Letter to the Editor

When it comes to accident or crime scene documentation, different site conditions call for different tools.

I have some comments to offer regarding the article “Reconstructing an Accident Scene: How RTK GPS equipment makes collision reconstruction faster and easier” (Evidence Technology Magazine, Volume 10, Number 4, July-August 2012, pages 10-15).

Indeed, GPS Real Time Kinnematic (RTK) is an excellent tool for site survey and documentation for areas depicted in the images shown in the article—that is, wide-open skies and no trees. However, catastrophes that occur in urban canyons and wooded areas are not suitable for site surveys with GPS RTK equipment due to signal multipath and obscuring objects. In those cases, total stations, robotic total stations, LIDAR scanners, and photogrammetry are more suitable choices of instrumentation for the task at hand.

The inference for the need for two GPS receivers is applicable to the Canadian provinces listed, but in most of the United States and much of densely-populated Canada, GPS RTN (Real-Time Networks) allow the user the freedom (and economy) to use only one dual-frequency GPS receiver. The precision and accuracy is the same and oftentimes better than using a single base station used in the article due to parts-per-million corrections based on rover distance to base station.

The facility for an investigator to work alone without a helper with GPS RTK or GPS RTN, however, also applies to the use of a robotic total station at a single site of less than a few hundred meters—and works well in urban canyons and wooded areas, also.

GPS is a superb tool, but is not a panacea. LIDAR scanners and photogrammetry can be appropriate tools, also. Different site conditions call for different tools; the only panacea is the investigator’s IQ.

Clifford J. Mugnier, CP, CMS
Chief of Geodesy
LSU Center for GeoInformatics
Department of Civil Engineering
Louisiana State University
Baton Rouge, Louisiana

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the editor, Kristi Mayo.

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Item of Interest

The language barrier between English-speaking investigators and Spanish-speaking witnesses is a growing problem. (Updated 28 February 2011)