Fresno Criminal Defense Attorney | Blood Spatter and Criminal Defense (5) | Forensic Analysis of Blood
Practicing criminal defense in Fresno has allowed me the opportunity to view blood spatter and how forensic analysis is performed. Forensic analysis of blood spatter can be critical in mounting a successful defense to an allegation of a violent assault or homicide offense.
Many times the position of the alleged victim at the time the blood spatter occurred, the direction of travel, and numerous issues that can be reconstructed through forensic analysis of blood spatter will help a criminal defense attorney reconstruct the events and properly analyze the possibilities that occurred.
It is important that the attorney you hire has an understanding of blood spatter analysis if blood spatter is involved in the allegations you face. As a criminal defense attorney I felt an obligation to gain further insight these exact issues to further my abilities as a defense lawyer.
Fresno is infamous for having a large occurrence of violent crimes, therefore I took the time and effort to gain this education. I am learning about blood spatter as part of my Masters of Science in Forensic Science program which I enrolled in after completing the Masters of Science in Forensic Toxicology. If this topic interests you, please read about it as the modules from my program are routinely posted to help education people on this topic.
Point of Convergence
The origin of a bloodstaining event can be determined in a number of different ways, and may include the overhead examination of the spatter as a means of locating a point or area of convergence where several stain patterns cross or intersect. The spatter can be viewed from both overhead and from the side, and spatter pattern analysis can be carried out using the stringing technique, or more commonly nowadays, specialized forensic software.
When the flight path of a specific stain is well defined, it's relatively easy to extrapolate a line backwards along the path in which the drop fell in an attempt to identify the area in which the drop may have originated. Looking from overhead at a target surface, a falling blood droplet generally follows a straight path from its origin to its destination. The origin of that stain should theoretically originate at a point somewhere along the reverse flight trajectory of that stain, which can be determined by the directionality of the droplet. The only deviation from that path could be caused by intermediate obstacles or maybe the size limitation of the room. If only a single spot is under investigation then the origin of that stain could be anywhere along the reverse path. Bear in mind, the reverse path may be quite extensive depending on where the scene is located. If there are two separate stains, each with slightly different directionality, then extrapolation of the reverse path may lead to a point of intersection between the two flight paths indicating a possible point of origin for both stains. Unfortunately, this very simple approach may miss the fact that the two stains are actually the result of two separate and different events, and the point of convergence for those stains is wrongly identified. In this situation, the point of convergence may be purely coincidental and have no significant investigational value.
Stains that are widespread throughout a scene and show no common point of convergence can generally be considered to have occurred from separate actions. The closer they are together, the more likely the convergence is a coincidence, but if only a few stains are present for evaluation then this kind of error may be overlooked. Similarly, when two patterns overlap, the point of convergence may be incorrectly identified. The more patterns available for examination, then the more confident we can be when defining a point of convergence. The more paths that intersect at a given point, the more likely the intersection is a true point of convergence. Multiple events that result in mixed spatter patterns should still have an easily identifiable convergence point. Paths may cross at several locations but the occurrence of clusters of intersecting paths can usually determine the primary convergence points of the spatter.
A limitation of the overhead technique is the method cannot be used to estimate the location of the origin (point of origin) of the spatter above the point of convergence. Using the overhead method, the point of convergence is only established in one plane and it is assumed that the flight paths of the droplets originate somewhere above that. Again, this assumption becomes problematic when multiple events may have occurred. An investigator may be looking at spatter patterns from more than one event that occurred in the same area in the same room, but actually occurred from a number of different events occurring at different heights.
The overhead technique still has many useful applications and is effective when combined with other techniques to compile graphic illustrations for point of origin estimation for blood droplets within a crime scene. The method is also useful for spatter that has a vertical target surface; however, adjustments have to be made to account for the parabolic flight path of the droplet before it hits the surface.
Side View Angles
To be able to determine the specific location of bloodstain origin above a point of convergence, the best method to use is a side view approach to the target. This method uses the angle of impact of the stain in the analysis.
We have already addressed the fact that there is a mathematical relationship between the length and width of a stain resulting from the angle at which a stain impacts a target surface. When well formed stains are available for investigation (by “well formed,” we mean a stain that, when divided along the major or minor axis, will produce two halves that are approximately equal to each other, or, in other words, a fairly symmetrical stain), we can apply the length-width ratio sine concept to determine the angle of impact and use this straight line geometry technique to define the origin of the blood staining event. The caveat of this process is the impact angle formula only provides an estimate of the angle of impact, since the parabolic flight characteristics and oscillations in the flying droplet prevent this value from being an absolute value.
