Photogrammetric Capture is the operation of capturing and representing planimetric or altimetric elements of a given region using specific instruments and techniques.
In aerial photographs, scale decreases from the center toward the edges and changes according to terrain elevation, but the cartographic product resulting from photogrammetric capture follows a single scale throughout the entire sheet.
Photogrammetric capture is carried out from stereoscopic models, that is, through the overlap of consecutive aerial photographs by means of Stereoscopy.
Stereoscopy
When a person observes an object, the images formed in each eye merge in the brain and provide a perception of its depth. By simultaneously observing two photographic images of the same area, obtained from distinct points, the artificial reproduction of this phenomenon is achieved, which is called stereoscopy.
Conventional Capture
This is Photogrammetric Capture in which the images of the stereoscopic model are on physical media — photographs from a conventional aerial camera fixed onto a stable, translucent base, called diapositives.
Conventional capture is carried out with analytical or semi-analytical restitution instruments, in which the images are projected via optical/electronic or optical/mechanical systems and are formed separately in each eyepiece of the instrument, providing stereoscopic vision, from which the desired elements are extracted.
Conventional photogrammetric capture has its days numbered thanks to the availability of increasingly efficient digital systems.
Digital Capture
This is Photogrammetric Capture in which the images of the stereoscopic model are in digital media, coming from a digital aerial camera or from the digitization (scanning) of conventional aerial film.
Digital photogrammetric capture takes place at high-performance workstations equipped with an image polarization system, where the combination of polarizing screen and glasses allows for the separate formation of images in each lens, providing stereoscopic vision, from which the desired elements are extracted.
Scale
The precision of photogrammetric capture is limited by the flight scale; despite the ease of enlargement that digital media provides, magnification factors of 3 to 5 times over the flight scale are adopted.
Thus, for flights at a scale of 1:30,000 to 1:40,000, capture is at 1:10,000; for flights at 1:15,000 to 1:20,000, capture is at 1:5,000; for flights at 1:8,000 to 1:6,000, capture is at 1:2,000; and for flights at 1:5,000 to 1:4,000, capture is at 1:1,000.
The choice of flight and capture scale takes into account the precision required for the cartographic product, as well as the degree of detail planned for the mapping.
Elements for Capture
The basic elements obtained by photogrammetric capture are point symbols and polylines (lines with several points).
Planimetric elements are captured by passing the stereoscopic mark along the outline of the feature, where the points and lines are recorded in a CAD system to compose the cartographic original. The stereoscopic mark must follow the surface of the feature for the correct recording of the X, Y and Z coordinates — the spatial representation.
Contour line (elevation) capture is carried out with the stereoscopic mark at a pre-fixed altitude; the mark travels across the terrain, recording subsequent points that follow this altitude, and these joined points form the isolines, or lines of equal elevation.
Professionals
The professionals who carry out Photogrammetric Capture are Photogrammetry Technicians, in addition to the Quality Control Technicians who verify and control the cartographic products. ESTEIO is proud to have highly experienced and qualified technicians on its staff.
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