The quality of positional agreement between pairs of ridge paths in two impressions should  consider the fact that friction ridge skin is elastic and can stretch and compress.  As a result  friction ridge skin elasticity stretch and compression thresholds should be defined.  

Friction ridge skin elasticity compression and stretch thresholds were defined based on the following study: 

A total of 39 different fingers from 39 different sources were used from male adults age 21-60.  A random 10mm section of normal flat finger friction ridge skin was measured on each finger.  The skin was then manually compressed and stretched as much as possible without folding or breaking the skin, and then re-measured.  The shortest (compression) and longest (stretch) measurements were recorded to the nearest millimeter.  The average compression and stretch threshold measurements were 8.923mm and 10.923mm, or a percentage difference of approximately .10 for each.  The average friction ridge skin elasticity threshold was conservatively defined as ±10%.  This tolerance range was used to define precise positional agreement between corresponding ridge formations and their nearest neighbors.

10mm sections of flat finger friction ridge skins from 39 different sources were marked and measured.  They were then manually compressed and stretched as much as possible.  The marked 10mm sections were then re-measured.  The difference in measurements were averaged and used to define friction ridge skin elasticity stretch and compression thresholds in terms of percent. As a result the average friction ridge skin elasticity threshold was conservatively defined as ±10%.  

Maximum elasticity compression and stretch measurements were recorded as 8 and 12mm respectively.  As a result, the maximum friction ridge skin elasticity threshold was conservatively defined as ±20%.  This tolerance range was used to define relative positional agreement between corresponding ridge formations and their nearest neighbors.

Friction skin was compressed and measured using a metric ruler.

The average compression distance was calculated to be 9mm.  The single maximum compression distance was measured to be approximately 8mm.

Friction skin was stretched and measured using a metric ruler.

The average stretch distance was calculated to be 11mm.  The single maximum stretch  distance was measured to be approximately 12mm.

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Distal friction skin elasticity threshold , i.e. compression and stretch, were combined with rotational friction skin elasticity threshold, i.e. ridge vector azimuth angle, in order to define pairs of ridge formations in "precise" or "relative" positional agreement.   See "Ridge Vector".