Accurate stain measurement when determining length to width ratio of a stain is actually very critical since the analyst must determine the correct length and width of a stain to provide an accurate angle of impact. Only the main portion of the body of the stain should be measured, and areas involving satellite spatter and spines should be avoided. By including the excess scalloping of satellite staining and elongated tail portions of stains in the basic stain measurements, the length to width ratio can be changed significantly, and as little as a 0.5mm deviation can greatly affect the estimated angle of impact.
To make the measurement, the length of the stain is measured along its major axis, excluding any measurement of spines and scallops along the stain edge. The width of a stain is measured across the minor axis of the stain, and again, any scalloping or elongated spines must be excluded. Ellipse templates are available and can be superimposed on a stain for purposes of angle of impact determination. These templates will only align correctly with a stain when the angle of impact matches that angle associated with the template stain shape.
Stains can also be measured using drafting dividers or rulers.However, the limitation of the template technique is the templates do not compensate for differences in blood volume that create wider or narrower stains. Remember, as the volume of a blood droplet increases, the width of a spot on a target surface also increases, so the range of templates would need to encompass all the sizes of stains that could possibly be encountered. Although this is costly and cumbersome, the use of these templates in new investigator training reportedly increases the level of confidence of analysts in choosing the appropriate closing point of a stain to enhance accurate measurement.
Once overhead information has been used to determine the point of conversion and the angle of impact is determined, this information can be displayed graphically to estimate the point of origin.
Side and Overhead Views
Say we have three stains with a common convergence point; we can measure the distance from the convergence point to the base of each stain, then combine that information with the angle of impact graphically to visualize the point of origin of the spatter.
The x-axis of the graph represents the target and plots the distance of the stain from the point of convergence. The y-axis represents the point of convergence and plots the height above the target. Each stain is marked on the x-axis at a point corresponding to its distance from the convergence point. Using a protractor, a line is drawn at the corresponding angle of impact back from the x-axis to the y-axis. The point where the line crosses the y-axis is the height of the probable point of origin. Once all the lines are drawn a common point of origin should be established. If the measured spatters occur from multiple events, each occurring at a different height, then that should become apparent.
The graphic process can be circumvented by defining the point of origin using the tangent function. After determining the point of convergence and the angle of impact the point of origin can be determined using the following formula:
tan I = H/D
Iis the known angle of impact
D is the distance form the spot base to the convergence point
H is the unknown distance above the target surface.
H = tan I x D
I = 35°
D = 30 inches
Tan 35 = 0.700
H = 0.7 x 30 = 21 inches
Point of origin evaluations come with several limitations. Both graphic and tangential approaches enable the investigator to establish a probable point of origin, but with the assumption that the droplets follow straight line flight trajectories. No consideration is given to the parabolic path of flight characteristics. As long as the investigator remembers this assumption, then both methods are functional since they provide an approximate point of origin, and an approximate flight path for each droplet. Each flight path comes with very specific limits though.
Consider a droplet that is measured to have struck a target surface at an angle of 65°. The droplet may have originally been projected towards the target at an angle less than, that is, more horizontal than, 65° (say 55°). Gravity would have brought the path of the drop closer to vertical as it fell, increasing its angle to 65° and creating a parabolic flight path. As a result, the droplet's point of origin might actually have been below that indicated by the measured angle of impact. However, the droplet, not being able to gain momentum on its own during flight, could not have been originally travelling at an angle greater than the 65° at which it was measured to have struck the surface. Therefore, the actual point of origin can not be above the measured one. This knowledge enables an investigator to develop possible, and impossible, flight paths for the droplets.
Three Dimensional Estimations
Three-dimensional estimations of point of origin can be conducted using the “stringing” technique or using computer-assisted analysis.
The stringing technique was used for many years as a basic investigational method by bloodstain analysts, but it is now being replaced with more sophisticated computer based techniques and is now used more as a visual aid in explaining the concept of point of origin in court room testimony.
In the stringing process, stains are selected and their angles of impact are determined. Based on directionality and convergence points, strings or threads are taped to the leading edge of the stain (the point on the long axis of the stain opposite the scallops or spine). The string is then extended back along the reverse travel path at the appropriate angle. The point in space where strings intersect marks the estimated point of origin.
There are several limitations associated with the technique of stringing: The technique is very time consuming since placing strings in the scene and finding appropriate attachment points can be very difficult. The process often involves the use of props, which result in clutter and limited movement around the scene.
The strings cannot be placed along direct flight paths (parabolic curving again) at accurate angles. Nowadays laser protractors are used to eliminate some of these issues, facilitating better angle calculations for the stringing technique.
Computer software programs are now available for the analysis of blood spatter patterns and may entirely replace stringing and other techniques. These programs can be used to determine the point of origin and include the parabolic flight characteristics in their analysis, as well as other factors such as air resistance, droplet volume and gravity.