Confirming RAW DATA COORDINATES  ~ Why the Mystery?

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:

  1. Slope distance  (units: feet/tenths) [accuracy: ppm]
    • From Instrument to evidence point/location
  2. Horizontal angle (units: degree/minute/second) [accuracy: arc seconds]
    • shows relationships between evidence on horizontal plane
  3. 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 toggle, from main screen

 

 
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