Close-Range Photogrammetry for Accident Reconstruction Measurements iwitness TM Close-Range Photogrammetry Software www.iwitnessphoto.com Lee DeChant Principal DeChant Consulting Services DCS Inc Bellevue, WA 425-637-1865 2008 All Rights Reserved DeChant Consulting Services
Preface If you utilize an EDM Total Station or you use the baseline offset approach (or both) that s fine. But, let s look at another technology that has proven its worth and value, and which is low cost, easy to use and readily available. Close-Range Photogrammetry is not a cure-all for forensic mapping tasks it is just another valuable dimensional measurement tool in the user s tool box. No one 3D metrology system is perfectly suited for all applications!
Typical AR measuring/diagramming tools OR Distance measuring Wheel (not 3D) EDM total station or the impulse laser The consumer grade digital camera & photogrammetry Speed Slow Moderate Fast Accuracy Marginal Very Good Very Good Set-up Time No Yes No Storage Easy More Difficult Easy Cost Inexpensive Very Expensive Inexpensive & Dual Purpose Ease of use Simple More Difficult Moderate Time at scene Slow Relatively slow Fastest Dual Purpose No No Yes Scene Real-time Yes Yes No Meas. after fact No No Yes
So, what does that word photogrammetry mean anyhow? The term photogrammetry is derived from three Greek words photos meaning light, gramma meaning drawn, metron meaning to measure. + +
Close Range Photogrammetry is the science and technology of generating 3D information from 2D measurements on images. mostly digital images nowadays, but film images can also be used.
The determination of 3D coordinates of these feature points happens via a process called: Photogrammetric Bundle Triangulation Image Chip Size ( Digital Camera) 4 Camera Positions Focal Node of Lens Spatial Intersections of Light Rays The 3D Object Points in object space to be measured
To accurately measure 3D points, certain information (internal to the camera) must be known. In iwitness TM this can be accomplished by fully automatic camera calibration. iwitness Camera Calibration
Best Practices for camera calibration and general image recording for photogrammetry - Camera calibration is performed with the camera s lens set at a fixed focal distance (typically widest angle field of view if zoom lens.) - It is best to have the autofocus turned off (i.e. use manual focus). - Focus the lens to infinity on the lens barrel, or via the electronic focus slider.
What happens if I have to work with digital images from another camera or from film negatives or prints? For digital images, there s usually an Exchangeable Image File Format (EXIF). - Photogrammetry software systems (e.g. iwitness) read-in the image EXIF tag. - The EXIF tag stores the image s format size in pixels and it s focal length. - Another key component for basic photogrammetric measurement is the pixel size (height & width) of the digital image getting this information is easy through the web, e.g., www.dpreview.com. Having the above information sets up the Camera Parameters correctly for the Image Format in relationship to the Principal Distance (Focal Length.) However it does not account for lens distortion. In brief, using the above is not as accurate as that of a Calibrated Camera. If the above process is accomplished properly, the measurement result is typically satisfactory for accident reconstruction purposes. Whenever possible, calibrate the camera!
The next slide is a combination of the EXIF Reader and the iwitness Camera Parameter Dialog box. The camera make is a Konica (Minolta) and the model is a DiMage Z10. Notice that the focal length is circled as 9.9mm. This information as well as the image format size in Pixels and the calculated Pixel Height x Width are core for close range photogrammetry. Photogrammetrically solving for the lens distortion is equally important, as without it, project accuracy can suffer depending on the specific lens being used. In certain cases where digital images are provided to you, they can be calibrated using a process called self calibration, but this requires a minimum of four images, with at least one being rotated 90 degrees, i.e. we need landscape & portrait shots.
EXIF Reader iwitness Camera Parameters Camera setup for zoomed focal length for use in iwitness
What about film (analog) photographs? The negatives or prints need to be scanned to digital format. The pixels must be square (a good software program for this is ZARAF used with iwitness (www.iwitnessphoto.com) The focal length of the lens for each image is required Scenario #1 Focal Length Known along with Image Format (un-cropped): - Proceed with the above Scenario #2 Focal Length Unknown and\or images cropped - Requires orientation via Control Points; iwitness has a feature called FOOM that allows one to work with images from an unknown camera, as well as solve for lens distortion
REAL WORLD CASE Lets review a real-world case where police used a digital camera (a Minolta DiMAGE Z10) at a car/pedestrian fatality in 2006. Noteworthy is the fact that the police agency did not take any measurements of this fatality scene, but they did photograph the critical road evidence as well as orange spray paint. The following slides illustrate how the police on-scene images were used with the iwitness photogrammetry program to assist in the reconstruction of critical evidence locations and to map a street intersection and over 500 feet of roadway all by one person.
