John Fiske Brown Associates, Inc.

       Forensic Engineering, 
  Science and Technology

Photogrammetry

"Photogrammetry allows investigators to map a crash scene in one hour, vs. traditional methods that take three hours. It alleviates congestion and reduces secondary collisions from traffic snarled during a crash scene investigation." Federal Highways Administration

In the pursuit of greater precision, safety and flexibility, John Fiske Brown Associates has adopted photogrammetry. Unlike direct measurement of a collision scene, photogrammetry allows us to recreate and measure the scene using the digital images we typically acquire during our scene investigations. These images are safely processed in an office computer away from the mayhem of live traffic. This reduces the errors made in a hostile environment and allows a great deal of data to be acquired from the photographs taken at the scene. Where the use of a total station requires the user to specify all the data acquired at the scene, photogrammetry allows the user to select data from within the photograph, at the office .

Photogrammetry is not even limited to accident scenes. It may be used on any physical object that can be thoroughly photographed.

Briefly, photogrammetry is a method of surveying and measuring the physical world with a camera. Objects in the physical world are photographed from multiple angles so that software may be used to triangulate positions in space of any object. It is the intersection of two (or more) rays between the camera's sensor and a particular object that define the object's position. This allows us to transform the two-dimensional images from a camera into a 3-dimensional representation using coordinate points.

Let's get right to it and show an example. Take this mundane ladder for example. It would be nearly impossible, or at least very difficult, to make measurements of this ladder from the photograph. This stems from the fact that no two parts of the ladder are parallel to the plane of the camera sensor. Since this is true, that means that the scale of the ladder in the image changes from one point to the next. One would have to use a changing scale or non-linear ruler to measure objects in this picture.

The following image is captured from iWitness Pro, our photogrammetric software suite.

Observe that the left pane of the image shows the other photographs used to reconstruct the ladder within the computer. Also note that the image of the ladder in the main pane has been marked up with reference points. These reference points will be synonomous with the same points of the ladder in the other photos as long as those points are visible.

Finally, the image can be displayed as a wire frame representation as in the following photograph.* The scaled coordinates of this image can also be exported to a CAD program to render the final drawing or exhibit.

Obviously, the 3D wire framed representation is not yet complete. The fourth leg and some cross braces have not been referenced in this project. By importing the other photos taken of the ladder this project can easily be completed, adding to the detail of the wire frame.

While not the intial intention of the project, we decided inside office to measure a few items in the background. This can be done on non-discrete points once the project is referenced sufficiently for the software to make accurate predictions of the whereabouts of the non-discrete points. Take the parking sign in the background, for instance. No discrete points are available on the curves describing the corners of the sign. However, this project is well referenced by a number of discrete points; so much so that the non-discrete points will be positioned accurately.

The following image has the parking sign referenced by points so the corner to corner distance of the red border can be measured.

Of course, it would be nice if the sign border and arrow could be changed from white to red for better identification. A closeup of the sign reveals which points were referenced, the lines joining these points and that we can indeed change the line color of the arrow and border.

The cross corner distance of the red border was calculated by the photgrammetric software as 19 and 3/4 inches. The actual measurement (made later since the sign is just outside our office door) is 19 and 5/8 inches. 1/8 inches over a true 19 5/8 inches is within 1/2 and 3/4 percent error. Considering that the sign was not initially designated for measurement  really highlights the flexibility and power of photogrammetry. 

So, here's the final wire frame rendition with the sign.*

Photogrammetry is a modern technology with real world application! This page was written to highlight the technology's flexibility. We can document and measure a collison scene just as easily as a defective product.

* - The dashing of the lines was caused by compressing the clipboard image for web presentation. The image has solid lines in its uncompressed format.

John Fiske Brown Associates, Inc. (2010)