Introduction
Footwear leaves its marks and impressions through direct physical contact, in or on the surfaces over which it tracks. They are left in a large variety of two-dimensional and three-dimensional forms and are accurately sized representations of the footwear that caused them. Some impressions are more easily detected than others. Most are remarkably durable and contain sufficient detail for meaningful forensic examination.
Footwear impressions are most valuable as forensic evidence, in crimes of burglary, assault, robberies, rape, homicide and other crimes, where proof of an individual’s presence is incriminating. They provide valuable information to the scene of crime officer and investigators, including information concerning the number of suspects, their path through the scene, their involvement in the crime, and their shoe type and size estimates. Impressions can reveal the point of entry and exit, often leading to other evidence.
The role of the forensic footwear examiner is to determine whether a particular item of footwear did or did not cause the marks or impressions recovered from a crime scene. This effort to either eliminate or identify the footwear evidence involves obtaining the best evidence from the scene of the crime, producing test prints from suspect footwear, conducting a laboratory examination and providinga written report. If necessary, the footwear examiner will offer this evidence and opinion in a court proceeding.
Forms of Marks Two-dimensional footwear marks
The term ‘footwear mark’ is used to describe the resulting evidence an item of footwear leaves when it makes contact with a non giving surface, such as a floor, kicked door or walkway. It is synonymous with the terms ‘two-dimensional impression’ and ‘shoe print’.
The mechanics of a shoe making a two-dimensional impression through physical contact with a receiving surface is complicated by the fact that innumerable forms of trace residue are picked up, carried and later deposited by the shoe; these include all forms of dust, residue, grime, soil, blood, grease and oils, etc. This is further complicated because these varied materials combine with the features of virtually all porous and nonporous substrates, resulting in marks ranging from highly visible to latent, and from those that are highly detailed to those with minimal detail. The many possible combinations also complicate the process of choosing which is the best method for recovery.
Transfer of trace or residue materials With many two-dimensional impressions, there is a transfer of trace materials or residue between the footwear and the substrate. An impression that results when a shoe sole accumulates and then redeposits material is representative of the areas of the sole that actually came in contact with the ground surface. These are known as ‘positive impressions’ and account for most two-dimensional impressions. A ‘negative impression’ is produced when the contact areas of a shoe remove residue from a surface, leaving the residue where the areas of the sole did not come in contact with the substrate. This occurs less frequently.
Two-dimensional impressions can occur when the shoes are wet or dry and can occur on a large variety of receiving surfaces. When a dry shoe leaves an impression on a dry surface, the impression is known as a ‘dry origin’ impression. The term ‘wet origin’ is used to describe an impression made when the shoe and/or surface contains moisture at the time the impression is made. Some examples of the many possibilities are the following.
Clean dry shoes on dry surfaces A shoe with a perfectly clean sole can still leave a trace of its impression on paper items, pieces of glass and polished surfaces by making an impression on that surface, or by removing the residue or film from those surfaces. For example, the physical contact of a clean shoe on paper items can result in impressions that can be developed chemically and physically.
Dry residue impressions When shoes track across a dirty surface, the bottom of the soles will accumulate a coating of residue. If they then track on to a relatively clean surface, that residue will be deposited in the form of footwear impressions.
Wet impressions Impressions made when a shoe sole is wet or damp constitute another category of impression. Most of these impressions dry out before they are found, but must still be considered as impressions of wet origin for the purposes of recovery. When there is rain, heavy dew or snow on the ground, most of the impressions, particularly at the point of entry, will be wet impressions. Many shoe soles retain moisture for a period of time and consequently leave wet origin impressions well inside interior areas. Impressions can also occur when the sole is wet and contains visible residue, such as mud or wet residue.
Depressed marks
The term ‘depressed mark’ is used to describe the evidence an item of footwear leaves when making contact with a deformable surface, such as soil, sand and snow. It is synonymous with the term ‘three-dimensional impression’. Depending on the qualities of, and moisture present in, the substrate, the impression can be deep or shallow and it can retain detail ranging from excellent to one that cannot even be recognized as an impression of a shoe. Three-dimensional impressions marks are located primarily on exterior surfaces, but can include those found inside on deep-pile carpet, spilled materials and other deformable surfaces.
