All flight paths, when viewed from above, appear as straight lines. This means that strings are perfect replicates of the flight paths when viewed from above and the top view analysis is not affected by the curvature of the flight paths. The average of the top view string intersections therefore yields values for CPx and CPy that are not dependent on the parabolic arcing due to gravity and air resistance. BackTrack/Win uses the top view analysis for CPx and CPy.
The errors in CPx and CPy , as determined from the top view analysis, depend only on the experimental errors of measurements of the widths, lengths, directionalities and positions of the stains and not from unknown quantities such as speed and size of the droplets.
On the other hand, the value of CPz, the height of the source position, does depend on the curvature of the flight paths. Since the speed and sizes of the blood droplets are generally unknown quantities, one can only compute a reasonable upper limit for CPz. This is done with BackTrack/Win using the side view or end view.