TOTAL STATION SURVEYING TECHNOLOGY FOR FORENSIC MAPPING OF FIRE AND EXPLOSION INCIDENT SCENES

Transcription

1 TOTAL STATION SURVEYING TECHNOLOGY FOR FORENSIC MAPPING OF FIRE AND EXPLOSION INCIDENT SCENES Brian C. Dunagan, CSP, CFEI, CFII IFO Group - Incident Free Operations, Inc., USA ABSTRACT Total stations (electronic surveying equipment) are frequently used in traffic accident investigations to collect data for reconstruction specialists. For more than 20 years these devices have dramatically reduced the time and labor required to document and map vehicle accident scenes. The data collected by a total station can be easily imported into modelling and sophisticated mapping software. This technology can be deployed in other forensic applications and can be readily used to assist investigators in systematically and accurately mapping fire and explosion incident scenes. This paper summarizes the role a total station plays in forensic mapping. First, an overview of forensic mapping using total stations and associated equipment such as prisms and data collectors is presented. Second, the constraints and the legal considerations of the technology are discussed. Third, a case study using forensic mapping of an explosion scene is presented. Finally, it will be demonstrated that the use of these techniques can assist the savvy investigator in building a compelling case narrative that builds on and complements other evidence collected while satisfying the ever increasing standards for reliable and accurate documentation of scenes. INTRODUCTION TO FORENSIC MAPPING Forensic mapping is the science of documenting an incident scene using a highly accurate and reproducible methodology to document evidence, produce scene diagrams and drawings and collect data for modelling. The goal of forensic mapping is to provide a fair and accurate representation of the scene, in addition to collecting accurate data that can be used to construct computerized reconstruction models and simulations. As noted in the current edition of NFPA 921, Guide to Fire and Explosion Investigations, a critical part of every fire and explosion investigation is thorough and accurate documentation of the scene to support and verify investigative opinions and conclusions 1. Forensic mapping with total station technology as described in this paper can play a key role in the process of documenting fire and explosion scenes and provides a highly accurate reference grid for photography, videography, notes and other documents detailing scene information. The goal of this paper is to provide an overview of the rapidly expanding field of forensic mapping by examining the contributions of total stations. A survey of the equipment used will be presented and the development of diagrams, measurements, photography and other documentation techniques will be discussed as they relate to the practice of forensic mapping. This paper is offered as an introduction and general explanation of the equipment and methods used in forensic mapping with total stations. It is not a comprehensive guide to the process and related techniques. TOTAL STATIONS It is important to note that although commercial surveying equipment is commonly used in forensic mapping, investigators using total stations are not engaged in the profession of land surveying and are not required to be licensed as surveyors. A total station is a surveying instrument that combines a theodolite with an Electronic Distance Meter (EDM) to measure slope distances to a desired point. A theodolite measures vertical and horizontal angles from the instrument to the point being measured. The EDM uses a pulsed infrared light (laser) to measure the distance from the instrument to the point of interest and is combined with the angles

2 calculated by the theodolite to determine the slope distance. Many total stations available on the market today can measure distances up to 16,000 feet away by using a reflecting prism mounted on a rod and distances up to 1,600 feet away using the instrument s reflectorless mode. (See Figure 1.) Figure 1. Sokkia Total Station Total station operators can elect to use the reflectorless mode to map scenes that are compact enough to allow direct visualization of the instrument s aiming dot on individual points to be measured. A prism reflector mounted on a survey rod is usually used to map more distant points, or those that aren t directly visible to the operator. (See Figure 2.) Total stations are typically paired with a data collector, a handheld computer with software that allows operators to see the scene diagram as the data points are collected and assign descriptions to those points for later reference. (See Figure 3.) The data points collected by the total station can also be exported into scene modelling or engineering software such as AutoCAD and MapScenes.