For simplicity, the more general term ‘impression’ will be used hereafter to refer to all forms of footwear evidence.
Information from Impressions
Footwear impressions provide valuable information. At the scene of the crime, they allow for the following.
Identification of footwear A specific item of footwear, to the exclusion of all others, can be identified as having left an impression at a crime scene.
Elimination of footwear Items of footwear can be eliminated as possibly having left an impression at the crime scene.
Linking footwear to a particular time Impressions left on objects moved during the crime, such as on paper removed from a safe, or on broken glass at the point of entry, on newly fallen snow, or in the victim’s blood, provide restrictions on the time frame in which they were left and therefore relate the impressions to the crime.
Participation in the crime Footwear can be linked to a crime based on the time frame restrictions, and based on the location of those impressions and circumstances surrounding a particular event. They may also be linked to the crime if the owner of the footwear has no other explanation of how the footwear impressions otherwise legally occurred at the scene.
Location of impressions Impressions at the points of entry and exit provide a means of locating other impressions or evidence. Impressions also provide information about the path through the scene, and activities which occurred within.
Corroboration or rebuttal of alibis The presence and specific location of, or the absence of, impressions is used to corroborate or rebut the statements or alibis of suspected persons.
Classification Classification of the footwear impressions allows for the determination of the brand of manufacturer and style of the footwear that left the impressions. In most full-service laboratories this is done with a computer database, wherein the sole designs are arranged in a suitable manner for searching crime scene prints. This information often contributes to the development of a suspect.
Crime scene linking The same or a similar database is used by some laboratories to store impressions from crime scenes and successfully link crime scenes through the features of the crime scene impressions.
Estimate or determination of shoe size Crime scene impressions can also allow both estimates as well as precise determinations of shoe size. The linear dimensions of full or nearly full impressions allow for a general estimate of shoe sizing. A more specific method of sizing is possible if the manufacturer of the footwear is known, and comparisons can be made with samples of various sizes from that manufacturer. In this case, accurate sizing is possible with partial impressions as well as full impressions.
Estimate of height Based on the length of the footwear impressions, an approximate estimate of the perpetrator’s height can be made. This estimate may have some investigative value.
Number of perpetrators involved Impressions of different designs or sizes of shoes at the scene can provide information about the number of persons involved in the crime.
Location and association with tire treads of perpetrator’s vehicle Backtracking footwear impressions from the point of entry, or tracking impressions exiting the scene, can lead to the tire tracks of the perpetrator’s vehicle.
Gait characteristics Gait analysis is concerned with the systematic study of human walking. Measurements of gait characteristics include stride length, step length, stride width and foot angle. Footwear impressions at crime scenes rarely present a succession of impressions that allows reliable use of this type of information. Further, it is concerned itself with general characteristics and is not suitable for personal identification.
Tracking Tracking involves the use of observing ‘signs’ to follow and locate an individual. This includes the use of shoe prints, bare footprints, crushed debris or displaced material that has been stepped on, and any other evidence of the passage of a person. It utilizes the gait characteristics to help in locating the various ‘signs’ found in successive steps. It is most commonly used for illegal aliens crossing borders, and missing children, but has also been used to locate criminals by following their tracks from the crime scene.
Treatment and Recovery
Footwear impressions at the scene of a crime are utilized in cases where proof of a person’s presence is of some significance. The extent of how impressions can be used in an investigation and the results of the forensic examination depend critically on the treatment and recovery of those impressions. This process is easily divided into two steps: (1) locating and protecting the impressions; and (2) recovering the impressions.
Locating impressions
Most footwear impressions are on floor surfaces and are therefore subject to being tracked over by the shoes of other individuals. In order to avoid additional foot traffic over areas that may contain impressions of the perpetrator(s), the scene should be secured as quickly as possible. Details concerning the crime should be factored into the evidence recovery process. If footwear marks are of potential value in the proof of facts, attempts to aggressively search for, protect, preserve and recover those impressions should be set in motion.
Many impressions are obvious upon visual examinations of the scene. Others require special methods to assist in their location. These include darkening the room and searching for impressions with the use of high density oblique lighting and alternative light sources; the use of specular reflection to view the mark from the opposite side of a strong light source; and the use of electrostatic lifting in areas believed to have been walked on, such as the point of forced entry. Areas searched include not only flooring that may have been walked on but also any pieces of paper, broken glass or other objects on the floor, doors that may have been kicked in, and other surfaces, such as bank counter tops, desks, chairs, etc., that may have been stepped or walked on.
