Essentials of Autopsy Practice: Reviews, Updates and Advances

By Guy N. Rutty

This book covers topical subjects within the field of death investigation, where changes in practice have recently occurred. The topics embrace the multi-disciplinary approach required for death investigation, and address advances in the field of forensic photography, pathology, and 3D printing as applied to forensics. This volume includes chapters on high altitude deaths, the role of 3D-printing applied to forensic investigations, photogrammetry, commotion cordis (an uncommon cause of fatal cardiac arrest) and the cricoid cartilage.

Essentials of Autopsy Practice: Reviews, Updates and Advances is an educational and practical resource aimed at trainees and consultants, generalists and specialists, and multi-disciplinary teams.

• Language: English
• Publisher: Springer
• Release date: Jan 1, 2020
• ISBN: 9783030243302

Book preview

© Springer Nature Switzerland AG 2019

G. N. Rutty (ed.)Essentials of Autopsy Practicehttps://doi.org/10.1007/978-3-030-24330-2_1

1. The Application of Photogrammetry for Forensic 3D Recording of Crime Scenes, Evidence and People

Chiara Villa¹   and Christina Jacobsen¹  

(1)

Section of Forensic Pathology, Department of Forensic Medicine, University of Copenhagen, Copenhagen, Denmark

Chiara Villa (Corresponding author)

Email: [e-mail]

Christina Jacobsen

Email: [e-mail]

Keywords

3D documentationPhotogrammetry3D modelCTSkin lesionsCrime sceneCamera

Introduction

The famous expression ‘a picture is worth a thousand words’ can definitively be applied to a forensic context. Photographs are the basis of any criminal investigation. Photographs are taken routinely to systematically document crime scenes, evidence, and victims. It is essential to record the crime scene accurately and precisely, since this documentation is used during police investigations and it can be crucial to the outcome of a trial. Photographs, sketches and manual measurements are the standard investigation tools to record a crime scene visually. However, by using these tools the 3D reality is reduced to a 2D plane. In addition, all the objects and people in a crime scene can always be visualized in the photographs, but their dimensions and actual location in the 3D space cannot be recovered from a single photograph. Recently, new forms of documentation such as laser scanners, structured-light scanners, and photogrammetry are increasingly used in forensic investigations [1–8]. These techniques enable a 3D documentation of the actual crime scene and of any evidence, including bodies. Photogrammetry has been found comparable to 3D commercial surface scanner with the advantages of being a faster method and not requiring expensive equipment and training [5, 8, 9].

In the forensic context, the potentiality of photogrammetry is often unknown. The aim of this chapter is to introduce forensic experts to the principles and application of photogrammetry for 3D recording crime scenes, evidence, and people (living or deceased individuals). Some applications in forensic pathology, including the integration of photogrammetry with post-mortem computed tomography (PMCT), are presented.

Photogrammetry

Definition and Brief History

The word photogrammetry derives from Greek: photo = light, gram = drawing and metron = measure. Thus, photogrammetry is the science of making measurements from photographs.

The American Society for Photogrammetry and Remote Sensing (ASPRS) defines photogrammetry as the art, science and technology of obtaining reliable information about physical objects and the environment, through the process of recording, measuring and interpreting imagery and digital representations of energy patterns derived from non-contact sensor system. Photogrammetry is used to extract 3D data of an object in digital form (coordinates and derived geometric elements, e.g. surface, area) or graphical form (drawings, maps).

The term photogrammetry was used for the first time in 1893 by Alberecht Meydenbauer (1834–1921), even though we can trace the history of photogrammetry centuries earlier. Indeed, photogrammetry is based on principles of optical perspective and projective geometry form. Leonardo da Vinci was the first to define the perspective at the end of the fifteenth century; he wrote that the perspective is nothing else than the seeing of an object behind a sheet of smooth glass … All things transmit their images to the eye by pyramidal lines, and these pyramids are cut by the said glass. The nearer to the eye these are intersected, the smaller the image of their cause will appear.

