iWitnessTM Close-Range Photogrammetry Software

www.iwitnessphoto.com

Lee DeChant

Principal

DeChant Consulting Services – DCS Inc

Bellevue, WA

425-637-1865

Close-Range Photogrammetryfor Accident Reconstruction

Measurements

©2008 All Rights Reserved – DeChant Consulting Services

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!

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.

Preface

Distance measuring Wheel (not 3D)

EDM total stationor the impulse laser

The consumer grade digital camera & photogrammetry

Speed Slow Moderate FastAccuracy Marginal Very Good Very GoodSet-up Time No Yes NoStorage Easy More Difficult EasyCost Inexpensive Very Expensive Inexpensive & Dual PurposeEase of use Simple More Difficult ModerateTime at scene Slow Relatively slow FastestDual Purpose No No YesScene Real-time Yes Yes NoMeas. after fact No No Yes

OR

Typical AR measuring/diagramming tools

The term ‘photogrammetry’ is derived from three Greek words…

photos meaning light, gramma meaning drawn,

metron meaning to measure.

So, what does that word ‘photogrammetry’ mean anyhow?

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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 coordinatesof these feature points happens via a process called:

Photogrammetric Bundle TriangulationImage Chip Size ( Digital Camera)

Focal Nodeof Lens

The 3D Object Points in ‘object space’ to be measured

Spatial Intersections of Light Rays

4 CameraPositions

To accurately measure 3D points, certain information (internal to the camera) must be known. In iWitnessTM 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 isthe 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 aboveis not as accurate as that of a Calibrated Camera.

If the above process is accomplished properly, the measurement result istypically satisfactory for accident reconstruction purposes.

Whenever possible, calibrate the camera!

The next slide is a combination of the EXIF Reader andthe iWitness Camera Parameter Dialog box.

The camera make is a Konica (Minolta) and the model isa 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, theycan 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.

iWitness Camera Parameters

EXIF Reader

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 #1Focal Length Known along with Image Format (un-cropped):

- Proceed with the above

Scenario #2Focal 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 unknowncamera, as well as solve for lens distortion

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 photographthe critical road evidence as well as orange ‘spray paint’.

The following slides illustrate how the police on-scene imageswere 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.

REAL WORLD CASE

One of three on-scene police recorded imagesused in iWitness’ to photogrammetrically map critical road evidence. Reference thered 3D points in this image – all 3 imageswere zoomed, which is not desirable in photogrammetry, but still tolerableon a case-by-case basis.

Looking Northwest

Police Image One of Three in iWitness

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)

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.

Looking East

Plan View – iWitness 3D Graphics View

Camera positions, 3D Points, Lines, and non-contactFacet Form Polylines.

NORTH

Using a digital inclinometer, two measurements are madeand 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 foradditionalCAD work.

The two bold red lines are measured tape distances used for iWitness’scale

OPTIONAL network “Level To Gravity”

Looking West

In close-range photogrammetry, the key is strong image-overlap from different perspective view points

Isometric view

Looking East

Road Evidence

Looking West

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”.

RPM

Street intersectionat incident 500’ away

3D Facet Form Polyline

Looking West

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.

5 Ray Intersection on Point ID “TIP1”1

2 3

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

Looking West

Plan view of the 3D iWitness mapped entities

3D view of some of the iWitness measured “natural features”, with entities completedin 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 roadfrom the pedestrian running across the street.

Pedestrian CurbStep-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, roadlane widths etc.,) revealed iWitness 3D object point uncertainty of .5” (1.2 cm)RMS.

The measured 3D points areaccurate (overall) to better than 1.5” - mapping a single photogrammetric network of 500 feet.

• 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

SummaryiWitness provides a 3D measurement tool for accident recording and reconstruction that is: