The Directional Analysis of bloodstain patterns is a mathematical procedure, developed by the author, for finding the directions in space (virtual strings) that point from the bloodstains to a spot directly above the location of the blood source. When viewed from above, the virtual strings are seen to converge onto the source position. When viewed from the side, the virtual strings provide an upper limit for the probable height of the blood source. The virtual string direction is computed from the blood droplet's impact angle and the bloodstain's glancing angle.

  The theory relies only on the well-known physical laws of motion plus the resolution of a velocity into its 3 components and simple trigonometry. The theory does not depend on unknown quantities such as droplet sizes and droplet speeds. This procedure has a solid basis in physics and mathematics. Digital cameras are used for gathering the evidence and the analysis is carried out with a computer.

  Directional Analysis, therefore satisfies one of the main criteria for bloodstain evidence specified by the courts; i.e. “it must be shown that the evidence has as its basis sound scientific methodology”.

     The reconstruction of the flight paths of blood droplets with strings is a rather crude procedure that is taught, in one form or another, in most basic courses on bloodstain pattern analysis. The actual flight paths of the droplets of blood are represented by strings which are attached to a number of suitable stains and stretched into space. The result is a collection of strings that may or may not converge on the location of the source of the projected blood.

     This simple idea can yield good results when the bloodstains are due to fast moving droplets that have flat string-like trajectories. A variation of the string method that eliminates the time consuming work of actually mounting strings at the crime scene is the so called Tangent Method. It is taught at most 40 hour basic bloodspatter courses. The Tangent Method works well for horizontal surfaces such as floors and ceilings. The flight paths of the blood droplets, when projected onto these surfaces are straight lines. It is not valid for vertical surface such as walls. This is due to the fact that the flight paths of the droplets when projected onto vertical surfaces are not straight lines but curved lines. 

    Curved lines because the forces of gravity and air resistance, acting on the droplets while in flight, produce this curvature, often called parabolic arcing. Fortunately, the curvature is limited to the vertical plane that contains the flight path. i.e. the blood droplets move up and down, not to the left or right. Therefore, when view from above, all flight paths are straight lines (string-like). The strings for bloodstains found on any vertical surface should be analyzed from the top view or bird's eye view. The top view analysis of a string pattern is almost impossible to perform without the use of a computer.

     The Tangent Method , applied to vertical surfaces, will have errors in all three coordinates of the source position due to parabolic arcing while the Bird's Eye View method, confines the parabolic arcing error to one coordinate i.e. the height of the source position, CPz.