970944

Accident Scene Diagramming Using NewPhotogrammetric Technique

Stephen Fenton, Richard Kerr,Knott Laboratory, Inc.

ABSTRACT

One of the challenges for accidentreconstructionists is creating accurate accident scenediagrams from photographs. The biggest challenge iswhen only one photograph is available, and informationabout the camera that took the photograph is notavailable. The authors will present a unique techniquethat enables the user to create an accurate accidentscene diagram from only one unknown photograph ofthe accident scene, by using a combination of processescalled Inverse Camera Projection and PhotographicRectification. Inverse Camera Projection allows the userto determine the unknown camera characteristics, whichthen through Photographic Rectification, the photographcan be rectified and traced to create accurate scenediagrams.

INTRODUCTION

BACKGROUND - In accident reconstruction, itis often necessary to create an accident scene diagramto scale which includes the location of skid marks,vehicle positions, and impact location. By the time anaccident reconstructionist investigates the scene, theskid marks have faded and the debris has washed away.This information is usually available only in photographstaken by the police on the day of the accident. Typically,there is little or nothing known about the camera thatwas used to take these photographs, which makes itdifficult to gather information from the photograph.Utilizing photogrammetry, a new technique has beendeveloped by the authors to create scaleable diagramsthat incorporates one “old” photograph of the accidentscene with “new” photographs taken by thereconstructionist during the site investigation. The newphotographs need not contain the same information

such as skid marks and vehicles, but must have discretepoints in common with the old photograph such as lanelines, street corners or light posts. A discrete point is asingle point that can easily be identified in both the oldand new photographs.

THEORY - In order to utilize an old photograph,the camera characteristics need to be determinedthrough a process called Inverse Camera Projection.There are seven unknown camera characteristics thatmust be defined. These characteristics are as follows:

• The position of the camera (x,y,z)• The camera rotation about the x-axis, OMEGA (ω)• The camera rotation about the y-axis, PHI (φ)• The camera rotation about the z-axis, KAPPA (κ)• The camera focal length

In order to determine the unknown cameracharacteristics, four control points must be identified inthe old photograph. Control points are reference pointscommon in both the old and new photographs. The realworld coordinates (x,y,z) of these four control pointsmust be known. The relationship between the real worldcoordinates and the photograph coordinates establisheseight equations, four in the x direction and four in the ydirection. The seven unknown characteristics can bedetermined using the eight equations through a processcalled Inverse Camera Projection.

Photographic Rectification involves determiningthe relationship between the surface of the object inthree-dimensional space and the image in thephotograph. Once this three-dimensional relationship isdetermined, an orthographic projection of the surfacecan be generated. An orthographic projection is a re-mapping of the image on a flat surface with noperspective distortions.

DISCUSSION

METHOD - In order to create an accident scenediagram to scale requires the user to have access to theaccident scene, where new photographs of the scenecan be incorporated with the old photograph. Acommercially available software program calledPhotoModeler*1 allows the user to determine the cameracharacteristics through Inverse Camera Projection, andperform the photographic rectification.

Inverse Camera Projection - In order to determine the camera characteristics of the oldphotograph, the three dimensional coordinates of fourcontrol points within the old photograph need to bedetermined. This can either be accomplished bysurveying the points with either traditional surveyingmethods, or through photogrammetric methods. Thispaper discusses the photogrammetric methods.

PhotoModeler allows the user to determine thecontrol point coordinates utilizing photogrammetry, andallows the user to identify these control points in the oldphotograph to determine the camera characteristics.

Photographic Rectification - Once the camera characteristics (position, orientation, and focallength) have been accurately determined, areas withinthe photograph can be rectified. To rectify a portion ofthe old photograph the three-dimensional coordinatesthat outline the area must be known, thus it is best touse these as control points in the Inverse CameraProjection process. PhotoModeler allows the user tochoose any plane for rectification. When an area isrectified on a plane parallel to the ground, the rectifiedimage can be traced to create a plan view drawing. Thisis most useful in determining the length of skid markswithin an accident scene. If an area is not planar,multiple planes can be identified for rectification. Forexample, if a curved surface (i.e. crowned roadway orhilly roadway) is rectified, it should be broken down intoas many pieces as reasonable to best represent thecurved surface. Since a rectification plane can not becurved, multiple planes must be created that bestapproximate the curvature of the roadway. Each piececan then be rectified separately and combined afterrectification.

