Elevated Target Markers &
Spinner Target Markers

Elevated Target Markers (ETMs) and Spinner Target Markers (STMs) provide the means to connect independent photogrammetric networks that would otherwise not share common points. ETMs and STMs are imaged in more than one network, thus allowing multiple photogrammetry surveys to be linked into a uniform coordinate system. An example of where ETMs and STMs are useful is shown below. The task in this case was to tie together three iWitness surveys of a semi tanker explosion, one on the freeway overpass and two others on the roadway below the overpass.

Click here for the PDF with close-up images of ETMs and STMs

ETMs and STMs are applicable in many applications where intervisibility between surveys to be connected is otherwise difficult or impractical, a condition which can occur, for example, with crashes involving vehicles running over steep embankments.

Solutions for Complex Camera Geometry in Accident Reconstruction
In order to measure multiple photogrammetric networks and "link" them into the same common coordinate system, at least 4 "Tie Points" (TPs) are required. TP's measured in iWitness may be natural feature (discrete object) points or special targets, such as ETMs or STMs. It is important that TPs are well separated and distributed in two dimensions (ie they cannot be approximately collinear) to facilitate a geometrically strong join, which in turn is carried out via the iWitness Project Linking tool.

ETMs & STMs designed for work with iWitness
ETMs are offered as accessories for iWitness. These stand approximately 8’ (2.4m) off the ground when mounted to a standard tripod extended to a height of 58". The ETM's 3/8" diameter fiberglass rod is secured to a standard 1/4"-20 tripod stud. A special adjustment nut permits the assembly to be rotated about the vertical axis, so the target can be spun to face any horizontal direction at the base location of the ETM-to-tripod mount. The ETM target itself can then be imaged with a digital camera over 360 degrees, making it ideal for linking iWitness photogrammetry networks. The ETM is also removable from the rod for easy storage.

STMs also allow the photogrammetric target to be spun on its axis, horizontally through 360 degrees. The STM is weighted with anti-slip grip tape installed to the base. STMs break down for easy storage and deployment. A galvanized plate is velcro hook and loop secured to the base.

The target components for both ETMs and STMs are removable for easy storage. When combining both ETMs & STMs, the user can easily achieve the required 2D separation of the target points that is necessary for a strong join within the unified 3D object coordinate system.

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