Exterior surfaces are searched with both existing and oblique lighting, and should be extended to include areas in the location around any forced points of entry, the escape routes and any remote areas where the perpetrator may have hidden.
All impressions should be considered to be of value. Partial and faintly visible impressions are likely to be as valuable as whole impressions. Areas or items containing impressions should be protected until the impressions can be properly recovered.
Recovering impressions
Once impressions are located, the preservation and recovery of those impressions is of paramount importance, as their detail will impact on any subsequent examination. Notes, photography, lifting, casting and enhancement methods are all used to recover the maximum amount of information.
Written notes and documentation Notes providing information about each impression should be prepared. A description of the impressions (a residue impression on a piece of broken glass), their position and direction (at point of forced entry heading toward interior door) and substrate and/or weather conditions (exterior dew on grass, dry inside) only takes minutes to note. These observations are of value to the examiner in considering enhancement methods and may also have investigative importance later.
General scene photography General scene photographs are taken in series, from long to middle to close distances. Their zoom-in effect provides photographic documentation of the relationship of the impressions to the scene. General scene photographs are not intended for, nor do they contain the necessary detail for, use in forensic examinations of the footwear evidence.
Examination quality photographs Examination quality photographs are taken to recover the maximum amount of detail, specifically for use in a forensic footwear examination. To take examination quality photographs, a camera with a negative format of 35 mm or larger, and capable of manual focus, is placed on a tripod and positioned directly over the impression (Fig. 1). A finely divided scale, placed next to the impression, should be used in every exposure. The scale permits accurate photographic enlargement of the prints to a natural size, which is critical when conducting the examination. In the case of two-dimensional impressions, a very thin scale, at least 15 cm in length, should be set alongside the impression. In the case of three-dimensional impressions, the scale should be positioned alongside the impression and it must be set in the substrate on the same plane as the bottom of the impression. Oblique lighting should be used in the proper manner to increase the contrast in both the two-dimensional and three-dimensional impressions. Some three-dimensional impressions in light-colored substrates, such as snow and white sand, are particularly hard to photograph. For impressions in snow, Snow Print wax or an aerosol paint can be lightly sprayed at an oblique angle, from 50-60 cm away, to highlight the ridges or high spots of the impression. This will make the impression more visible, by adding contrast. This procedure is especially important with snow impressions because photographs of snow impressions alone provide little contrast and because subsequent casting of snow impressions is more difficult and less reliable. Very light application of colored aerosol paint of contrasting colors has also been used to highlight impressions in very light or very dark soils.
Figure 1 To take examination-quality photographs, a camera on a tripod should be positioned directly over the impression. A scale should be placed alongside the impression. A flash unit and long flash extension cord will allow the proper use of oblique light during exposure.
Original impressions Whenever possible, the original two-dimensional impressions, such as those on pieces of broken glass, paper, small rugs and the like, should be recovered and transmitted to the laboratory for examination. This should be done only after photography at the scene, and providing the impressions can be moved without damaging them. Moving them to the laboratory provides additional time and resources which often enable better recovery, enhancement and analysis. The removal of larger items containing impressions should be considered on an individual case basis.
Lifting techniques Lifting is performed (1) to improve the visibility and detail of the impression by transferring it to a surface providing better contrast; (2) to remove an impression from the scene to the laboratory; and (3) to search specific areas at the scene for latent footwear impressions.
Dry origin impressions To successfully lift a dry origin impression there must be certain differences between it and the surface it is on. A shoe tracking dust or dirt across a floor covered with the same dirt or dust does not leave a mark that can be separated by lifting. If a shoe tracks from a dirty or dusty surface to a relatively cleaner floor surface, or steps on items such as paper, a chair seat or desk top, the impression could be lifted from that surface.
Wet origin impressions Wet origin impressions, as well as impressions in blood or greasy materials, are more firmly attached to and partially penetrate the surface. They are thus more difficult to lift. The use of cyanoacrylates, followed by powdering and lifting, or powdering alone followed by lifting, or the use of gelatin lifters on the original impression, are methods which provide some success.
Techniques or materials used to lift impressions include the following.