3 Figure 2. Examples of Prism Reflectors Figure 3. Example of Data Collector

4 MAPPING WITH TOTAL STATIONS Once an investigator arrives on scene and a decision to use forensic mapping to document the scene is reached, it is important to do some planning to ensure that all of the needed data is captured. Some of the questions that should be considered are: 1. Is more than a simple hand sketched scene diagram needed? 2. Are there any vertical surfaces that require mapping to document damage patterns? 3. Is information for 3D mapping or modelling required based on the scene and related circumstances? 4. Is the scene very congested with obstacles that would hinder hand held tape measurement? 5. Is the scene very large and complex? 6. Does the scene involve a high value loss, high potential incident, serious disabling injury or a fatality? If the answer to any of those questions was yes, then measuring the scene with a total station is likely a great option for rapidly and accurately collecting the needed data for forensic mapping. Scene diagrams and maps developed from forensic mapping with total stations are constructed from points collected on scenes by electronically recording three measurements simultaneously. The theodolite measures the vertical and horizontal angles, while the EDM measures the slope distance to the point being measured. These measurements provide the information necessary for the instrument software to calculate the absolute polar coordinates of each point in relation to the instrument This allows the investigator to quickly collect the data needed to use polar or grid coordinate mapping to document points of interest and evidence locations in three dimensions relative to a fixed point, such as, the base of a utility pole or building wall floor corner. (See Figure 4.) Figure 4. Example of Polar Coordinate Map Diagrams, Drawings, and Models The use of total stations for documentation of fire and explosion scenes has a number of benefits, but probably the most compelling reason to use a total station is the ability to directly collect data that can be imported into mapping and modeling software. It is very difficult for most investigators to quickly hand draw scene maps and diagrams that meet the ever increasing expectations of jurors now conditioned to see elaborate exhibits at trial but using the appropriate software significantly reduces this burden. It

5 should be noted that total station data collection does not fully replace basic hand sketching, but enhances the quality and utility of final exhibits produced for the case file. Once evidence has been marked and photographed, it can be quickly and accurately mapped, even evidence that is located in difficult to reach areas or is irregularly shaped. An example of this is provided in the drawing below with an irregularly shaped spill pattern on the floor. (See Figure 5.) Figure 5. Example of Evidence on Flooring Surface (Irregularly Shaped Spill Pattern) The evidence in this example is easily mapped by either using the reflectorless mode on the total station or by moving a prism rod around the perimeter to mark the outer bounds of the spill. (See Figure 6.) This data can be easily uploaded into the mapping program of choice and used to create a 2D overhead drawing or a 3D model that be used to give the viewer a better understanding of the evidence location in relation to the room or building itself or to other evidence found on the scene. If adequate 3D data points are collected, this information can be used to build a Computational Fluid Dynamics (CFD) or related model to simulate the evolution and movement of overpressure, smoke, fire gases and temperature through a fire or explosion scene.

6 Figure 6. Example of Measurement Points for Evidence on Flooring Surface (Irregularly Shaped Spill Pattern) PRECISION, ACCURACY, AND ERRORS It is very common for various analyses to be performed based on measurements taken at the scene of a fire or explosion. The foundation for these analyses is premised on the assumption that proper and accurate measurements were obtained by the investigator. If any part of the methodology is later challenged and found to be lacking, the entire resulting analysis work can be subjected to a successful challenge as well. Therefore, it s very important that a scene is mapped with an appropriate level of precision and accuracy and that total station operators remain vigilant to recognize and prevent mistakes that can affect the accuracy of scene measurements. Total stations used for forensic mapping that are well maintained and calibrated in accordance with manufacturer recommendations are precision instruments with accuracy tolerances of approximately +/- 1 over 1,000 yards. 2 There are several known sources of errors that can affect the accuracy of measurements collected with total stations and these are human, natural, and instrument. Clearly, care and maintenance of the equipment, training, and proper calibration is key to collecting high quality data. Human Error The human errors associated with total station operation are related to both the station operator and the person holding the prism rod. Prism rod errors are typically caused by a failure to hold the rod plumb or to properly locate the center of the rod over the point to be mapped. Total station operators can cause small errors by not properly sighting the telescope sight hairs directly over the center of the target or prism and by failing to properly set up the instrument at the start of the scene mapping process. Natural Error or Uncontrolled Error Natural factors can affect the accuracy of measurements obtained by total stations, specifically temperature and barometric pressure. Extremes in temperature and high humidity can actually cause refraction of light and introduce small variances in measurements taken over long distances. These factors are not typically a concern for mapping of interior scenes or those of a more compact nature. Most manufacturers of total stations provide an atmospheric correction chart for high precision measurements over long distances.

