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Bloodstain Pattern Analysis · Tools
Estimate the height of a blood source above the convergence plane from a stain's distance and impact angle.
h = d × tan(α)
Single stain, measured from the point of convergence
Measured along the surface, from the stain to the point of convergence.
From the bloodstain impact angle calculator.
Height above the convergence plane
84.0cm
h = d x tan(a) = 120 x tan(35°) = 84.0 cm
The tangent method treats the droplet path as a straight line. Because gravity curves the real trajectory, the result overestimates the true height, and increasingly so with distance. Use several stains from different directions and treat the spread as the answer.
The tangent method gives a zone, not a point. It ignores the parabolic effect of gravity, so it overestimates height, and it assumes every stain used came from the same event. Stringing or software reconstruction with many stains is the accepted approach; a single stain proves nothing on its own.
Once the impact angle of a stain is known and the point of convergence has been established on the surface, simple trigonometry gives the height of the blood source: the horizontal distance multiplied by the tangent of the impact angle.
Because a droplet actually travels a parabola rather than a straight line, the tangent method systematically overestimates height, and the error grows with distance. That is why reconstruction uses many stains from different directions and reports a zone rather than a point.