Electrostatic lifters These are devices that utilize a high voltage source to create static electricity, causing the transfer of a dry origin impression to a black lifting film. The black lifting film provides better contrast and allows for improved visibility of the impression. Photography of the lift with oblique light provides increased detail. Electrostatic lifting can be used to lift dry origin impressions from any surface (Fig. 2). In cases where attempts to lift the impression electrostatically are not successful, the process will not harm the impression, and will not affect the success of other, subsequent enhancement or recovery methods.
Gelatin lifters A gelatin lifter consists of a thick layer of gelatin and other components, laid upon a vinyl or cloth backing and covered with a protective clear cover sheet. Although less sticky than adhesive lifters, they can lift impressions from any surface, porous or nonporous. They are often used in lieu of the black electrostatic lifting process to lift dry origin impressions. They are also reasonably successful at lifting some wet origin impressions.
Adhesive lifters White adhesive lifters are used to lift powdered impressions. They can also be used to lift certain residue impressions from smooth surfaces, after which they are chemically treated to enhance the impression.
Microsil Microsil is a silicone product that can be used for lifting impressions. Its black or white forms are normally used for lifting powdered impressions on surfaces that are uneven or otherwise would not permit a complete lift when utilizing adhesive or gelatin materials. Microsil will release from most surfaces, dries completely, and can be lifted in 5 min.
Lifting odors from impressions Dogs trained to recognize and compare human scents are used in the traditional way for searching and tracking a scent line laid down by a human. In some countries, the scent is also collected from footwear impressions at the scene, with a specially prepared sterile cloth. The cloth is laid over the impression for a period of time, after which it is placed in a glass jar. A specially trained dog can then be used to associate this scent with the scents of several suspects in a line-up.
Figure 2 A residue impression is barely visible on a piece of paper knocked to the ground and stepped on during a robbery. Above it is a photograph of an electrostatic lift of the same piece of paper, clearly showing the footwear impression.
Casting techniques Casts back up photography and provide a three-dimensional representation of the impression. The cast permits evaluation of certain physical size and shape features that are normally not captured in a photograph. All three-dimensional impressions should be cast using dental stone.
Dental stone Dental stone is a gypsum-based product that has been modified for use in the dental industry. Dental stones, in contrast to the softer plasters, have a high compressive strength and require a smaller water-to-powder ratio. Casting with dental stone provides a quick and easy way of recovering the three-dimensional features of an impression that has depth.
Casting with dental stone A popular method of using dental stone involves placing premeasured amounts, sufficient to fill a reasonably sized footwear impression, into reclosable plastic bags. When there is a need to cast an impression, a predetermined amount of water is added to the bag, which is then reclosed and used to mix the materials thoroughly. When completely mixed, the bag is opened, held at ground level, and poured so that the casting materials flows into the impression. The impression should be completely filled with the casting material until it overflows. The cast may be removed when hardened, usually in 20-30 min, or longer in colder conditions.
Release agents or fixatives Release agents are those materials, such as talcum powder or silicone spray, that are lightly applied to three-dimensional impressions to facilitate the release of the casting material from the substrate, resulting in a cleaner cast. They may be preferred by some but are not necessary with dental stone. Fixatives are those materials, such as dust hardeners or hair sprays, that are lightly sprayed indirectly on to fragile impressions to stabilize them, thus avoiding any degradation of the impression during casting. If not properly used, they may contribute more to harming the fragile impression then to preserving it. Most impressions do not require fixation.
Casting snow impressions Different snow types and variations in temperatures affect the amount of detail retained in impressions. Prior to casting, snow impressions should always be highlighted first with a light spray of Snow Print wax or colored spray paint. The spray should be directed at a low angle from a safe distance to avoid damaging the impression with the force of the spray. Once highlighted, the impression must be shaded from sunlight or bright indirect light as the colored material will accelerate melting. Examination quality photographs, with a scale placed next to the impression, but on the same plane as the bottom of the impression, should be taken. Snow impressions can be cast using Snow Print wax combined with cooled dental stone, cooled dental stone alone, or melted sulfur.
Enhancement of Impressions
Many impressions examined in the laboratory, or located at crime scenes, are indistinct, have poor contrast with the substrate or are altogether latent. A variety of enhancement techniques are routinely used to provide additional detail for comparison and, in some circumstances, as tools for developing and locating the impressions at the scene.
