At the crossroads of aerial photography and cartography we find orthophotography or orthomosaic.
The area to be captured is first broken into a grid using a special software that will plot a capture sequence and communicate it to a drone, which will take photos following this sequence. Each picture overlaps the adjacent ones by a proportion of 60% for standard orthomosaics or 80% and more for “true” orthomosaics.
Once the capture has been made, the reliefs are flattened and the resulting photographs chromatically adjusted, stitched together, then layered upon a two-dimensional canvas; a mosaic.
The final image is then superimposed, and its scale calibrated to exactly coincide with a mapped space, resulting into an accurately scaled photographic representation of a geographical area.
A standard orthophotographic map can show a few building facades in its layout, while a “true” orthophotographic map will depict the entire territory as if it had been photographed directly above each structure. And so, each building, regardless of its height, will occupy only the surface of its roof, freeing the neighboring surfaces such as sidewalks and streets, from any visual obstruction. Therefore, “true” orthomosaic requires greater overlap between the capture boxes, to make it possible to get a perfect 90 degrees top down vantage points for each portion of the area that is photographed.
Orthophotography is a process that was introduced in the 1960s. At the time, photographs were shot on film, from an airplane that made several linear passes over the territory to be recorded. The photographs were then developed and scanned in digital format, to finally be processed by industrial size computers. In addition to the processing time, the first iteration of the orthophotography also faced issues inherent to the scanning of negatives, i.e., dust and scratches, which also had to be removed before the final output. Orthophotography was therefore a long, expensive and complex process.
Today, technological advances and the miniaturization of electronic components make this methodology much more accessible. An area can be mapped using a drone rather than an airplane, digital imagery eliminates the costs and delays of analog photography, and the improvement of processing algorithms facilitates and refines the correction of optical aberrations, radiometric color adjustments (when the light conditions vary from one photo to another), the correction of deviations due to elevation and the stitching of the images for the final composite.
Up until now, cartography has always had to deal with the disparity between the illustrated representation of a territory and the real visual layout of the physical elements that compose it. This disparity requires an effort of visualization and contextualization on the part of those who consult the plan or the map. Orthomosaics throws a bridge over this disparity, while opening the door to new applications.
One of the best known applications of orthomosaic is the satellite rendering available on Google Maps, which combines a series of images taken from space into a single photographic background which is perfectly overlaid by the graphic elements of classic Maps, ie roads, buildings, rivers and forests. The advantage of orthomosaic by Virtuo Airviews? Much more precise and detailed images ranging from 3 to 5 cm of resolution per pixel can be delivered in less than a day , rather than 1 to 3 years for Google Maps updates.
GIS (Geographic Information System) can use orthomosaics to contextualize layers of information of geographical relevance like statistics, frontiers, landmarks or demographic data, by creating a scaled background photography of the area. It is a visualization tool that makes the assimilation and understanding of large data models that much easier and natural.
An elevated vantage point is by itself not sufficient to showcase a geographical area in a clear and homogeneous way. Terrestrial curvature and reliefs are all distortions that hinder the summative understanding of a large-scale physical space. Orthomosaic allow these distortions to be removed in order to produce a flat and geographically exact model which perfectly synthesizes the reality of the territory in an immediately assimilable format.
Photographic renderings of geographic regions are used as visual complements to GIS, or geographic information systems. A GIS is used to display several layers of information related to a geographic context, in a visually significant way. Roads, flood zones, demographic information or landmarks, there are plenty of data sets that can benefit from this kind of spatial visualization.
Orthophotography makes it possible to contextualize layers of information over a photographic background representing the corresponding geographical area, in accordance with the true physical distribution of the territory on a flat and precisely scaled plane, regardless of the actual variations in elevation.
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