Ridge Formation Type and Ridge Path Agreement  

In order to refine quantitative-qualitative values for the agreement of ridge formations found in two impressions, a quality of agreement assessment scale is used based on reduced levels of agreement between level II ridge formation shapes and positions.

A ridge formation type, i.e. ending ridge, bifurcation, scar, and so on, is a class characteristic that does not define the many ways it may slant, curve or angle.  For example, one ending ridge unit may end with a sharp point while another may end with a flat edge.  As a result, pairs of corresponding ridge formations in two impressions that fail to display relative agreement in ridge shape and/or position should be subject to reduction.  

A ridge feature quality of agreement grade scale was designed to assign reduction factors to mated pairs of "corresponding" ridge features in two impressions displaying reduced levels of quality of agreement.

To View Quality of Agreement Grading Scale click HERE

Agreement of Ridge Type (Connectivity)

The T-Model defines agreement of "ridge feature type" based on whether or not the ridge units display similar connectivity or non-connectivity characteristics.  For example, an ending ridge unit does not (by definition) display connectivity with an adjacent ridge, however a bifurcating ridge unit does display connectivity.

Agreement of Ridge Path (Slant/Angle)

For purposes of simplicity, the T-Model defines the agreement between ridge formation shapes in two impressions in terms of relative similar slant and/or angle, which may be supported by physical measurement (if needed). 

It is significant to note here that relative level II ridge path agreement should not be confused with level III ridge unit width and ridge unit edge contour in agreement.   The agreement of level III ridge detail should expand the quantitative weight for a continuous ridge unit in agreement (pending validation studies this Level III value is tentatively set at 1.15).

Agreement of Spatial Relationship to Nearest Neighbor

Distal Agreement

Agreement of spatial relationship (e.g., in terms of falling within distal, e.g., stretch and/or compression, friction ridge skin threshold tolerances to include) between nearest Level II ridge feature neighbors is determined by a distal difference (from exemplar to latent) as not to exceed 20%. 

Based on experiment, it has been determined that 20% is the maximum friction ridge skin stretch and compression tolerance threshold for normal friction ridge skin (see Friction Skin Elasticity).  As a result, any distal measure between a Level II ridge feature and it's nearest  neighbor that falls outside of this stretch or compression threshold, e.g., is greater than 20%, is deemed to be not in agreement and therefore subject to reduction, i.e., has no discriminating value.

It is significant to note here that in general spatial agreement within tolerance of a 20% distal measurement threshold between Level II ridge features and it's nearest neighbor can be accurately determined from visual examination by a trained latent print examiner.  However, careful measurement is necessary  when the distal agreement between a Level II ridge feature and it's nearest neighbor is “borderline” or fails to speak for itself.

Rotational Agreement

Based on results from experiments, Ridge Vector Azimuth Angle displacement of 10 degrees served as the threshold for relative rotational agreement between ridge vectors.  As a result, agreement of rotational positions between pairs of corresponding ridge vectors and the nearest neighbor for each was defined as ±10 degrees.

For purposes simplicity the angle present between the ridge feature in question and it's 2 nearest neighbors may also be used to determined whether or not the rotational displacement is within friction skin elasticity thresholds, i.e. does not exceed 10 degrees.

Otherwise, mated pairs of latent vs. exemplar ridge vectors with azimuth angle displacement greater than 10 degrees to the nearest neighbor of each are considered out of tolerance and therefore subject to a minimum reduction of 0.50.  The reduction factor applied was set as the same as that for ridge features displaying distal displacement (see Friction Skin Elasticity). 

It is significant to note here that in general the angle of rotation can be often accurately determined from visual examination by a trained latent print examiner.  However, careful measurement is necessary when the angle of rotation between a Level II ridge feature and it's nearest 2 neighbors is “borderline” or fails to speak for itself.

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Example

A diminishing area ending ridge in a latent fingerprint slants slightly to the right.  The ending ridge counterpart in the exemplar is straight or slants slightly to the left.  The ridge feature type in the latent and exemplar are the same.  The positional agreement to the nearest 3 neighbors for each all agree.  As a result thequaity of agreement grade is "B2" or "Above Average" and the reduction factor is subsequently defined as .75.

The initial value for the diminishing area ending ridge is defined as 10.  Based on a qualitative reduction factor of .75, the total value for the pair of diminishing area ending ridges is refined to 7.5.