The photogrammetryía It is a technique to extract spatial information from images, especially from aerial photos, but also from those taken on land or even under the sea. From this information the dimensions and positions of the represented objects are quantified.
The photographic images are flat, like the one shown in figure 1, but through them it is possible to estimate, for example, the height of buildings or rocks, either with respect to the road, the sea or another point reference.
The creation of images that are very close to reality is not new. The great Leonardo da Vinci (1452-1519) was a pioneer of perspective, perfecting his principles through the use of so-called leak points.
Vanishing points are the places on the horizon where parallel lines converge, giving the viewer a sense of depth.
Leonardo did it with paintings and drawings made by hand, but from the moment photography was invented, in the 19th century, photos also began to be used for technical purposes.
So did Aimé Laussedat (1819-1907) and Albrecht Meydenbauer (1834-1921), considered the fathers of modern photogrammetry. Laussedat built detailed topographic maps in 1850 by superimposing different perspectives on a plane.
For his part, Meydenbauer, who was an architect, applied the technique to document buildings, which, if destroyed, could be completely rebuilt thanks to the information stored.
In the 80s of the 20th century, modern computing made photogrammetry a great leap, by minimizing the time required for image processing.
Broadly speaking, the method consists of taking images of objects, processing them and finally interpreting them. The main elements to describe the basic principle are indicated in figure 2:
First of all, you need a sensor to capture the image and also a lens, so that each ray of light coming from a point, hits the sensor in the same place. If this does not happen, the point is registered as an overlay, resulting in a blurry or out of focus image..
In order to reconstruct the object, only the rectilinear ray drawn in black in figure 2 is of interest in photogrammetry. This is the one that passes through the point called perspective center in the lens.
SOn that ray, which goes directly from the object, passes through the lens and reaches the sensor, there is the distance that is sought.
The natural vision of human beings is stereoscopic. This means that we can know the distances at which the objects are, thanks to the fact that the brain processes the captured images and evaluates the reliefs..
That is why each eye captures a slightly different image and then the brain does the work of interpreting them as one, with relief and depth..
But in a flat drawing or photograph it is not possible to know how far or how close an object is, since the information on the depth was lost, as explained graphically in figure 3.
As we have said, the point is on the main ray, but there is no way to know if it is closer because the object is small, or if it is further away, but it belongs to something bigger..
So, to fix the closeness problem, two slightly different pictures are taken, as shown below in figure 4.
Knowing the intersection of the rays by triangulation, the position of the object from which they come is discovered. This procedure is called “point matching” and it is done using specially designed algorithms, since it is necessary to repeat the procedure with all the points of an object..
Details such as position, angle and other characteristics of the camera are also taken into account to obtain good results..
Depending on the way the images are acquired, there are several types of photogrammetry. If the images are taken from the air, it is aerial photogrammetry.
And if they are taken on the ground, the technique is called terrestrial photogrammetry, which was the first practical application of the technique..
Aerial photogrammetry is one of the most widely used branches today, as it allows the generation of highly accurate plans and maps. The images can also be acquired through a satellite, in which case we speak of space or satellite photogrammetry.
Likewise, photogrammetry is classified according to the instruments used and the treatment given to the image, and may be:
In analog photogrammetry, imaging and processing are completely optical and mechanical.
In analytical photogrammetry, the frames are analog but processed on the computer. And finally, in digital photogrammetry, both the frame and the processing system are digital..
The topography also aims to represent the rural or urban terrain on a plane, highlighting points of interest. And conversely, if necessary, take the points of the plane and place them in space.
For this reason topography and photogrammetry have a lot in common, however the latter has some advantages:
- It is almost always cheaper.
- The acquisition of the data - survey - is faster, suitable for large areas.
- Works best on very rough terrain, unless covered by thick vegetation.
- All points are recorded equally.
- The information can be saved and it is not necessary to return to the field to obtain it again.
In general, it is not possible to reconstruct a photographed object from a single photograph, unless some other additional information is used, because as we have already seen, in a flat image there is no record of depth.
Still, the images still provide valuable information, albeit with some restrictions.
As an example, suppose you want to identify a bank robber. A surveillance camera image can be used to determine the height and build of the person who committed the crime, by comparing it to the known size of furniture or other people in the image.
Photogrammetry is widely applied in various disciplines, such as architecture, engineering and archeology, to name a few. As explained before, it is applied in forensic sciences and of course, for the special effects of the films.
In engineering, good images can reveal information about the relief and configuration of a terrain, for example. Here are some specific areas of great interest:
-Study of communication routes.
-Establishment of paths.
-Study of hydrographic basins.
-Aerial surveys for mining prospecting.
Additionally, photogrammetry is a very appreciated tool in:
-Architecture: in the raising of monuments and buildings.
-Archeology: to rebuild old buildings from the remains preserved today.
-Zoology- Helps make three-dimensional models of current and extinct animals.
-Mechanics: in the modeling of cars, engines and all kinds of machinery.