Supplying the Proof

June 1, 2000
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Forensic surveying requires extreme precision

Forensic surveying and the use of forensic graphics to portray survey data have been around for a long time. Forensic surveying deals with the science of measuring the size and relative position of natural and artificial features. Forensic graphics illustrate data to ensure that non-technical people can understand the findings.

Professional surveyors performing forensic surveying may be working on litigation, lawsuits, trials or hearings. The cases may range from criminal cases to civil and personal injury litigation. They are typically tasked with collecting measurements consisting of distances, angles, elevations, locations and identifications of site features. The data and related interpretations are then prepared for inclusion into evidence. They may also be used to provide support to an eyewitness or an expert witness. It is at this point that the data must be presented in a manner that can be understood by all. The surveyor must then stand ready to testify to his or her collection techniques, reduction algorithms and findings.

Day in the Life of a Forensic Surveyor

A forensic surveyor’s work is similar to that of a land surveyor. They both research preliminary data, establish control, oversee field crews collecting data and are responsible for reducing data into final form. However, three things may differentiate the two types of surveyors:

  • The findings presented by a forensic surveyor are subject to an extraordinarily high level of scrutiny. A majority of cases have an opposing surveyor working for the other side. Few land surveyors have their work contested by peers on a daily basis.

  • Techniques used in collecting or reducing data are often questioned. For example, if surveyors use a least squares analysis to reduce their control traverse, they must understand how those computations affected their data and be able to communicate that if asked.

  • Accuracy, although always important to a surveyor, receives paramount attention in forensic surveying.

For instance, in a routine roadway survey, the surveyor may shoot an existing guardrail every 25' if it appears to be on a straight line. He or she may then indicate that the guardrail has a 2.7' typical height. Of course, it would be duly noted if significant deviations occurred. Taking an example from our archives, a forensic surveyor collecting information for an accident reconstruction took a different approach. Nothing about the guardrail was assumed to be “typical.” Every guardrail post was shot and the height of the post and guardrail was measured. The shape of the posts and rail was described and drawn in detail. The main idea: the surveyor must not add any assumptions to the data set.

These case studies examine a variety of forensic cases. One case was a homicide involving aerial survey data of a landfill, another was a multi-million dollar civil litigation involving a site survey for an accident reconstruction, and a third entails survey work done on the differential settlement of a foundation for a large commercial building. In each case, surveyors collected the data to prove the case. Forensic graphics were then used to portray the survey data in ways that non-surveyors could understand.

Case Study-Homicide

Police had reason to believe that a murder had been committed and the body had been dumped into a sanitary landfill from a garbage truck. The landfill was very large and served a large metropolitan city, hence excavating the entire landfill was not feasible. We were requested to identify areas of disturbance that may have occurred during the past year. We were supplied with an aerial survey of the topography collected annually for the current and previous two years. We analyzed the terrain features and developed three-dimensional maps showing where and when the trash had been placed in the landfill between aerial surveys. We also computed and produced an isopach (a map representing the differences between two surfaces) showing areas that were disturbed over that year’s time. With these maps, excavations could then be focused on likely areas for dumping and corresponding stockpiles.

Figure 1. An example of the source data, the rendered data and a profile view of the surveyor's data.

Case Study-Civil Suit

In this case, a land developer was sued because a driver ran through a stop sign and was hit by an oncoming car. Millions of dollars were at stake because the developer was accused of building a road that did not have the appropriate sight distance to see the stop sign over a vertical curve in the road. There was no evidence that alcohol or lack of sleep caused the accident. We were to depict exactly what the driver saw or should have seen on the night the accident occurred. The stop sign at the intersection was replaced by a traffic signal at some point after the accident and therefore had to be reconstructed digitally. Our approach was to produce renderings and animations from the driver’s eye height along the roadway. We took the surveyor’s field-located data and produced an exact, three-dimensional model of the roadway and associated features. Everything was re-created precisely from case evidence and included eyewitness and expert witness testimony, police reports, maps and survey data.

Figure 2. Perspective views are shown from the animation.
The surveyor measured the location, elevation, height and size of every feature. For instance, the location and elevation of each side of every paint stripe was identified. Three-dimensional curvature of the roadway pavement and curb sections was located in detail. Extremely accurate renderings and animations were developed from this extraordinary data. The results showed that the driver could have or should have seen the stop sign at least 800' before the intersection (See Figures 1 and 2).

Figure 3. The survey data consisted of spot shots taken throughout the building from which contours were developed.

