Discussion of the Matching Method
Viewing the images on a large monitor makes this process easier although
even a typical laptop can be used. The degree of zooming that works best
may be an individual preference. Zooming in closer may make it easier
to find a match by blocking out extraneous details in the image (details
that will not seem to match between the two prints and can lead to confusion)
but it carries a penalty as well – you have a greater tendency to lose
your overall sense of reference, resulting in a lot of wasted effort trying
to match areas of the print that come from radically different parts of
the foot pad. Using ImageJ with 100% zoom on a 17 inch monitor with 1280
x 1024 resolution works quite well. This setup will display almost the entire
main portion of the metacarpal pad simultaneously. Zooming to 200 % sometimes
helps when examing particularly difficult images.
As mentioned above, the easiest images to use at first are those
made from the museum specimens. Three more pairs (NOTE: these are larger
images than those found on the introductory page - those with dial-up connections
be warned.) are provided here [1A, 1B] [2A, 2B] [3A, 3B]. A large percentage of the marks will match,
covering practically the entire image. Notice the similarities and differences
between the prints from different feet. Keep this in mind later when
you encounter right foot prints. Being able to tell rights from lefts
is critical to application of this method. Right footprints don’t
always occur on the right-hand side of the contact paper!
To satisfy your curiosity, try to match two prints from different
feet. You can probably find 3 or 4 dots that seem to align but will not
be able to match 10 or more neighboring dots. Note also that you can align
rows of dots fairly well in some cases. Although differences can sometimes
be discerned at this level of detail, feet do not differ sufficiently in
the pattern of rows to adequately distinguish between individuals.
As on the introductory page, here are some more inked impressions
of live fishers. [4A, 4B] [5A, 5B]
And here are some more prints from track plates, with alignment marks.
First, a straight forward pair [6A, 6B].
The next prints actually include three images. While it can be
readily determined that 7A and 7B are impressions of the same foot, these images
are so similar that it is not as obvious as with some of the other examples.
7B Rotated is a copy of the second
image but rotated digitally by 4 degrees. It is especially easy to match
prints when a small amount of rotational misalignment is present and you
can see this effect by matching both the rotated and non-rotated images.
These two images [8A, 8B] show that, while often it is very difficult,
it is sometimes possible to get an unambiguous indication of a match by
comparing the “wings” of the metacarpal pad.
Tips for the Advanced Print Matcher
Before proceeding with the training exercises, a few advanced tips for
matching fisher prints are in order.
- Put some effort into developing a comfortable, ergonomic setup.
Matching prints with this method requires lots of tabbing and mouseing,
so anything you can do to ease the physical strain associated with repetitive,
precise motions will help.
- It is much easier to match prints if the initial alignment is close
before trying to shift images. That way you know you should only need to shift
by relatively small amounts. Two clues will help: the overall outline (including
the two auxiliary "wings") and the curvature of the rows. Start by getting
the overall outline as close as possibe, using a zoom level that lets you
see the entire metacarpal pad - perhaps 33% or 50%. Then zoom in a
bit - to perhaps 75% - and check out the row curvature. Make whatever adjustments
seem appropriate and zoom to 100% or 200% to compare dots.
- When zooming, keep in mind that clicking on the image with the zoom
tool (magnifying glass) active causes the place beneath the cursor to move
to the center of the window. Because of this, after getting your initial alignment,
click once to zoom, tab to the second image without moving the mouse and
then click to zoom the second image. This way you preserve the alignment
at the higher (or lower, depending on which direction you are zooming, in
or out) zoom level.
- Recognizing distortion when it occurs is very helpful.
Actually, it occurs in all prints from live animals to some degree, but
certain relatively rare cases are extreme and difficult to deal with.
The degree of distortion present can be seen by noting extent that the
row patterns differ from those of the inked, severed feet (which serve
as our zero-distortion reference). Prints with obvious gross differences
can only have been made by the same foot if at least one of the images
is distorted. Clues that can indicate distortion are atypical row
patterns and images composed of small, light dots.
- It’s easy to fool yourself into thinking two prints match when
comparing areas of the print that feature rows where the dots blend together.
Recognize that all left metacarpal pads are basically similar and that
row shape alone is not a definitive characteristic. Concentrate on
trying to match areas with distinct dots, not where they blend together
into an unbroken row.
- Depending on the degree of precision employed when the images
were scanned or photographed, digitally rotating one of the images to achieve
the required alignment may be necessary. The need for this is usually
readily determined by examining the overall outline of the metacarpal pad
image. Simply rotate one image until the pad shape seems to match between
the two images. While some rotational misalignment helps in matching
prints, more than 10-15 degrees will make the process more difficult, if
not impossible.
- In contrast to these matching exercises, when testing prints gathered
from the field (where you don't know ahead of time whether they match or
not), perhaps the most difficult skill to learn is when to give up trying
to match two prints. A positive indication that two tracks match is reasonably
clear. If two tracks don’t seem to match, though, one must consider whether
it is because they are from different feet or because the matching was not
done properly.
If the overall size of the metacarpal pad and general pattern of rows
seem to correspond between two tracks and yet no obvious alignment
can be made when the images are overlaid, a “brute force” approach will
often work. It is somewhat tedious, but sequentially shifting one image
relative to the other by small amounts and checking for correspondence (that
is, the sense of motion when switching between images) may reveal a match
that might be missed otherwise. The distance the image is moved for each
shift should correspond approximately to the spacing between typical pair
of neighboring dots.
The shifting process must take place in both horizontal and vertical directions
until all possible combinations of alignment within some limits are tried.
The process is perhaps best learned through attempting it with images
known to have been made by the same foot. Begin with the two images aligned
by gross characteristics as well as is possible. I suggest picking a prominent
dot in the reference image and mark it, say with an “x”. Switch back
and forth between the reference and candidate images, periodically moving
dots in the candidate image so they align with the marked dot in the reference,
until all possible combinations within reason have been tried. With practice
and appropriate software this process takes less than 5 minutes.
Now, move on to real-world images in Advanced Matching Practice II.
Back to Basics 
Advanced Matching Practice II 
Fisher Print Home 
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