Photogrammetry has its beginning with the invention of the photography by Daguerre and Niepce in 1839. Ten years later (1849), Aimé Laussedat carried out the first experiment with photogrammetry, using terrestrial photographs for topographic map compilation. For this, he is considered the father of photogrammetry. Photogrammetry has been a very active field, especially in mapping and topology, and during the last centuries underwent several technological improvements. These developments can be divided into four cycles or stages: (1) Plane table photogrammetry, from about 1850 to 1900, (2) Analog photogrammetry, from about 1900 to 1960, (3) Analytical photogrammetry, from about 1960 to 2010, and (4) Digital photogrammetry, the present period. For more details on key historic figures, methodical and instrumental developments, please refer to e.g. Luhman et al. [10] and Foster and Halbstein [11].

Application Areas and Classifications

The expansion of photogrammetry has been rather dynamic, particularly in recent years. As mentioned earlier, the first broad application of the photogrammetry was in mapping. Today, photogrammetry is applied in many different fields: automotive, machine and shipbuilding industries; architecture, heritage conservation and archaeology; civil engineering; medicine and odontology; bioarcheology and forensic anthropology; natural sciences and geology; animation and film industries; and only recently, forensic investigations.

Photogrammetry, independently from its application, can have different classifications [10]. It is worthwhile to mention the classification based on the camera position:

Satellite photogrammetry: processing of satellite images;

Aerial photogrammetry: processing of aerial photographs acquired from plane or drones;

Terrestrial photogrammetry or close-range photogrammetry: processing of photographs measurements from a fixed terrestrial location.

Another classification is based on the number of measurement images:

Single image photogrammetry: single image processing, mono-plotting, rectification, orthophotographs;

Stereophotogrammetry: dual image processing, stereoscopic measurement;

Multi-image photogrammetry: more than two images.

Furthermore, photogrammetry has been classified based on the number of cameras used to take the photographs:

Single-camera system: photographs obtained from one single camera;

Multi camera system: photographs obtained from several cameras at the same time.

How Does It Work?

The fundamental principle used by photogrammetry is triangulation. By taking overlapping photographs from at least two different spatial locations, lines of sight can be pointed from each camera to points on the object (Fig. 1.1). From the intersection of at least two corresponding lines, a point can be located in three dimensions. In stereophotogrammetry, two images are used to achieve this. In multi-image photogrammetry, the number of images can be unlimited. Thus, photogrammetry uses the position of the camera as it moves through 3D space to calculate 3D coordinates (x, y, z) of the objects; for that is also known as structure from motion (SfM) photogrammetry. In practice, an accurate, true-scale 3D model of an object (e.g., a room, a car, a knife, a body) can be created from a series of overlapping images taken from different positions (Fig. 1.2). Please refer to Luhman et al. [10] for the mathematical fundamentals.

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Fig. 1.1

Schematic representation of fundamental principle used by photogrammetry to calculate 3D point positions

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Fig. 1.2

The Gallarus Oratory (Irleand): (a) overlapping photographs, (b) camera positions, (c) 3D model (software: Visual SFM)

The final products of a photogrammetric analysis can be 3D models (point cloud or mesh), lines (sketches, maps), distances and areas. Volume and surface can be also calculated. The 3D models obtained can be used in combination with 3D models obtained from other equipment (surface scanner or CT scanning), and for different purpose (e.g. animation, 3D printing).

Software Applications

There are several commercial and open-source photogrammetry software packages. In forensic and archaeological contexts, the most used ones are PhotoModeler (https://​www.​photomodeler.​com/​), Agisoft Metashape (https://​www.​agisoft.​com/​), previously known as Agisoft PhotoScan, and Reality Capture (https://​www.​capturingreality​.​com). There are also free programs, e.g. Visual SFM (http://​ccwu.​me/​vsfm/​) or Python Photogrammetry Toolbox (https://​github.​com/​steve-vincent/​photogrammetry). 3D models from the photographs acquired from a smartphone can be quickly obtained from online apps e.g. Scann3D or Qlone, although, they are not suitable for forensic work, due to the fact of sharing online confidential data. In addition, the accuracy of such apps is lower than professional applications.