PROCEDURES - In order to create an accuratescene diagram from a photograph with unknown cameracharacteristics, the following four steps must be taken.

1) Determine the Real World Control Point Coordinates - The following steps allow the user to determine the real world coordinates of the control pointsthrough photogrammetry.

a) Identify the area of the old photograph that ascaleable diagram is needed. This area will be rectified.To do this, the camera characteristics must bedetermined. Case Study: In this case study, a scene diagram containing the skid marks in lanes two andthree in Figure 1 is needed. Therefore the portion of thephotograph that contains these two lanes must berectified.

b) Investigate the scene, and identify controlpoints that border the area that will be rectified. Controlpoints are identifiable points in the old photograph that

can be easily identified at the scene. These controlpoints will be used to determine the cameracharacteristics and to rectify the image. At least fourcontrol points are needed. Case Study: From the field investigation, it is determined that the original lane linesare still in place. The original lane lines will be used ascontrol points and will border the area that will berectified. The control points are identified on the oldphotograph in Figure Two. In this case, more than fourcontrol points were identified, which will increase theaccuracy of the project. PhotoModeler utilizes aniterative algorithm to determine the cameracharacteristics. The algorithm adjusts the positions andangles of the camera station and the positions of themarked control points on each iteration. A greaternumber of control points, increases the amount ofadjustment the algorithm can make, therefore increasingthe accuracy of the process.

c) Photograph the scene ensuring that thecontrol points in the old photograph can be identified inat least two new photographs taken at the scene. It isimportant that the control points can be clearly seen inthe new photography. The closer the photographs aretaken to the objects, the easier it will be to identify thecontrol points, and the greater the project accuracy.However, one should realize that there is a balancebetween accuracy and efficiency. Photographs takenclose to the object can be cumbersome to manage anddifficult to tie together later. Case Study: This project was broken down into smaller sections when it wasphotographed during the field investigation so that thelane lines could be easily identified.

d) Have the negatives processed to Photo-CDwhere they can then be utilized by PhotoModeler.Scanning to Photo-CD is the most efficient andeconomical way to import the images into PhotoModeler.PhotoModeler supports all types of camera formatsranging from film cameras to digital cameras to videocameras. Film cameras deliver acceptable accuracy ata low cost. High resolution digital cameras areexpensive and are typically used when a quick turn-around is needed. Video cameras are used whenaccuracy is not a priority.

e) Import the new photographs intoPhotoModeler. Identify the control points in at least twonew photographs, and process the project.Photomodeler will determine the coordinates of thecontrol points. Case Study: The accident scene consisted of skid marks, gauge marks, and damagedvehicles. In order to clearly see the lane lines over theentire length, the lanes were photographed in sections.For this case study, photographs closest to thebeginning of the skid marks are used to explain theprocess of determining control point coordinates.PhotoModeler will allow the user to relate similar pointsbetween different photographs. Figure 3 consists ofthree photographs that contain similar control points.Once all the control points have been identified in atleast two new photographs, PhotoModeler can processthe information and determine the three dimensionalcoordinates of the control points. The coordinates arestored in an AutoCAD DXF file. This information will be

used to determine the old photograph’s cameracharacteristics.

2) Determine the Camera Characteristics - Once the real world coordinates of the control points aredetermined, they must be referenced in the oldphotograph. Once referenced, the unknown cameracharacteristics can be determined through InverseCamera Projection.

a) Have the old photograph digitally scannedinto the project. Identify the control points in the oldphotograph and reference them to the same controlpoints in the new photograph. Case Study: To assign the control point coordinates to the old photograph,import the DXF file previously created. Assign thecontrol points to the old photograph with the control pointidentification tool (see Figure 4).

b) After all the control points in the oldphotograph are referenced in the new photographs, theunknown camera characteristics can be determinedthrough Inverse Camera Projection. Case Study: PhotoModeler can determine the camera characteristicsthrough Inverse Camera Projection. The results of theprocess are graphically depicted on the screen with thecontrol points laid over the old photograph. If the controlpoints do not overlay properly, there probably is an errorin the referencing of points.

