Using iWitnessPRO to automatically map a homicide scene

DCS conducted a staged homicide scene measured and mapped with iWitnessPRO using a Nikon D200 digital camera, Coded Targets, and Feature Point Targets. This staged-scene was setup to illustrate the mapping of a "suffocation by pillow" in a bedroom 10' x 14' in size. A movie link of this staged scene is noted below.

Survey with iWitnessPRO
At the time of conducting the photogrammetric survey, a quantity of 142 beta version Feature Point Targets (FPT’s) were placed on the objects of interest in the room with 48 iWitnessPRO Coded Targets randomly distributed for iWitnessPRO’s automatic camera orientation and 3D point determination. In addition to the 48 standard coded targets, 3 Spinner Codes were randomly placed in the crime scene. It took approximately 25 minutes to pre-target the room with the FPT’s and iWitnessPRO Coded Targets.

A total of 96 images were recorded for use in the mapping of the 142 FPTs in the room, with photography taking about 10 minutes. As the photographer moved around the room, the three Spinner Codes were rotated into the camera’s field of view.

Once the crime scene images were imported into iWitnessPRO, with the push of one-button, the fully automatic measurement process of camera orientation and 3D object point determination was performed. It took only six and a one-half minutes for iWitnessPRO to automatically compute the photogrammetric bundle-triangulation of all 96 camera positions, coded targets and FPT’s.

As a post process in iWitnessPRO (accomplished in less than 5 minutes), photogrammetric scale was applied, along with rotation of the 3D model into the chosen reference coordinate system. At this stage, the operator can optionally use iWitnessPRO’s advanced features (e.g., facet and freeform polylines, or photo texturing) to “trace” geometry in the images and to model these entities for later use in CAD (after export from iWitnessPRO in DXF or VRML file formats.) It should be noted that once iWitnessPRO automatically orients the camera positions, the user can simply click the same “natural feature” entities in two or more images and the 3D point determination in the digital images is automatically carried out.

The 3D accuracy produced by iWitnessPRO for the FPTs was better than .02" (.5mm) RMS (about a five-fold increase in accuracy over natural feature points measured in iWitness.)

iWitnessPRO measured the pre-targeted FPT “key features” required to diagram the scene fully automatically from RGB (color) imagery to an accuracy level beyond that attainable with other low-cost image-based measuring systems. Furthermore, the time for the photogrammetric data processing was reduced from hours to a few minutes, the time saving being as a consequence of iWitnessPRO’s fully automatic 3D measurement capability.

The image below is the 3D Graphics View in iWitnessPRO of the automatic 3D point cloud with the new coordinate axes grids.

To watch a 6 minute video clip of this staged scene using the 96 measured images, click here (9Mb WMV format movie download).

Footnote on digital camera technology:
A 10MP Nikon D200 was used as the digital camera for this project and it is always recommended that digital SLR cameras with resolutions of 6 megapixel or greater be used with iWitnessPRO. A key advantage of iWitnessPRO and color imagery is that one can simply use the digital camera in “Program Mode Exposure”, but it has been found that using the camera’s onboard flash in Aperture Priority mode (f-stop being F8 or F11), produces a better depth of field and better illumination of the iWitnessPRO coded targets and FPTs.

Also, use of either a fixed-focal length lens of 14-18mm focal length or a zoom lens of, example, 17 - ??mm set to its widest angle setting (fully zoomed out) and focus manually fixed to one setting is recommended. A wider field-of-view makes it easier to cover the scenes and to achieve satisfactory image-overlap with favorable geometry.

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