The edges of historical aerial photographs are often slightly warped and roads are re-routed or adjusted over time. It can be challenging to make sure everything gets lined up properly. It is like a jigsaw puzzle, with the various pieces overlapping and fitting together to form the complete image.Ī Series of Georeferenced Photographs Pieced Together to Form the Complete Image The georeferencing tool in the City’s ArcMap program then rectifies the aerial photograph by stretching and rotating it to fit in the desired spatial location on the present-day map. The eye is fooled into combining the two images and the result is that landscape features suddenly have the illusion of depth.īy using a tabletop stereoscope, mapping technicians would have been able to determine the height of the Port Mann Bridge - shown in these aerial photographs as it was being built in 1963. Today, mapping specialists use complex computer-aided design (CAD) technology to create these three-dimensional digital representations of what’s on the ground but before computers came along, photographic prints were examined using a tabletop stereoscope that allowed for viewing two separate overlapping images at once. These overlapping images allow mapping specialists to get a sense of the relative height of buildings or natural features. The overlapping nature of the photographs (what is referred to as "stereo") enables three-dimensional features to be perceived from two-dimensional photographs. The plane flies at a set speed and photographs are taken at specific intervals to ensure the images overlap. Aerial photographs are taken in such a way as to provide overlapping coverage of the ground. This is called stereoscopic vision.Īerial photography mimics this same process but uses an external viewer to put the images together. Our brain takes the two separate images from our eyes and combines them in a way that shows depth. Did you know that human eyes can only see in two dimensions? It is an incredible trick of our brain that allows us to see in three dimensions, giving the world its depth.
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