Cameras and Calibration Process

Digital single-lens reflex (DSLR) cameras are to be preferred over compact cameras. Indeed, DSLR cameras have a better resolution, i.e. more points and thus more details in the images used for creating the photogrammetric model. In addition, the lenses in DSLR cameras are assembled more rigidly, thus the distortion of the lenses can be calculated with more accuracy and precision. Each lens introduces a distortion of the images, i.e. a deformation of the images, in a more or less evident way. In a distorted image, straight lines are visualized as curved lines. The two most common lens distortion are barrel distortion and pincushion distortion (Fig. 1.3). In barrel distortion, straight lines bend outward from the centre of the image; in pincushion distortion, straight lines bend or pinch inward from the centre of the image.

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Fig. 1.3

Illustrative examples of Barrel and Pincushion lens distortion

The lens distortion and other internal camera parameters (e.g. focal length, format aspect ratio, and principal point coordinates, radial and tangential distortion coefficients) are calculated and adjusted during the process of camera calibration. The camera calibration process improves the accuracy of the photogrammetric outputs and needs to be performed for each lens. To a small extent, internal camera parameters change also when the focus changes. A high accuracy is obtained with a well calibrated camera.

Photogrammetry programs, such Photomodeller or Photoscan, tackle the lens distortion using mainly two strategies: calibration through calibration sheets/targets or automatic calibration. The first way to calibrate the camera-lens system is based on calibration sheets, targets or calibrated scale (https://​www.​agisoft.​com/​pdf/​tips_​and_​tricks/​CHI_​Calibrated_​Scale_​Bar_​Placement_​and_​Processing.​pdf). For example, the calibration sheet of Photomodeler software is a printable sheet with targets and dots that needs to be printed out on paper and fixed on a floor or on a wall; four photographs around the sheet with the camera in different positions need to be taken (Fig. 1.4). The software then processes these photographs in order to calculate the lens distortion and internal camera parameters. To obtain the best calibration results, the calibration sheet should be printed on the size closest as possible to the subject of interest, e.g. 25.0 × 25.0 cm for small objects or 2.0 × 2.0 m for rooms. A multi-sheet calibration is more appropriate in case of larger subjects or scene. A similar process can be also performed with Agisoft, using, e.g. a chessboard.

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Fig. 1.4

Example of camera calibration using PhotoModeler software

Alternatively, the software can perform an automatic calibration by using the photographs of the subject of interest. In that case, the software calculates the cameras internal parameters during the alignment process identifying common points identified from the photograms.

How to Take Good Photographs for Photogrammetry Processing

In this section we provide some general guidelines for taking photographs to be used on photogrammetry projects based on our experience. It is assumed that the reader is acquainted with the basic concepts of photography. The results of the photogrammetry processes will depend heavily on the quality of the photographs.

A good photograph needs to be in-focus with the lowest noise possible and with a balanced exposure. DSLR camera with fixed lens or primer should be preferred. Image stabilization, and chromatic aberration need to be turned off. The camera should be set to aperture priority. The aperture should be kept fixed for the duration of the entire session. It is preferable to use a higher f-number, thus to guarantee a greater depth of field. Play with shutter speed, and ISO or increase the light. It is not advisable to use the flash because the shadows move on the subject between photographs and create artefacts in the reconstructed 3D model. Ring flash or external lights are a good alternative. The use of a tripod may also be helpful in some conditions. In our experience, a shutter speed equal to or faster than 1/100 should be used with a hand-held camera to assure an in-focus photograph. However, this is very subjective and depends on the experience of the photographer. It is advisable to calibrate the white balance or use a colour checker during the taking of photographs (https://​xritephoto.​com/​colorchecker-passport-photo).