Specialized lighting and photography Specialized lighting and photographic techniques are nondestructive and are therefore used first in attempts to enhance impression evidence. Alternative light sources, high intensity lighting and oblique lighting are routinely used, at both the crime scene and in the laboratory, to provide increased detail and visibility of the impressions. The use of high contrast black and white film photography and the use of appropriate color filters with black and white film, for the purpose of selectively darkening the impression, are also routine. The additional use of computers, to increase the visualization of scanned original or photographed impressions, is also successful in increasing the contrast in some cases.
Physical methods These include (1) methods of transferring an impression to a lifting film that provides greater contrast; and (2) powdering with both conventional and fluorescent powders, on nonporous surfaces, to enhance or detect impressions.
Chemical methods There are many commonly used methods of chemically treating a footwear mark for detection or enhancement purposes. Those reagents that are used to enhance blood impressions include, but are not limited to, leuco crystal violet (Fig. 3), amido 10 black, diaminobenzidine, luminol, fuchsin acid and patent blue. Those reagents which are used to enhance residue impressions include, but are not limited to, physical developer, safranin 0, bromphe-nol blue, potassium thiocyanate, diazafluoren (Fig. 4) and 8-hydroxyquinoline. Cyanoacrylate, followed by powdering or dyes, is also used to enhance wet origin impressions.
Known Shoes of Suspects
Impressions recovered from the scene of the crime can include both those of the perpetrator(s) and those of the victim, police officers, medical personnel and other innocent persons. Once footwear impressions are recovered from a crime scene, comparisons to either eliminate or identify those impressions with shoes will follow. The actual shoes are needed to make known impressions for comparison, and to allow for a detailed and thorough examination of both class and individual characteristics.
Figure 3 Leuco crystal violet is a clear, colorless solution, that can be easily applied at the crime scene or in the laboratory, for the enhancement of bloody footwear impressions. The first picture depicts multiple footwear impressions in blood on a light-colored tile floor. The second picture depicts the same impressions, but after enhancement with leuco crystal violet.
Footwear from suspected individuals
Footwear seized from suspected individuals, obtained within minutes from the time of the crime, might appropriately only include the shoes that the individual is wearing. However, persons normally own more than one pair of shoes, and may even possess several pairs of shoes that are very similar in design. Additionally, impressions of additional designs, not known to the investigator at the time of the seizure of the shoes, may be detected or developed on evidence in the laboratory at a later date. To avoid any chance that the wrong footwear is seized, footwear obtained from suspects, hours, days or weeks later, should include not only the footwear the suspect is wearing, but all footwear in that individual’s home or vehicle.
Eliminating footwear of nonsuspects
Elimination examinations are important, in some instances, to insure that unaccounted footwear impressions are not those of the perpetrator or accomplice. Although it may not be possible to account for all impressions recovered, consideration as to whether a case warrants obtaining either a photographic record or elimination prints of shoes of all persons at the scene should be made early on. Photographs or test prints of footwear from known innocent persons at the scene are satisfactory for elimination purposes, based on differences in design and other class characteristics with the questioned impressions. Only if the elimination footwear is coincidently similar to the design of a questioned impression is it necessary to have the actual shoes.
Known impressions
A variety of methods exist for creating both two- and three-dimensional known impressions of suspected footwear for use in the comparison with the scene impressions. They include methods that utilize inks, fingerprint powders, casts and other materials. Known impressions should reflect a high degree of detail and do not need to attempt to duplicate the exact conditions of the crime scene impression. The known impressions are often made in, or converted to, a transparent form, so that they can be used as a superimposition tool by the examiner to assist in the evaluation of class and individual characteristics.
Abandoned or discarded footwear
Most footwear is removed from the feet of a suspect when he or she is apprehended or interviewed. Other shoes, which a suspect admits to owning, are found in the suspect’s house or car. However, occasionally footwear used during a crime is abandoned or intentionally discarded by the perpetrator, to be located later during the investigation. If that footwear can be linked to the crime scene through its impressions, or the presence of the victim’s blood, then it becomes important to associate or disassociate those shoes with the feet of potential suspects. Comparisons can be made between that footwear and a suspected owner’s feet.
