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By Mick
Capman,
PSFM President (Kalamazoo County Sheriff – Retired)
Bottom
line:
> Position Instrument
CLOSE to Evidence
> Red Aiming-dot DOESN’T represent “footprint” of actual
infrared beam
> Assure that ‘shot’ was to CORRECT object,
evidence or surface
> Offer a PERPENDICULAR Surface per “shot”
> Does Evidence Appear Correctly Positioned on data
collector???
> Confirm Red-Aiming-Dot alignment with
Instruments Crosshair
 
In the
February, Volume 19, Issue 2 of GIM International, an
interesting article caught our attention in their
Technology in Focus section.
As
more and more forensic investigative agencies initially
purchase a “reflectorless” electronic Total Station, or
trade-in their older Instrument, the persisting question is
“how does it work” or “how does it capture the
distance without a prism”. It just can’t all be “FM”
(!) can it? ((call for what FM means!!))
The
process is pretty simple really. You first make sure the
Instrument is set for ‘reflectorless’ use (mode) and that
the “target height” has been adjusted, if needed, then point
the instrument towards the relevant evidence point, say a
body location or tire mark, or shell casing.
Then
you aim the crosshair precisely at the object and press the
data collector button to collect the ‘shot’ and ~ as quick
as a passing motorist(!), the Instrument has measured the
slope distance … WOW, and without a prism or someone
standing over it (further contaminating the scene?) or
walking through your scene!! S-w-e-e-t!!
But, how does it work?
As
most of us have experienced, the EDM portion of the
Instrument permits distances to be measured well beyond what
most crash or crime scene investigations really need.
Instruments can measure to single prisms, in some cases,
thousands of feet away in most all directions.
A few
prism-less (reflectorless) total stations also can measure
to certain objects from virtually right up against its scope
to several hundred, and in some instances, a thousand feet
or more away from the RP location.
-
note-
before purchasing ‘reflectorless’ total station, make
sure it will measure to the surfaces you might be called
upon to document say, for example, a blood pattern off a
nearby wall. Some total stations can not be positioned
closer than 6-10 feet from a surface! A real NEGATIVE
fact is you plan to electronically diagram interior
crime scenes!! What to know what instruments won’t,
just email us!
This
makes a perfectly tailored Instrument for a variety of scene
assignments, from documenting facts about a hard to access
location of a reluctant witness, hazardous area as near
moving vehicular traffic, or securing blood droplet evidence
off a nearby wall.
Forensic Mapping Specialists
should be knowledgeable about how such measurements are made
using this method. Certain underlying characteristics and
cautions should be explored.
 