Police Image One of Three in iwitness Looking Northwest One of three on-scene police recorded images used in iwitness to photogrammetrically map critical road evidence. Reference the red 3D points in this image all 3 images were zoomed, which is not desirable in photogrammetry, but still tolerable on a case-by-case basis.
Police digital camera images were used to measure the areas marked with spray paint. Note the road evidence of vehicle at area of POI (the red points) Looking East 3D Mapping of the roadway, one year after the incident using iwitness, a digital camera and RPMs
Looking East 3D Mapping of the roadway with iwitness, one year after the incident
Looking East iwitness survey one year after the incident note: Reference Point Markers (RPMs) are not in the road they were randomly spaced about 20 to 50 feet over the mapped single network distance of 500 feet. The RPMs are placed on the grass, shoulders, curbing, etc. The markers are simply there to assist in camera orientation. RPMs assist in making the data processing work faster - leading to a more precise photogrammetric result.
Plan View iwitness 3D Graphics View Camera positions, 3D Points, Lines, and non-contact Facet Form Polylines. NORTH
OPTIONAL network Level To Gravity Using a digital inclinometer, two measurements are made and used within iwitness. The coordinate system is then mathematically leveled to gravity (X,Y horizontal, Z vertical) The DXF output file from iwitness was then used for additional CAD work. The two bold red lines are measured tape distances used for iwitness scale Looking West
Isometric view Road Evidence Looking East In close-range photogrammetry, the key is strong image-overlap from different perspective view points
Street intersection at incident 500 away The markers (RPMs) are placed anywhere in the incident scene, where they re observed from multiple camera view points. The RPMs are best not used as evidence markers per se. iwitness allows the user to trace the images using advanced tools such as Facet Form Polylines and also other image marking tools for natural feature marking. Looking West RPM 3D Facet Form Polyline
The camera aim point for these three closest camera positions is looking down the road in an east to west direction. Notice the cameras aimed in the opposite (west to east) direction. The key in imaging is semi-rings of images and waves of these same semi-ring images. 1 5 Ray Intersection on Point ID TIP1 Looking West 2 3
Looking West The result of the iwitness 3D entities as they appear in a CAD diagramming program; iwitness works with any CAD program that reads the DXF file format. In this project we are illustrating the iwitness DXF results displayed in CAD Zone s Crash Zone. www.cadzone.com
Plan view of the 3D iwitness mapped entities
3D view of some of the iwitness measured natural features, with entities completed in the Crash Zone diagramming software Looking East
The pedestrian was struck and killed after stepping off this sidewalk and then running across the street in a north to northwest direction to catch a bus. The sidewalk corner was imaged for possible sightline obstruction for the pedestrian (and also for the driver). The 3D photogrammetric model from iwitness, upon importation into CAD, allows one to stand on the sidewalk at the crosswalk, or to sit in the drivers seat. iwitness was used to measure all entities including the crosswalk (curb step-off.) Looking East oncoming car westbound
A TIME & DISTANCE STUDY was performed using the information from witnesses at the scene, along with the measured police images, with post-site survey measuring / modeling using iwitness. iwitness measurements are now displayed in CAD - creating a 3D view of the scene as viewed from the drivers perspective. Time & Distance calculations were made from driving speeds of 30 and 35 mph at specific distances of the car in the road from the pedestrian running across the street.
Pedestrian Curb Step-off to cross street Looking West
What about accuracy of the photogrammetric measurements?
Just how accurate is the iwitness 3D point data for this project? Internal accuracy estimates:.4 (1cm) RMS Check distances (tape and measuring standards, (i.e., stop signs, sidewalk widths, road lane widths etc.,) revealed iwitness 3D object point uncertainty of.5 (1.2 cm) RMS. The measured 3D points are accurate (overall) to better than 1.5 - mapping a single photogrammetric network of 500 feet.
Summary iwitness provides a 3D measurement tool for accident recording and reconstruction that is: Fast minimizing traffic disruption and road closures Comprehensive any feature of interest can be measured, at any time Accurate can measure to fractions of an inch within large-area accident scenes Reliable the inherent redundancy of multiple images makes iwitness a very reliable 3D measurement tool Flexible digital or film recording can occur in most conditions Easy to use needs only an amateur camera & the easy-to-use iwitness software Inexpensive total system cost can be as low as a few thousand dollars, or about 10-20% of the cost of a total station Widely use hundreds of users employ iwitness in various local, state and federal police and traffic agencies, as well as in other application domains such as engineering measurement and heritage recording Capable of providing a re-measurement or further measurement at any time after the fact from the archived images and it is noteworthy that iwitness was developed especially for accident reconstruction and 3D forensic measurement