Ballistic Imaging (2008) / Chapter Skim
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PART III: Implications for a National Reference Ballistic Image Database, 8 Experimental Evidence on Sources of Variability and Imaging Standards
PART III: Implications for a National Reference Ballistic Image Database, 8 Experimental Evidence on Sources of Variability and Imaging Standards
Pages 197-222
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PART III Implications for a National Reference Ballistic Image Database
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(2004) exhibit set, and thus getting a direct impression of automated systems' ability to detect sameness amidst a vast array of exhibits with highly similar class characteristics.
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We describe these sources below, along with the steps taken to acquire two-dimensional and three-dimensional images and measurements; for ease of reference, the basic design of the exhibit sets is summarized in Box 8-1.
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Chosen makes were Ruger P95D, SIG Sauer P226, and Smith & Wesson 9VE. The SIG Sauer pistols bore consecutive serial numbers; the Ruger pistols bore closely proximate serial numbers; the Smith & Wesson pistols included 3 with close serial numbers.
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From the pistols known to be of the SIG Sauer P226 model (some of the 600 pistols were very similar to the P226 but not that exact model) , the committee randomly selected 10 pistols; all 7 casings for each of those guns were extracted from the exhibit set for further analysis.
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The full NBIDE exhibit set has 144 elements: three repetitions of each of four ammunition brands, fired through four guns from each of three makes. However, the NIST analysis (Vorburger et al., 2007)
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Figure 8-1 contrasts the greyscale photographic images collected by the current IBIS platform with representations of three-dimensional surface measurement data, for both the breech face and firing pin markings of a particular cartridge casing. The raw data for the three-dimensional surface measurements are just that -- numeric distance measurements over a fine array of spatial coordinates; for graphical purposes, these can be rendered in many ways, using colors to suggest "height" or "depth" or simulating lighting from any desired angle.
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Images are from the DKT exhibit set, the Federal casing from pistol number 535. The region-ofinterest delimiter circles are superimposed on the IBIS images.
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• Filtering: As a rough means to try to emphasize individual characteristics rather than class characteristics in the three-dimensional images, NIST applied standard filters in noncontact optical profilometry, based on spatial wavelengths in the topographic image data. Spatial wavelength calculations are based on distance between consecutive peaks after subtracting out a mean surface depth; in this particular case, both very short and long wavelengths are subtracted out, removing effects that can be thought of as corresponding to system measurement noise and broad structural features (class characteristics)
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8–B.1 Two-Dimensional and Three-Dimensional Performance, DKT Data As shown in Box 8-1, each exhibit in the DKT exhibit set has six possible same-gun matches. Table 8-1 summarizes the same-gun entries found in the top 10 rankings in a standard IBIS search against the 69 other DKT exhibits, and Table 8-2 provides the same results based on NIST's analysis of three-dimensional topographic data.
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Though the DKT data provide a glimpse at only one firearm type -- SIG Sauers -- Tables 8-1 and 8-2 do underscore variability -- in propensity to leave clear, identifiable, and computer-matchable marks -- from gun to gun and across ammunition types. The firing pins from pistol 7, for instance,
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8–B.2 Two-Dimensional and Three-Dimensional Performance, NBIDE Data Tables 8-3 and 8-4 summarize the results of two-dimensional and three-dimensional analyses of the NBIDE exhibit set. Again referring to the design in Box 8-1, and given that NIST withheld casings from one ammunition type (Speer)
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210 TABLE 8-3 Number of Same-Gun Matches Found in Top 10 Ranks, Two-Dimensional/IBIS Analysis of NBIDE Exhibit Set Gun Type Ruger SIG Sauer Smith & Wesson Mark and Ammunition R1 R2 R3 R4 Avg.
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TABLE 8-4 Number of Same-Gun Matches Found in Top 10 Ranks, Three-Dimensiona/NIST Analysis of NBIDE Exhibit Set Gun Type Ruger SIG Sauer Smith & Wesson Mark and Ammunition R1 R2 R3 R4 Avg.
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For the pure SIG Sauer DKT exhibit set, both the two-dimensional and three-dimensional systems appeared to do a better job at finding same-gun matches using the firing pin mark than the breech face; the opposite appears to be true for the NBIDE dataset. The success of the three-dimensional system in finding the same-gun matches in the top 10 ranks on breech face is excellent; indeed, it is near perfect.
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The cutoff at rank 29 corresponds to a strict 20 percent threshold on a sample size of 143; the actual effective sample size for any of these comparisons in a standard IBIS run would be somewhat larger because both breech face and firing pin images are considered, but we use the 29 cutoff for simplicity. For purposes of generating ranks, tie scores in the score lists are assigned final ranks by sorting by the NIST-assigned ID number for the test (in-database)
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The empirical distribution of the cross-correlation scores can be derived separately for the matching (same-firearm) and nonmatching pairwise comparisons in a dataset of topographic "images"; Figure 8-2 illustrates such a distribution for the scores generated from the firing pin scores using the NBIDE exhibit set.
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, a casing-specific p can be derived from the 107 pairwise comparisons using that casing as the reference, 8 of which are same-gun matches and 99 of which are nonmatches. Table 8-6 summarizes estimates of casing-specific values of p from the DKT and NBIDE exhibit sets.
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Much work also remains to done on streamlining the acquisition and data processing steps. As a first foray -- one geared to ensuring proper calibration of equipment and to testing different algorithms and computer programs for generating comparison scores -- the data acquisition process was time consuming and comparisons took many hours to run to completion.
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This set of casings was supplemented by a small extract of eight casings from the DKT exhibit set. Two DKT pistols (numbers 535 and 68) or the archive cases, exhibits were drawn from 2004 forward, since boxes containing F those exhibits were accessible near the IBIS entry room.
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8–C.1 Basic IBIS Results, NAS Exhibits Table 8-7 reports the IBIS breech face and firing pin scores and ranks for the eight NAS exhibits, extracted from the DKT exhibit set. Practically, these comparison runs looked at the performance of the current IBIS in finding elements of an eight-exhibit set, nested within a database of effective sample size 15,082 of casing images from new firearms of the same caliber and basic demographic characteristics.
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that was less than 11 -- none of these lower than 27, and most of them greater than 100 -- but still merited inclusion in the "full" correlation report. • The balance, 9, failed the coarse comparison pass and 20 percent threshold.
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The (# of results) entries represent the number of entries included in the "full" IBIS comparison report and are the number of exhibits that survive the coarse comparison and 20 percent threshold steps (see Chapter 4)
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For instance, for the NAS02 casing, the top 16 ranks by firing pin include four entries from one of these runs, entered in 2003 (two other nonrelated SIG exhibits from 2001 are also highly ranked on firing pin) ; however, none of these pistols appears in the highest ranked possible matches by breech face. Independently, a committee subgroup visited the New York City Police Department forensic laboratory and ran tests on NAS01–NAS04; the Albany test had the effect of seeing how these same-gun casings were handled in an RBID of images from new firearms, while the New York City test contrasted that with performance in a large database of crime scene evidence.
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of 571. The effective sample size was 8,106, so the link from NYSP01 to itself was not in great danger of being excluded by the coarse comparison and 20 percent threshold steps.
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