Accuracy Performance of the Close-Range Photogrammetry System iWitness: Results of a Measurement Comparison with a Surveying Total Station

Preamble

The iWitness photogrammetry software system, combined with a consumer grade digital camera, was assessed for accuracy potential against a total station measuring device within a simulated traffic accident reconstruction scene. As will be described, the results indicated an accuracy agreement to within one inch over a test range area spanning approximately 100 feet (wide) by 150 feet (long).

Background

This test of 3D measurement accuracy was carried out in conjunction with members of the organization Forensic Accident Reconstruction of Oregon (FARO) during their Spring 2004 conference in Eugene, Oregon (http://www.accidentreconstruction.com/faro/FARO2004Spring.pdf). The aim of the measurement project was to assess whether a photogrammetric measurement approach utilizing a consumer grade digital camera and the powerful iWitness software system for close-range photogrammetry could yield a similar level of accuracy to the much slower total station measuring approach which is at present widely used in accident reconstruction (AR).

The simulated accident scene, set up in the parking lot of the Eugene Fire Department, as shown below, comprised a 'crashed' vehicle, artificial skid marks, road line markings and street signage.

iWitness user interface showing referenced image with measured points and lines.

The overall AR measurement process comprised two stages: Firstly the 3D coordinates of feature points of interest in the scene where measured with both the iWitness system, and with a surveying total station. The results were then compared for accuracy assessment. Secondly, the 3D point data was imported into CAD Zone’s Crash Zone diagramming software system to prepare line drawings for evaluation by FARO members.

Feature Point Measurement

Two different total station measuring devices were used to survey the scene. One total station employed a target prism; the other utilized a 'reflector-less' mode of 3D data acquistion. To facilitate a relatively straightforward comparison of the measurement results from the reflector-less total station and iWitness, both measuring techniques utilized the same feature points, which in this case were DCS Evidence Markers, orange traffic cones and additional signage and road markings.

Time Requirements for Image Recording for iWitness Measurement

It took approximately 18 minutes to set the evidence markers and photograph the scene – during a light rain. The digital camera used was an Olympus C-5050. The camera had been previously calibrated using iWitness. Twenty-three images were acquired with the camera, but only eleven images were subsequently used in the iWitness image-based data processing (redundant imagery is a beneficial feature of the photogrammetry approach – additional photographs can always be used if needed). The data processing was accomplished away from the scene, though it could have been carried out if required at the site on a notebook PC. The accident scene was recorded in mid afternoon, and by 6:30 PM the iWitness data processing was complete and all measured 3D point and line data was exported in DXF file format to the Crash Zone diagramming program.

Scene Recording by Total Station

The total stations were set up and all points of interest were recorded without a 'survey move'. The two total stations were operated by Derik White (The CAD Zone Inc), Verne Hoyer (FARO Treasurer) and Patrick Higgins (FARO Vice President). Mr. White operated the reflector-less total station, while Messrs Hoyer and Higgins used the Prism Pole method. The feature points of the staged AR scene points were intentionally placed so that a clear line-of-sight was available for total station operation. It took approximately one hour for each total station to be set up and for full acquisition of the 3D data points.

Comparison of 3D Measurement Results

The measured 3D coordinates from iWitness were compared to those measured with the reflector-less total station as part of an accuracy assessment of the photogrammetric approach (ie the total station was accepted as the accuracy standard). The actual coordinate comparison was performed within the Crash Zone diagramming program. In a 'best fit' of one 3D data set to the other, coordinate discrepancies were in all cases less than 2 inches, with the root mean square error being close to 1 inch. This '1 inch' level represents the measurement accuracy of the iWitness operation, which is quite impressive given that the images were recorded with an inexpensive, off-the-shelf, consumer-grade digital camera. As previously mentioned, this study was not aimed at producing a high- accuracy comparison of the two 3D measurement methods, but more a comparison within a routine AR environment. As documented elsewhere, iWitness is capable of performing 3D measurements to better than 1:50,000 of the size of the object given the right combination of camera, targets and strong network geometry.

The following two figures provide illustrations of, firstly, the feature point/line and camera layout within the iWitness 3D graphics view, and secondly, the corresponding CAD drawing produced in Crash Zone.

iWitness 3D graphics view of measured entities (plan view).

Overlay of iWitness and Total Station Measurements within CAD Zone Crash Zone.

Summary

The impressive accuracy capability of iWitness in AR surveys was confirmed in the comparison of measurement results between the photogrammetric and total station approaches. Moreover, iWitness demonstrated that it provides a very fast 3D measurement tool for the accident reconstructionist or police officer; the acquiring of digital images at the scene for subsequent 'image-based mapping' takes only a matter of minutes. iWitness has been specifically designed for crime scene and accident scene mapping purposes, though it can also be applied in many other fields for 3D measurement. The system is very easy to use by non-specialists and it is by far the most robust close-range photogrammetry software available for applications within accident reconstruction and forensic measurement.

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