Case Study-Survey for Differential Foundation Settlement

In this case, the foundation of a commercial building was failing. Severe differential settlement was occurring throughout the structure. It was so noticeable that a marble placed in the center of a room would roll rapidly to the wall. The surveyor collected spot shot data throughout the building. After reducing the data, the surveyor then produced a very fine interval set of contours to illustrate the foundation activity. When this was presented to the attorney for the case, a concern arose that a lay jury may not understand the concept of contouring. We were requested to prepare the data from another perspective that would attempt to illustrate the problem more clearly to non-technical people.

Figure 4. The end result in 3-D.
We processed the data in three dimensions and produced an isometric, perspective view. We then developed a horizontal plane that sliced through the foundation and computed cross sections of the resulting difference. These were then merged together and animated. The result was similar to a knife cutting through a stick of butter, except the butter did not have a flat top surface (See Figures 3 and 4).

Types of Forensic Graphics

Forensic graphics take many forms. In two dimensions, they include charts, maps, exhibits, posters and handouts. In three dimensions, renderings of both two-dimensional models and three-dimensional models may be generated. Depending on the scope requirementsforaccuracyand assumption, materials, lights, shadows, reflection, refraction, luminosity and retro-reflection may come into play. The term we use for this is “Technical Visualizations.” They are accurate, consist of verifiable data and are usually technical in content.

Included in this mix are animations, photo-montages and multi-media presentations. Animations involve frame capturing, movie formats, videotapes and CD-ROMS. Photo-montages combine raster data (photographs) with vector data and merge them together with geometric and perspective accuracy. Multi-media presentations generally combine full motion animations with audio or sound.

The advent of GIS links map data and database data to produce informative results. These may comprise shortest path traces, evacuation routes, overlays and escape routes. For a recent murder trial, we used GIS to consider the potential escape routes of a sniper who murdered a police officer in a wooded area.

There are many options for displaying models and views: 8-1/2" x 11" black-and-white or color prints, engineering plans, posters, maps, charts and graphs. Most courtrooms have ready access to a VCR, hence videotapes can be an excellent choice. Tape can be submitted into evidence easily, but the equipment to produce the tape can be costly and the skill level involved quite high. Another fairly new output type is CD-ROM and DVD. CD writers and re-writers are fairly inexpensive, but DVD writers can be pricey. CDs can be submitted into evidence easily, but the court may not have the computer to run them on. They may be better in depositions where computers may be more accessible.

The Benefits of Using Forensic Graphics

The benefits of using forensic graphics to illustrate forensic survey data have increased rapidly during the past several years. In the past, graphics used for litigation support tended to be hand-drawn depictions. Bar charts, pie charts, graphs, photographs, maps, and engineering and surveying drawings were used to graphically illustrate a point or concept. But, as our computer processing abilities increase, we can ask more of our software. As software improves we can analyze data in ever-imaginative ways.

Two-dimensional charts and graphs have given way to multi-dimensional charts and graphs. The third and fourth dimensions of depth (or height) and time are routinely added to better convey a sense of the ambient conditions. Photographs are augmented by motion pictures, VCRs and audio capability. Motion pictures now have the benefit of post-production and digital video editing. Further, by merging raster-based, aerial photographs with the vector line drawings, maps can be significantly enhanced in their ability to portray ideas. And only quite recently has the idea of viewing engineering and surveying data been augmented by animation, infinite viewpoints, flight paths, kinematics, realistic lumination, shadowing, textures, reflection and translucence. When these depictions display fact-based evidence, they can be admissible in court.

Forensic surveyors must be extremely precise in data collection and data reduction, and add no assumptions to the data set. Aside from that, they should apply a strong imagination in developing the visual aids that accompany the data. The field is wide open for suggestions.

Sidebar: Benefits of Computer-generated Graphics

  • People tend to trust computer data.

  • Many studies show that pictures can be recalled easily.

  • Pictures can relay complicated concepts at a glance.

  • Pictures are language independent and are understood by non-English speakers.

  • Pictures can summarize mass amounts of data and reduce the focus of study rapidly.

  • Fact-based information can be surrounded by subtle or ambient conditions slanted toward a specific perspective.


Sidebar: Drawbacks of Computer-generated Graphics

  • Computer models and their theories and algorithms are not readily understood and accepted by the lay public.

  • Details surrounding the creation of these graphics can confuse the issues when superfluous minutiae are brought into the analysis.

  • Facts can be routinely twisted and distorted at dizzying speeds.

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