The
article of reference was titled, in part, “…Laser Distance…”
measuring. Most of the law enforcement members reading this
will recall from their Laser Radar days that L.A.S.E.R.
means “Light Amplification by Stimulated
Emission of Radiation”.
It was
in the early 1970’s that many of us recall our departments
first being introduced to “laser” and “radar” and other
speed measuring/monitoring devices. Total stations, many
first used by our military, were generally introduced
commercially in the late ’50-60’s although this is always a
disputed time period, depending on the manufacturer.
Reflectorless total stations, it has been reported, were
first introduced commercially in the United States, by
Leica Corporation, in the latter portion of the 1990’s.
They began to be used by collision & crime scene
reconstructionist in 1999*. Since then, nearly all of the
commercially available total station manufacturers have
their own version of the ‘reflectorless’ instrument on the
market.
Most
of us are accustom to using a prism or retro-prism, mounted
on a telescoping prism pole. We would then position the
prism pole at a given and documented height, directly over
or next to the “object of interest” (shell casing, body,
blood, etc.).
The
distance measured in this fashion from the RP
(reference point) location and its associated horizontal
angle, indicating relationships between other
evidence points, and the vertical angle registering
any changes to the scenes’ geometry (or shape), all
necessary facts to pin-point where certain evidence was
discovered accurately.
The
article, written by Henk Key, contributing editor,
GIM Inter., and Dr. Mathias Lemmens, editor,
GIM Inter., indicates that basically, “…two laser
principles are common in …” the sort of measurements we
and other professions require: phase shift and pulse, both
known to Einstein and made, according to the article,
operational in the ‘60’s.
They
continue “…phase shift method is considered to be the
most accurate one; it allows a very narrow beam, but its
measuring range is limited. The pulse method has a much
wider range but has the disadvantage of poor performance on
short ranges.”
The
more recent EDM systems (Sokkia, Leica) reportedly use a
“hybrid method” which isn’t as limited to its built-in
clock and the width of the spot size.
They
explain that the prism-less EDM’s in use today require high
energy pulses at a level generally in the broad range from
“1 to 20” watts. Where, most total station EDM’s
that depend on the use of prisms (classified as
‘electro-optical’) only emit their signals at a level “of
a few milliwatt.”
Comparatively, it’s like trying to get a good reflection off
something using just a tiny AAA-battery pen light as opposed
to a 12-cell, heavy duty flashlight!
“This
high energy level..”
they continue to explain, “.. enables the detection of
reflections on bare surfaces without the need of using
prisms.”
As
illustrated below, another consideration is the size
of the emitted dot’s “footprint” on the reflective
surface itself.
“Beam divergence as a function of the
distance from the EDM Instrument.
(Courtesy of Leica)”:
Distance
Spot size
20m 7x14mm
100m 12x40mm
200m 25x80mm
300m 36x120mm
400m 48x160mm
500m 60x200mm
-
This is not to be confused with the visible,
red-aiming-dot emitted by the instrument to aid the
investigator in aligning the cross-hair, without
focusing, onto the “object of interest”.
Beam divergence, or the size of the transmitted beam,
depends on the “line of sight” distance between the
Instrument’s setup position and the ‘target’ itself. See
above chart.
In the informative article, the authors point to two
potential sources of errors associated with the
process of documenting any evidence “reflectorless”. An
“error” indicated here might only mean not being able to get
a return signal, such that the evidence point itself might
not be recorded.
First, it could be the generally unfavorable makeup of
the object/surface itself, and second that the
object/surface isn’t ideally situated perpendicular to
the Instruments “line of sight”.
Another “phenomena” that could occur when documenting
interior crime scenes and crush measurements, is the S
M E A R I N G-effect (see below) of the visible
red-aiming-dot. What does that in itself reveal about
the laser’s footprint on the surface?
The article indicates that that portion of the surface
(plane) closest to the Instrument will “reflects the beam
first, whilst the part that is farthest away will reflect
last.”
IMPORTANT OBSERVATION- One important topic the article didn’t cover,
due to its intended reader outside forensic scene
reconstruction, is the absolute and vitally important
procedure of verifying the red-aiming-dot’s
alignment with the Instruments Crosshair!
The above commercially available ‘red-dot aiming
target’ should be placed at nearly the same height
as the Instruments’ scope, and approximately 15-20 feet away
(where permitted). Then by sighting through the Instruments
scope, the operator first lines up the instruments’
crosshair to the small cross seen at the center of the
‘target’ and then second, activates the red-aiming-dot
itself.
If the visible red-aiming-dot is seen to line up with
the small cross seen at the center of the ‘target’, the
operator can then be assured that where the ‘red-dot’ is
visually seen to project onto the selected evidence point or
surface, that is where the Instruments cross-hair is pointed
and the resulting coordinate will be satisfactory.
Once the recommended
Red-Aiming-Dot Protocol
has been completed, then most reflectorless instruments so
equipped, can be quickly aimed at the relevant piece of
evidence or surface negating the prior need
for the Operator to physically focus on each evidence item!
The procedure has proven to save time at the scene
and permits more flexibility in the overall scene
electronic documentation – setup process.
An additional exciting & positive feature inherent to
the red-aiming-dot from a reflectorless total
station, is that the Instruments height ~ whether at its
extreme extended tripod height or absolute “spread out
the tripod legs and get it down” height ~ won’t matter
as the Operator .. working with the red-aiming-dot
for aiming at evidence targets .. will not need the
ability to focus on the actual object itself, but instead,
just aim the red-aiming-dot at the evidence point!
Ø
Instrument Caution! It has been demonstrated that some Trimble
instruments have their “red-aiming-dot” feature affixed to
the top of the scope, as shown below
(model 5600 DR200+).
When the Trimble, as seen here, projects its’
red-aiming-dot, it is NOT in line with the
Instruments’ crosshair! All other instruments witnessed
(Sokkia/Leica), have the red-aiming-dot
literally built in the scope of the Instrument
and thus not the parallax effect introduced as with this
reflectorless total station.
In conclusion, here is the bottom line when
using “reflectorless” total stations.
-
Be sure to have available all necessary equipment when
you start your scene work. Reflectorless total station,
tape measure, data collector/EvR, HTs, pencil/paper,
small, white circular stickers (dime size), spray paint,
etc.
-
Start with the basics. Draw a rough sketch of the scene
itself, taking some preliminary “safe” measurements by
conventional means.
-
Position the RP (instrument setup location)
within close proximity of the evidence points to
be documented ‘reflectorless’. Shown here is a complex
shooting scene where many shell casings were observed
laying in the street. The RP was situated such
that all shell casings, vehicle and body positions, and
structure locations could all be documented ‘reflectorless’.
Prism pole (electro-optical method) was used to document
other distance scene features, i.e. EPs, cross-walks,
etc.
-
Provide for each ‘shot’, a surface perpendicular
to the ‘line of sight’ of the EDM, as seen here
(fingernail, business card, etc.):
  
The authors basically support this by indicating “…the
effect is negligible as long as the plane has homogeneous
reflectance characteristics.”
-
Try to be sure the surface/object is “clean” to the
extent possible. If documenting, for example, the lower
portion of a vehicle’s frame or other parts, they may be
greasy or dirty which presents a condition not always
suitable for ‘reflectorless’ EDM measurements.
As shown below, small white circular
stickers can be used to aid in the collection of these
important vehicle features.
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