What Is the Raw Data

In the forensic mapping process a total station is most often used to measure the physical evidence, roadway geometries, crime scene information and other important details. There are a number of manufacturers of total stations. These stations may be infrared stations requiring a prism or newer models that also use lasers to measure directly to the evidence being collected without the need for a prism. Irrespective of the total station type, they are all constructed with two basic components – the theodolite and EDMI. The theodolite is that portion of the instrument that measures angles. The EDMI, short for Electronic Distance Measuring Instrument, which obviously measures distance.

Every point of geometry that is measured and recorded is comprised of three elements. The theodolite measures the horizontal angle, usually referenced to magnetic north, and the vertical angle. These angles may be referred to as observations in some systems but the terms angle and observation are used synonymously. The third component of the geometry point is the slope distance that is measured by the EDMI. These three pieces of data comprise the geometry for each point that is measured. Many of the software systems allow graphical attributes to be assigned to each geometry point that is measured. These graphic attributes can cause lines to be drawn between points or symbols to be associated to points. These attributes do not in any way change the actual geometry or location of the measured point. Usually, the point number is automatically assigned as each piece of geometry is measured. Collectively, this information forms the raw data associated to each point in our map.

Why Is Raw Data Important

Each raw data point that is measured is typically stored in some type of data collector. These devices vary from PDA's, data collectors, to some systems that store data on the total station itself. Eventually all the raw data is taken from the data collector and transferred electronically into a computer for processing with a variety of available software. Some software programs have the ability to display raw data while others do not. One software, MapScenes Pro tracks every raw data point and records any changes to any part of the data point to create a permanent history or record for the point. This type of software feature greatly enhances data security and integrity. When taking the measurements we can compare the data on the total station with the data the data collector is recording to verify the accuracy of this stage of the process. Once this raw data is captured and recorded it becomes the basis for all the coordinate calculations that the CAD or mapping software uses to draw the map.

The question becomes, particularly for court, how we know or have verified that this data conversion from the raw data measured in the field by the instrument to the coordinate data calculated and used by the computer software has been done correctly. The only way we can verify this calculation process is to use the raw data. Thus, the importance of the raw data is simple. It is the only way to validate the accuracy of the project.

What Should We Be Doing With the Raw Data

Since the raw data is the only way to validate the map accuracy, it should be clear that we should be using it to do just that. Through basic trigonometric and mathematical functions we can validate the calculations performed by the computer with a scientific calculator. Generally, about ten percent of the points should be randomly selected and validated for a given project. Starting with the slope distance, vertical angle, horizontal angle (the raw data), total station and prism target heights that we measure, and our initial elevation and coordinates we can validate every point to make sure our computer software has made the correct calculations and properly positioned each geometry point on our scale diagram. We can validate any point to make sure our computer software has made the correct calculations and properly positioned each geometry point on our scale diagram. This is accomplished with the slope distance, vertical and horizontal angle, ( raw measured data ) total station and prism target heights that we measure along with the station coordinate and elevation that we've assigned. Specific instructions on how to perform these calculations can be found in the article by Steve McKinzie titled “Forensic Mapping Challenge, Part II”.

The MapScenes software offers redundancy in the form of data protection. Not only is every point time stamped but a copy of original data is archived to your hard drive.

Conclusion

The raw data is the information obtained from the total station for each physical point that we measure at our scene. Our mapping software then takes this raw data and calculates coordinates and an elevation for each point that it displays in the form of a map. Depending on the software it may even connect lines and insert symbols based on the graphical attributes assigned to each point so that we have a finished product scale diagram of our scene. We must check these calculations in order to verify that our computer software has correctly performed these calculations and our points of geometry are where they are supposed to be. If we do not check these calculations, then we are making the assumption that the points are where they should be when in fact they may or may not be. When these validations are properly performed, we can testify in court that our map is in deed, a fair and accurate representation of what we observed at the scene. You should feel comfortable adding your name and “to scale” as annotation.

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