7 Instrument Error The manufacturer of each model of total station has published the known accuracy error rates for their instruments and it is typically provided with the owner s manual and calibration certificate. There are well established protocols that should be utilized at the beginning and the end of mapping a scene to verify the initial and final accuracy of the instrument. LEGAL ISSUES AND CONSIDERATIONS Traffic accident investigators and reconstruction experts frequently use total stations and have for than 20 years. The captured information is then used to develop diagrams, models, and other analysis based on that scene information. All of the information gathered can be used to fight spoliation concerns and in supporting expert testimony. Based on the role scene data has in a case, it is critical that investigators utilizing total stations not only understand the underlying technology, but also be capable of clearly explaining the basics of how it works to others. Admissibility of Data Obtained with Total Stations Investigators should be prepared to address questions regarding the admissibility of data collected with total stations and how to respond to the gatekeeping factors laid out by the Frye and Daubert standards for admitting scientific expert testimony. In general, these factors include: Can the technique be subjected to empirical testing: is the theory or technique is falsifiable, refutable, and/or testable? Answer: Yes, the underlying mathematical theory is testable and can be validated by using trigonometry. In addition, the data collected on scene can be verified in real time by using the reference measurement protocol. Has the technique or process been subjected to peer review and publication? Answer: Yes, since 1991 numerous papers have been written and research studies completed addressing the use of total stations in forensic mapping. Additionally, the technology has been accepted by U.S. state and federal courts as a valid scientific process. Is the potential error rate known and is it reasonable? Answer: Yes, measurements obtained with a total station are within reasonable accuracy levels. The sources of error are known to trained practitioners of the process and are controllable. Are there standards for the application of the technique or process? Answer: Yes, the Professional Society of Forensic Mapping (PSFM) has adopted minimum standards for the documentation of scenes utilizing total station technology. Has the relevant (scientific) community accepted the theory or technique? Answer: Yes, since 1991 numerous papers have been written and research studies completed addressing the use of total stations in forensic mapping. Additionally, the technology continues to be accepted by U.S. state and federal courts as a valid scientific process. Electronic data collection must meet the same proof of documentation that is required for manual data collection and maintaining the integrity of the data is paramount. Data collected on a scene with total stations is initially stored in a raw format before it is uploaded into mapping or modelling software. It is appropriate for investigators to save and preserve this raw data file with their other investigation case