Figure 4 (A) A muddy print on paper before treatment. (B) The same print enhanced with DFO and photographed with an alternative light source. The print was lifted, and activated, with a black gelatin lifter. Courtesy of M J M Velders, Eindhoven, The Netherlands.
Method of Comparison
Basis for comparison
The physical contact of an item of footwear with a substrate results in the direct physical transfer of class and individual characteristics of that footwear to the substrate. Those characteristics, when retained by the substrate, accurately represent the class and individual characteristics of the footwear that made them. Those characteristics can be compared with footwear alleged to have made them.
Examination
Examination involves the comparison of the questioned crime scene impressions with the known shoes of suspects by a competent and qualified forensic footwear examiner. The forms of the questioned impressions examined can include original impression evidence, casts or lifts of the scene impressions, and enlarged natural-size photographic prints of the examination quality photographs of the impressions. The comparison utilizes side-by-side and superimposition methods, assisted by low magnification, specialized lighting and known impression exemplars of the shoes in question. Similarities or dissimilarities between the class, wear and individual characteristics form the basis for the examiner’s resulting opinion. Results are dependent on the extent of individuality of the shoe as well as the quality of reproduction of detail in the crime scene mark or impression.
Comparison of both partial and full impressions is made with the respective areas of the suspect shoe. The areas examined include:
Design The design or pattern on the sole bottom is the first area of comparison. There are many thousands of shoe designs and those designs are constantly being added to, while others are discontinued. The specific details of each design may also vary from mold to mold, due to the specific manufacturing process, thus adding to the already large number of possible choices. In addition, many designs also incorporate texture that is applied through a hand stippling or acid etch process. The texture permits even further distinction between molds. If the examination reveals the specific design is different from the crime scene impression, the suspect shoe could not have possibly made the impression and an elimination can be made. If the specific design corresponds with the respective area of the suspect shoe, the shoe cannot be eliminated and the examination proceeds. A conclusion that the design corresponds is very significant, because of the large number of choices of shoe designs.
Physical size and shape Each shoe design is made in many sizes. The soles of different sizes will have different dimensions and/or proportions throughout the size range. Further, different styles of shoes are made with different-shaped soles, allowing for further distinction, even between shoe soles of the same general length. The subsequent physical differences in the size and shape features of the sole is of further comparative value in associating or disassociating scene impressions with suspect footwear. It is noted that this portion of the footwear comparison pertains to the actual physical size and shape, and whether these correspond with the scene impression. It is not about the manufacturer’s shoe size designation. In cases where a plain sole bottom does correspond in its physical size and shape with the scene impression, this constitutes a limited but still meaningful conclusion. In those cases where a shoe sole has a design, and both its design and physical size and shape correspond with the scene impression, the association is highly significant. This is because of the extremely large number of possible design and size and shape combinations, and the obvious fact that any specific shoe design/size combination would represent only an extremely small fraction of 1% of the overall shoe population. It should be noted here that minor discrepancies in the physical dimensions of recovered impressions, when compared with known impressions of suspect shoes, are normal. These occur for a variety of reasons resulting from the normal impression-making process and the recovery process. Minor discrepancies are insignificant. Larger discrepancies are cause for concern.
Wear As shoes are worn, their soles are altered by the abrasive forces created each time those soles make contact with the substrate. These abrasive forces slowly remove selected areas of the rubber. The areas that contact the surface with every step, and where pressure and movement of the foot are transmitted, receive the most wear, while areas that do not contact the surface receive little or no wear. As wear progresses, and the abrasive action removes the rubber, it alters portions of the design. The difference between the worn and unworn areas becomes obvious and is sometimes referred to as the wear pattern. As long as the shoe continues to be worn, the degree of wear and the wear pattern will gradually change. Wear is a function of time and, therefore, as a routine matter, the date of crime scene impression and the date suspect shoes are seized should be known by the examiner. The degree of correspondence in wear between a crime scene impression and the perpetrator’s shoe depends on the amount of additional wear the shoe has received.
