This is a mapping product including metrics and scale that is obtained through the manipulation of vertical aerial photography in a completely digital surrounding.
The production of digital orthophotos is basically the digitalisation of flight photograms and the manipulation of the image, pixel by pixel in a process known as orthoprojection, until placing each one in their true position within the tolerances set at each scale. The final effect achieves what would be achieved if each on-site object had been photographed from its own overhead at a variable height; thus, all appear to the same scale.
Analogical and Analytical Photogrammetric Plans:
Photogrammetry allows a model similar to the terrain to be achieved with on-field images registered.
Some points are necessary (known as 'support points') to carry out the transformation from images to reality. Once this model is done, it is relatively easy to get the coordinates of all necessary points from it with a homogenous precision and an output that goes well beyond what can be obtained on-site.
This is a technique that is in continual evolution and has many uses that are only now coming to light: video photogrammetry, images obtained with non-visible radiation, art, industry, virtual reality, interactive flights, etc.