8 files so that if necessary, the underlying data can be examined and reviewed by other experts at a later date. 3 CASE STUDY 2012 PEMEX KM 19 EXPLOSION AND JET FIRE On the morning of 18 September, 2012 a massive gas release within the metering unit of PEMEX s KM 19 facility near Reynosa, Mexico fuelled a large explosion and subsequent jet fire that ultimately burned for more than 90 minutes before extinguishment. The explosion and fire killed 31 people and seriously injured another 47 people working in the facility at the time. An investigation into the cause of the incident was launched immediately and the first investigators arrived on scene later that day to assume custody of the site. A significant share of natural gas production in northern Mexico flowed through this facility for processing and distribution and it was deemed critical that the site be released back to the site owner as quickly as possible in order to restore production, while not compromising the integrity of the investigation. Clearly, detailed and accurate data was critical and the need to quickly and accurately document the site was obvious. The investigators elected to use total stations to perform the forensic mapping of the site. (See Figure 7.) On the first day of the site investigation, the team received a thorough overview of the incident s known facts from PEMEX staff; reviewed pre-incident photos and drawings of the facility; completed a safety briefing; ensured that all incoming and outgoing process streams and other energy sources, such as utilities, were secured; and formulated a work plan for the conduct of the site investigation activities. The outer scene perimeter for the initial search and survey was identified and the investigators were briefed on protocol for marking, documenting, and recovering evidence. High priority was placed on documentation and recovery of failed piping and process components that were displaced from their installed locations within the perimeter of the primary investigation scene, specifically identified as the metering pad area across from the tank farm areas. The areas between the outer boundary of primary scene perimeter and the outer boundary of the secondary scene perimeter boundary were searched in a grid pattern utilizing investigation team members, PEMEX staff, and members of local security forces available at the site. A large number of failed process piping sections and components identified during these searches were documented and marked for recovery and mapping. The incident scene was thoroughly recorded, documented, and mapped by use of photography, hand sketching, and total stations. This site presented a number of challenges to mapping that included: Surface area of the overall scene (approximately 530 acres). Multiple total station system moves due to visual obstructions on the site. Uneven terrain and elevation changes throughout the site. Very large number of evidentiary items marked and recovered from the site to include failed process piping and components, human remains, personal protective equipment, electrical components, and metallurgical samples collected from undamaged piping as exemplars. All work had to be completed during daylight hours due to concerns about the security of the general area.

9 Figure 7. Overhead View of Pemex KM 19 Fire and Explosion Scene Despite these challenges, all forensic mapping activities on the site were completed within 5 days of the date of the accident; a task that would have previously taken investigators several weeks to accomplish with ladders, man lifts, tape measures, measuring wheels, and handheld electronic distance measuring devices. The data collected during the mapping activities allowed the investigation team to build accurate 2D and 3D computer models which were used to re-construct the events immediately following the piping failure to include modelling of the gas dispersion, ignition of the cloud and the subsequent deflagration explosion and jet fires. The mapping data was also used to develop other exhibits, diagrams, and maps suitable for use in the civil and criminal cases that followed the completion of the investigation. CONCLUSIONS Total station technology is a valuable tool for fire and explosion investigators faced with the need to map large or complicated scenes and provide accurate data for the building of scene diagrams and models. The appropriate use of these techniques can further assist the savvy investigator in building a compelling case narrative that builds on and complements the other evidence collected while satisfying the ever increasing standards for reliable and accurate documentation of scenes. Total stations are not a replacement for basic scene sketching and other scene evidence documentation methods but do represent an ideal way to improve the accuracy and precision of forensic mapping while easing the investigator s workload on and off scene. Large fire and explosion scenes are also generally highly complex and present many challenges for accurately and quickly cataloging victim locations, photographs, evidence, safety concerns and reconstruction data. Forensic mapping with total station technology can play a significant role in helping the investigator address these issues and challenges quickly and effectively.

10 ABOUT THE AUTHORS Brian C. Dunagan, CSP, CFEI, CFII is a Principal Consultant with the IFO Group and is a Certified Safety Professional and a Certified Fire and Explosion Investigator and Instructor. He is also a Professional Member of the American Society of Safety Engineers with inclusion in the Fire Protection and Oil and Gas Practice Specialties. Mr. Dunagan has a record of successfully leading and managing high profile / high value loss incident investigations involving process related explosions and fires, process equipment failures, human factors / errors, and acts of terrorism / sabotage. In addition, Mr. Dunagan is a frequent industry event speaker and has been recognized as an expert witness for cases filed in state, federal and international courts. ENDNOTES 1 National Fire Protection Association, NFPA 921, Guide to Fire and Explosion Investigation, 2014 Edition 2 Sokkia Corporation, (2012), CX Series Operator s Manual, Tokyo: Sokkia Corporation 3 Boots, K. & Salinas J. (2010), Fundamentals of Forensic Mapping, Rocklin: Kinetic Energy Press