly be transformed to a digital image with a specific pixel resolution.
3. Ground Control: Ground control is required to rectify (geo-reference) the imagery to its true geographical position on the earth’s surface.
4. Digital Elevation Model: A suitable DEM must be obtained to provide a vertical datum for an orthophoto.
5. Data Merge and Radiometric Correction: The concluding phase of the orthophoto method is the merger of the digital image and the DEM along with corrections in pixel intensity throughout the image.
6. Tiling and Formatting: The conclusive orthophoto image is in the end broken into smaller areas that are more convenient to handle by the end user.
METHODS USED TO GENERATE TRUE ORTHOPHOTOS
Method based on Z-buffer: The Z-buffer algorithm is a method based on the fact that objects in lesser depth put out of sight those with greater depth with high opinion to the viewer.
Method based on dense digital terrain models: This technique was introduced by Ecker (1992) and further investigated by Skarlatos (1998) and uses digital terrain model with fine resolutions.
Method based on the merging of terrain and buildings orthophotos: the digital terrain model (DTM) and the digital building model (DBM) are processed independently. Buildings are correctly rectified using the DBM. A DTM is used with the original image, but where buildings are masked, to create a conventional orthophoto. The merging of the two orthophotos will give the final true orthophoto (Amhar, 1998).
Method based on a segmented digital surface model: It is based on enhance digital surface models acquired by correlation or LIDAR systems. (Boldo, 2003).
Method that generates orthophotos from a sequence of oriented images: all the images covering the area of interest are used in one step to generate the orthophoto.
In summary, technological developments of recent years in the digital photogrammetry field have facilitated invention of reliable digital orthophotos, essential for various surveying and Geographic Information (GI) applications. These developments are driven by the demand for high resolution data for research and engineering projects -properties that digital orthophotos provide. This changed lot of believe that orthophotography as a technology in search of an application. The introduction of digital photogrammetric cameras opened up the prospect of a fully digital work-flow, from camera to orthophoto. The production of orthphoto became a moderately simple procedure and a routine of photogrammetrists. The use of orthophoto being quick to produce and cheap is substituted for a conventional line-map.
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Also in recent years, Light Detection and Ranging (LiDAR) technology became a significant factor in producing advance and accurate topographic data. LiDAR technology is, based on airborne laser scanners, enabling acquired dense and accurate 3D data of the surveyed area, i.e., the Digital Surface Model (DSM). Orthogonal projection using a Digital Surface Model that takes into accounts the sudden elevation changes of artificial structures. Such as detection of occulted areas and merging of adjacent images to fill gaps and missing parts of the surveyed areas.
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