Wear is examined to assess: (1) the position of the wear, i.e. does the position of wear on the shoe match that of the crime scene impression; and (2) whether the degree of wear corresponds, which is more relevant in cases where the shoes are obtained shortly after the crime, versus shoes that may be obtained months later. The correspondence of wear characteristics offers an additional means of eliminating other possible shoes of the same size and design that are owned by members of the general population but which exhibit different wear. Therefore, correspondence of wear further reduces the possible number of shoes that could have made the impression. In some cases, when shoes and impressions exhibit very little or no wear, when the detail or other limitations in the recovered impression preclude an assessment of wear, or when a significant period of time has elapsed between the date of the crime scene impression and acquisition of the suspect’s shoes, the relevance of wear in an examination may be minimal or insignificant. The absence of the ability to utilize wear during the examination in no way minimizes the conclusions that may result from comparison of the other aspects, i.e. size, design and identifying characteristics. The general wear features of footwear, even if matched precisely with the scene impression, are not a basis for identification of the shoe with the impression. When present, however, wear provides a significant contribution to the overall basis and results of the examination. In some cases, wear can be used to eliminate a shoe as having made an impression, should its condition of wear be clearly different, and should it be impossible for it to have been in the same condition of wear as that producing the crime scene impression. The wearing down of the sole can also result in the erosion or creation of individual identifying characteristics. For instance, as a shoe continues to wear, any individual identifying cuts, scratches, etc., can be altered or altogether lost. At the same time, if a shoe wears excessively, the wear may create individual identifying characteristics, such as the rough edges of a hole worn through the sole, or may produce microscopic abrasion wear patterns which are highly unique.
Individual identifying characteristics Individual identifying characteristics include virtually anything that has been added to or removed from the shoe sole in a random fashion, thereby contributing to or giving a shoe sole uniqueness, and allowing it to be distinguished from other shoes soles. They most commonly include cuts, scratches, gouges, etc. sustained by the surface of the sole as the shoe is worn (Fig. 5). They also include materials that randomly become attached to the shoe sole, such as tar, gum, tacks or nails, stones imbedded in the design, etc. What makes individual identifying characteristics valuable is the fact that they occur randomly and are therefore unlikely to be in the same position on another shoe of the same design. It would be a remarkable coincidence to find even one similar feature in the same position on another shoe of the same design. Even more significant are the features of the characteristics, e.g. a V-shaped scratch, an irregularly shaped hole, even further individualized by its orientation (direction) and size. These features add incredible individuality to an item of footwear and serve to distinguish that item of footwear from all others. Individual identifying characteristics can also be transferred to shoe soles during the manufacturing process, but these are less frequent, and their value is often more difficult to assess, as in some instances this might require assistance from a particular manufacturer.
When a crime scene impression and a suspect shoe share sufficient random individual identifying characteristics, a positive identification is possible. A positive identification means that only that shoe, and no other shoe in the world, made the crime scene impression. There is no minimum number of random individual characteristics needed to establish an identification: one characteristic alone could be used to identify a shoe, providing the characteristic was sufficiently clear and detailed, and reflected sufficient significant features in common with the scene impression. An example would be a feature generally shaped like an ‘X’ with one stem of the ‘X’ longer than the other, and terminating in a slight curve. If a characteristic such as this were of the same size, shape, position and orientation on both a crime scene impression and a shoe sole of the same design, it could be sufficient alone to permit a positive identification. However, should a characteristic be less clear or contain fewer features, such as a very tiny hole, more than one characteristic would be needed for identification.
Figure 5 An enlarged area of (A) a shoe and (B) the respective known inked impression of that shoe, depict the many individual cuts and scratches. Individual random cuts, or groups of cuts, for example adjacent to the arrow on the top right, reflect sufficient size, shape, orientation and position features to make this shoe sole unique. Others, like the cut by the arrow on the left, although highly significant and important, reflect fewer features and would not be used alone to identify a shoe.
The Footwear Impression Expert
Footwear impression examinations should be made by persons who have received additional specialized training and are experienced in that discipline. Being qualified as an expert in other forensic disciplines is not, in itself, qualification to conduct examinations between questioned footwear impressions and known shoes. Examiners in this field should be afforded specific training and experience under the direct supervision of an experienced and qualified footwear impression examiner. This normally includes working cases and practical exercises of sufficient quantity over an extended period of time to enable the trainee examiner to acquire the necessary skills and knowledge related to this forensic discipline. Reading the available literature, attending specific courses, seminars and workshops, touring footwear manufacturing facilities and conducting research are ways of obtaining further knowledge.