c) The accuracy of the processing can bechecked by comparing a known distance measured inthe field to a processed distance in the photograph.PhotoModeler developers claim that the relativeaccuracy of the software is around one part in twothousand (1:2,000). A relative accuracy of 1:2,000means that for an object with a 20ft largest dimension,PhotoModeler can produce 3D coordinates with 0.01ft(approximately 1/8 inch) accuracy. Not all 3Dcoordinates will be of the same quality. PhotoModelerdepends heavily on the user marking precise locationson the images. If a point appears in two photographsand the user guessed or approximated the location ofthe point in one or both of the photographs, then theresulting 3D coordinate will probably be inaccurate.Case Study: Field measurements were taken to determine the accuracy of the project. A comparisonbetween the field measurements and those determinedby PhotoModeler were within one percent. Therefore,the length of the longest skid mark, which was measuredto be 281 feet, is within three feet.

Comparison BetweenField Measurements and

PhotoModeler Processed Measurements.Item Field

Dist.Proces

sDist.

Percent

Lane Line Length 10.0’ 9.91’ 0.9%Gap between Lane Line 17.0’ 17.13’ 0.8%Lane width 12.0’ 12.07’ 0.6%Barrier Wall Length 10.0’ 9.96’ 0.4%

3) Rectify the Photograph - Now that the camera characteristics are known, the area of interest onthe old photograph can be identified and rectified.

Identify surfaces on the photograph forrectification by choosing control points. A surface isidentified by connecting three control points. An entiregroup of surfaces can be identified to rectify, but onlyone rectification plane can be chosen. Case Study: PhotoModeler allows the user to identify a surface usingthe “surface tool”. Connect all the control points with thesurface tool and then rectify the image (see Figure 5).The image resolution can be defined in the rectificationwindow.

4) Trace the Rectified Image - Now that the photographs is rectified, it can be traced to create ascaleable accident scene diagram.

Import the rectified image into any drawingpackage like CorelDraw or AutoCAD, and trace over theimage. Use the control point data to determine thedrawing scale. Case Study: Import the rectified image into CorelDraw and trace over the image to create anaccident scene diagram (see Figure 6 and 7).

BENEFITS AND LIMITATIONS - The biggestadvantage of this process is that the objects, such asvehicles and skid marks, do not need to be in the sceneduring the field investigation, they only need to be visiblein old photography. This is a typical situation foraccident reconstructionists. Reconstructionists rarelyfind that skid marks and gouge marks are still identifiableat the accident site. This method allows thereconstructionist to incorporate old photographs taken ofthe accident scene, with new photographs of theaccident site, to create an accident scene diagram toscale. Utilizing the principles of photogrammetry, there isno need to survey the scene with traditional surveyinstruments. Therefore, there is no need to get intotraffic, which can be dangerous and time consuming.This method can be used for all types of roadwaysurfaces from flat to curved. The software can usecropped photographs, which is typically the case whenonly prints are available.

The major limitation of this process is that theresults are only as good as the quality of thephotographs and the camera used. Cameras with poorlenses, such as point-and-shoot cameras, will notgenerate good results. However, standard 35mmcameras produce good results. The process cannot beused when the scene has changed significantly.Common points in the old photography must beidentifiable in the new photography.

CONCLUSION

Utilizing desktop photogrammetry software, acommon problem of creating accurate accident scenediagrams from a single photograph can be solved. Thisprocess requires that the accident scene be available,and that new photographs of the scene have a minimumof four points in common with the old photograph. Theadvantage of this process is that the entire process canbe accomplished on a desktop computer, utilizingscanned images and Photo-CD images. Updated

technologies in computing hardware and software nowallow photographs to be rectified on desktop computers.This new technology has been incorporated intoPhotoModeler software. The software is flexible enoughto handle cropped photographs, uneven roadwaysurfaces, and multiple images.

ABOUT THE AUTHORS

The authors have been utilizing PhotoModeler softwaresince it was first introduced in 1993, and are listed byPhotoModeler as Authorized Service Providers. Theauthors can be reached at the following address:Knott Laboratory, Inc.2727 West Second AvenueDenver, Colorado 80219-1605Phone (303) 936-2700Fax (303) 936-2739e-mail knottlab@ix.netcom.com

ACKNOWLEDGMENTS

*1) PhotoModeler, developed by Eos SystemsInc., 2040 West 12th Ave., Vancouver, B.C., Canada.

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