**2.1 Technological evolution: film to digital**

Photogrammetry has undergone three digital revolutions. The first took place in the 1980s with the digitization of the mathematical model that resulted in analytical stereoplotters (process input, frames, remained analogue). Analogue stereoplotters that solved the mathematical model by mechanical analogy disappeared.

The second occurred in the 1990s with digitization, using powerful and accurate photogrammetric scanners, analogue images from the aircraft that resulted in digital stations. This revolution was possible when personal computers had sufficient capacity to efficiently handle digital images, and represented the extinction of analytical stereoplotters and, with them, the stereoplotters themselves; that is, of the specific physical machines used for photogrammetry.

The third revolution took place in the first decade of the 21st century thanks to the development of digital cameras that began to compete with the large format (230 x 230 mm) of analogue cameras. These cameras were already used in terrestrial photogrammetry, where the small and medium formats were enough to develop projects. The need to cover large areas of land in aerial photogrammetry made the large size of the camera the only possible solution between the two photogrammetric requirements: accuracy (requiring long focal length), and performance in object coverage (requiring either short focal or large focal planes). This phase involves the disappearance of analogue cameras and, consequently, films. Therefore it will also represent the disappearance of photogrammetric scanners (**Table 2**).

The main advantages of digital versus analogue images are [2]: the ability to establish an entirely digital workflow (suppressing the scanners), a considerable improvement in radiometric quality as well as the possibility of simultaneously acquiring panchromatic images in the different color bands and in the near infrared; as well as the ability to generate real-time mapping [3]. The color depth of digital cameras (12-bit) should allow flights to be carried out in poor lighting conditions [4].

A first approach to digital cameras for photogrammetric use allows them to be classified into two large groups: frame and pushbroom. The first can be classified according to the size of the sensor [5]: small format cameras (up to 16 megapixels); medium format cameras (from 16 to 50 megapixels); and large format cameras (50 megapixels or higher). More recently, medium format can be located at 80–100 megapixels [6], and large format larger than 100 megapixels.


#### **Table 2.** *Evolution of digital photogrammetry.*
