**3.1. From photography to photogrammetry**

By themselves, photographic images, whether analogue or digital, do not have metric properties. That is, we cannot infer direct measures of the objects displayed in it, or obtain the distances that lie between them. We can visualize, interpret, or analyse, but not exploit them metrically. This is because images are built on the base of the conical perspective, rather than an orthogonal projection. As a consequence scale in images is variable. Therefore, performing accurate measurements on images must be done by taking some precautions.

Photogrammetry deals with obtaining metric information (two‐dimensional or three‐dimen‐ sional) from photographic images, allowing crime scene 2D or 3D reconstruction while preserving strictly their geometric characteristics [8]. The basic principle for photogrammetry is the geometric relationship that takes place between the points of the object (3D element in space), the corresponding image points (2D), and the point of view (placed in the centre of the camera lens). In essence, it is noted that each point of the object, the photographic image, and the viewpoint verified the colinearity condition, that is, they define a straight line (**Figure 4**).

In the photogrammetric workflow, we can distinguish the acquisition phase and the phase of restitution. In the first one, from a three‐dimensional real object, the image is built. The restitution process is just the opposite: from the image will attempt to reconstruct the object, that is the mapping or virtual representation.

**Figure 4.** Basis of colinearity condition.

**3. Geomatics and Forensics: overview**

8 Forensic Analysis - From Death to Justice

This section will describe the main geomatic technologies currently applied in forensic analysis. Since the early 1960s, we are witnessing a quick evolution of sensors and algorithms in geomatic science. That evolution was supported by the electronic advances such as electro‐ magnetic distance and angular measurement, computer miniaturization, increase of computer power or imaging semiconductor circuits. Nowadays, the drive is now reinforced by the

telecommunications advances. **Figure 3** shows a quick summary of this evolution.

**Figure 3.** Chronological chart showing geomatic advances related to forensic science.

By themselves, photographic images, whether analogue or digital, do not have metric properties. That is, we cannot infer direct measures of the objects displayed in it, or obtain the distances that lie between them. We can visualize, interpret, or analyse, but not exploit them metrically. This is because images are built on the base of the conical perspective, rather than an orthogonal projection. As a consequence scale in images is variable. Therefore, performing

Photogrammetry deals with obtaining metric information (two‐dimensional or three‐dimen‐ sional) from photographic images, allowing crime scene 2D or 3D reconstruction while preserving strictly their geometric characteristics [8]. The basic principle for photogrammetry is the geometric relationship that takes place between the points of the object (3D element in space), the corresponding image points (2D), and the point of view (placed in the centre of the

accurate measurements on images must be done by taking some precautions.

**3.1. From photography to photogrammetry**

For the cartographic restitution of a photograph, it must be solved three problems of geometric nature. First, the position of the viewpoint with respect to the photograph (or digital image) must be identified. This implies that the internal geometric characteristics of the camera (essentially its focal length, distortions and deformations) should be known. This process is known as *internal orientation*.

Second, we must determine the location of the photograph viewpoint regarding the object. This process is called *external orientation* if the location of the object is referred to a ground coordinate system, or *relative orientation* when not working with a real reference system. When documentation of indoor scenes is done, the Cartesian reference system is characterized by the fact that the Z axis tends to coincide with the vertical direction and *XZ* (or *YZ*) plane usually matches to some of the façades or walls object (if there is possible, on the contrary it could be arbitrary). The positioning of each photograph in space is determined by the three spatial coordinates (*X*, *Y*, *Z*) and the three spatial angles (ϖ, ϕ, χ) of the viewpoint.

Finally, the reconstruction of the object is performed through the intersection between different correspondent and perspective lines (collinearity condition) by using at least two photographs. In geometric terms, this means a difficult task, especially in those cases where a full automation is required. To solve this problem, photogrammetry, without prior knowl‐ edge of the object, acts similar to the behaviour of human vision, in which images are obtained from two converging intersections.
