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By Mick
Capman,
PSFM President (Kalamazoo County Sheriff – Retired)
It
is pretty straight forward actually …
-
if
you were at the scene in question,
-
if
you walked the scene yourself examining relevant
evidence to be documented,
-
if
you assisted or otherwise documented the scene yourself
serving as the ‘pole-person’,
… you
would, in all likelihood, know where all relevant ‘shots’
were taken from and would quickly recognize ‘shots’ (aka:
coordinates or points) taken in error.
Currently there are three classifications of electronic
“total stations”. Microwave, electro-optical and
electro-optical + laser.
 
As it
relates to Crash and Crime scene documentation, the more
modern instruments can measure quickly, 3-immediate
“facts” about a particular evidence location:
-
Slope distance
(units: feet/tenths) [accuracy: ppm]
-
From Instrument to evidence point/location
-
Horizontal angle
(units: degree/minute/second) [accuracy: arc seconds]
-
shows relationships between evidence on horizontal
plane
-
Vertical angle
(units: deg/min/sec) [accuracy: arc seconds]
-
shows elevation (geometry) changes
-
shows relationships between evidence on vertical
plane
In lay
terms, light waves are used by the total station as a
carrier of electromagnetic energy. Each transmitted light
wave carries a unique “signature” that the Instrument
recognizes after being refracted back towards the instrument
by a prism or reflective surface/object.
Each
time a prism or object is measured to, eight facts
about the particular evidence point is collected or
calculated by the data collector/EvR.
A
‘shot’ number; code or description of the evidence;
horizontal angle
(generally a compass direction of ‘zero’ or
North);
elevation; vertical angle
(with
‘zero’ being overhead);
“northing” or Y-position of the Cartesian coordinate system;
the linear slope distance from the instrument
(RP
location)
to the
evidence point itself; and the “easting” or X-position of
the Cartesian coordinate system.
Elevation changes are calculated first by the data collector
based on the measured height of the Instrument above the
ground, the indicated prism pole (or target) height, and the
vertical angle measured by the Instrument.
The
coordinate data that the total station measures per ‘shot’
is called a “spherical” coordinate, and consists of a
measured distance and two angles (horizontal & vertical).
While on just a horizontal (flat) plane, a “polar”
coordinate is offered for determining the X and Y
coordinates.
When
deciding to confirm a coordinate data regarding a particular
‘shot’ or evidence location, the “polar” coordinate will be
meaningful.
In the
below scene example, the “point/shot of interest” is number
261. It is a ‘shot’ taken to a corner fence post
seen in the left quadrant, just to the left of a utility
pole (cad-symbol).
By left-clicking on the corner post
point, a ‘Point Editing’ dialog appears when using
MapScenesPRO.
This data is consistent with what is seen on a data
collector from either pressing, (on a EvR) Verify >
Review points, then scrolling to ‘shot’ 261, or on a SDR
by pressing View > Enter > F1.
To
assure what some have called “software consistency”, and
from within the same referenced software, other manners of
verifying the measurements are accomplished as seen here:
Scene Measurements,
Active Drawing Technology
Point
Editor,
Active Drawing Technology
View Log
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