PHOTOGRAMMETRY: HISTORY, METHOD, TYPES, APPLICATIONS - SCIENCE - 2025

The fotogrametr ed is a technique for extracting spatial information from images, especially aerial photographs, but also those taken on land or under the sea. From this information the dimensions and positions of the represented objects are quantified.

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.

Figure 1. An aerial image taken to perform a photogrammetric survey. Source: Wikimedia Commons. Photograph by D Ramey Logan

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 vanishing points.

Vanishing points are the places on the horizon where the parallel lines converge, giving the viewer a sense of depth.

Leonardo did it with paintings and drawings made by hand, but from the moment that 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 plan.

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 1980s, modern computing made photogrammetry a great leap forward, minimizing the time required for image processing.

Photogrammetry method

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:

Figure 2. Basic principle of capturing an image. Source: F. Zapata.

First of all, a sensor is needed 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 the center of perspective in the lens.

If that ray, which goes directly from the object, passes through the lens and reaches the sensor, is the distance that is sought.

Stereoscopic vision

The natural vision of human beings is stereoscopic. This means that we can know the distances to which the objects are, thanks to the fact that the brain processes the captured images and evaluates the reliefs.

So 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 depth information 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.

Figure 3. In a flat image, the depth of objects cannot be determined. Source: F. Zapata.

So, to fix the closeness problem, two slightly different images are taken, as shown below in figure 4.

Figure 4. The intersection of the two lines allows us to find the real location of the point in space. Source: F. Zapata.

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.

Types

Depending on how 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, which may be:

-Analog

-Analytics

-Digital

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.

Photogrammetry vs. topography

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 much in common, however the latter has some advantages:

- It is almost always cheaper.

- The acquisition of data - survey - is faster, appropriate for large areas.

- Works best on very rough terrain, unless covered by thick vegetation.

- All points are registered equally.

- The information can be saved and it is not necessary to return to the field to obtain it again.

Single Image Photogrammetry

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 robber in a store or bank. An image from the surveillance camera can be used to determine the height and build of the person who committed the crime, by comparing it with the known size of furniture or other people in the image.

Figure 5. The chairs are the same size and we immediately know which is the closest. On the other hand, the parallel lines on the floor that converge in the distance, provide the feeling of depth in the photo. Source: Pixabay.

Applications

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 science and of course, for special effects in movies.

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 routes.

-Earth movements.

-Urban planning.

-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 reconstruct old buildings from the remains preserved today.

- Zoology: helps to make three-dimensional models of current and extinct animals.

- Mechanics: in the modeling of cars, engines and all kinds of machinery.

References

1. Adam Technologies Team Blog. How Does Photogrammetry Work? Recovered from: adamtech.com.au.
2. Armillary, Applied Geomatics. Photogrammetric techniques. Recovered from: armillary-geomatica.blogspot.com.
3. Photomodeler Technologies. How Does Photogrammetry Work? Recovered from: photomodeler.com.
4. Quirós, E. 2014. Introduction to Photogrammetry and Cartography applied to Civil Engineering. Published by the University of Extramadura.
5. Sánchez, J. Introduction to Photogrammetry. University of cantabria. Recovered from: ocw.unican.es.