Importantly, either a ruler must be used during the photograph session or a precise distance of reference, in case larger objects or scenes, need to be photographed. This measurement is fundamental during the post-processing of the photographs to scale the project, i.e. the set of photographs. It is advisable to have more than one measurement, preferably in the different axis (x, y, z), to check if the project is correctly scaled. The ruler does not need to be in all the photographs, but in a minimum of 6–10 photographs.

The photographs must be taken with a good overlap: each point in the scene should be clearly visible in at least three photographs; the more, the better (Fig. 1.5). An overlap of around 80–90% generally provides good coverage of the scene. Shiny surfaces, mirrors, glasses and white walls can be difficult to be 3D reconstructed. Surface scanners have identical problems with such surfaces. A solution could be the use of targets or as a chessboard patterns [12].

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Fig. 1.5

Example of good photogrammetry coverage of a shoe; each colour square represents a photograph

Some basic ideas about camera positions in the different scenarios are shown in Fig. 1.6. It is desirable to repeat the steps at different heights (Fig. 1.7). Alternatively, a turntable table can be also used for small objects or a multi-camera system for living individuals.

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Fig. 1.6

Recommended camera positions in the different scenarios

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Fig. 1.7

Recommended camera positions at different heights

Applications

3D Documentation of Injuries and Bodies

Photogrammetry has already been demonstrated a valuable tool in forensic and clinical pathology. Photogrammetry has been used to record skin lesions [5, 13], imprint marks on the skin [14, 15] and also internal organs and bones [5]. Entire bodies can also be 3D documented using a multi-camera system [16, 17]. Photographs can be taken using a single camera [5, 6, 18, 19] or a multi-camera system [16, 17], Recently, the use of video recording has been suggested as a possible alternative [20].

Photogrammetry enables a permanent 3D documentation of the injuries that can be reassessed with great accuracy and precision at any time. Measurements gained by photogrammetry are very accurate, reliable and repeatable [6]. An example of 3D model of a stab wound can be seen in Fig. 1.8.

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Fig. 1.8

3D model of a hand obtained using PhotoScan software

The photogrammetry workflow we use [6, 20, 21] fits easily into the routine autopsy algorithm and does not significantly influence the workload in terms of time. The time necessary to take photographs of a lesion to process using photogrammetry varies from one to 15 min [6], using a video camera is five times faster [20]. We found out that the documentation of lesions using photogrammetry is faster and easier than that obtained using surface scanners. Importantly, it does not require extensive training, only some basic knowledge about photography; personnel in the autopsy room do not need to remain stationary for a long time and photography is not subject to any room influence, differently to surface scanners that can be influenced, for example, by light conditions or metal objects [3].

As a general guideline, we suggest taking photographs of lesions after the body has been cleaned and completely dried. If opportune, two photograph sessions, one before, one after cleaning, can be carried out. Similarly, lesions in hairy body regions, e.g., head, legs, or arms should be photographed before and after the shaving of the regions. Any skin lesions can be documented using photogrammetry, even though some cases can pose more difficulties than others can. For instance, the mid- to distant range shotgun wounds, are often devastating for the body and should be photographed more than one time. Particular attention should be paid to the colours in case of bruises. Indeed, bruises are blurred, especially at the edges; thus, small differences in colours not completely visible at a naked-eye examination or not completely captured by the camera could generate a large difference of measurements. A solution would be the use of a colour check during the photographs. Stab wounds should be photographed undisturbed on the skin (‘open’), and in a closed state (‘closed’) thus to enable measurements of the lesions as routinely performed by the pathologist. It is not advisable to close the lesions with the help of fingers; the fingers partially obscure the lesion and can introduce movement artefacts preventing the good orientation of the photographs. A solution could be to put a representative stab wound into a closed state by cutting a square around the surrounding skin to release the tension in the surrounding tissue, as suggested by Catanese [22]; alternatively, the edged of the lesions can be closed using tape. Internal organs and bones can also photographed and 3D reconstructed (Fig. 1.9). However, the resulting 3D image may be more noisy because of the presence of reflective moist surfaces [5].

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