High-Speed Scene Documentation: 7 Key Questions to Ask Before Buying a Laser Scanner for Your Agency
Law enforcement agencies and crime labs are increasingly adding 3D laser scanning reality capture to their technology tool box, and for good reason. Laser scanners capture comprehensive data on scenesthrough 360–degree images and millions of accurate measurement points collectively known as point clouds, allowing investigators to document scenes quickly and analyze evidence long after the incident occurred. But as the popularity of the technology has accelerated, so, too, has the proliferation of scanner types and brands. A quick search online or in social media pulls up a number of different options, with various claims regarding speed, accuracy and quality. How can you determine which scanner is the best fit for your agency?
A little knowledge on the key considerations in choosing a laser scanner can go a long way toward helping you make a wise investment that will make your agency more efficient and more effective for years to come. Here are seven questions to ask before you make your purchase.
1. What is the scan speed, and does it includeHDR image capture?
One or two million points per second sounds impressive, but what does that mean in real-world terms? What happens to the scan speed when you want a full-dome image capture of a scenealong with the scan data? What if you need full-dome high-dynamic-range (HDR) imagery to see minute details in shadows or bright light conditions – how does that affect the speed of scene documentation?
Knowing the field performance of ahighly rated laser scanner can be useful for comparison. For example, the Leica RTC360 laser scannerscans at two million points per second (pps) and captures a full-domeHDR image in one minute. This meansyou can complete a full high-resolution scan (3 mm) of your scene at a 10-meter range in less than three minuteswith trueHDR imagery. Running the scanner at maximum speed without HDR images allows you to complete scans in less than 30 seconds.
True vs. laser HDR: If you plan to use HDR as part of your scanning procedure to document your scenes, be sure to check the source of the imagery. With real HDR imagery, high-resolution cameras collect high-quality source images that are overlapped so they can be blended together easily. Some laser scanners combine non–HDR imaging with laser intensity values to create more contrast in the colorized point cloud and a panoramic “HDR” image. While this approach can speed up HDR image capture in the field, it slows down processing in the office when working withthe high–resolution scans most often required for crime and crash scene investigations. Laser-based HDR images also lack natural colors, which can be important to the investigation. When evaluating a laser scanner, look at how the scanning speed translates to the work you do every day.
2. What is the data qualityof the scans at high speed?
Data quality is important in completing investigations, closing case files and creating courtroom visualizations, but it’s not always an easy parameter to measure when comparing laser scanners. To get an accurate assessment of data quality, ask for a high-resolution demo scan of an area at a 10–meter range that includes complex surfaces, and evaluate the data on the following key points:
Completeness.Is any data missing? Focus especially on areas where the angle of incidence is flat; the data should be just as complete here as in other areas of the scan.
Cleanliness.The data should be clean and crisp, with no invalid points “floating” in the scan data.
Geometric Accuracy.The geometry of scanned objects should be correct. Watch for rounded edges.
Range Noise. On some laser scanners, achieving clean, high-quality scan data with low range noise requires significantly slower scan speeds.A laser scanner is not really high speed if it takes hours to complete a high-quality scan. Check to see if the laser scanner requires you todetermine the amount of noise acceptable in your data and then choose the corresponding quality setting. If so, be aware that this process will add time and affect the geometry of your resulting scans. A better approach is to use a laser scanner thatdelivers the best quality automatically, without adjustments and without compromising scan speed.
Mixed Pixel Filtration.How well does the scanner filter out invalid points (mixed pixels)? Look for groups of invalid and unfiltered points in the data. The data should be clean and well filtered.
3. What is the image quality of the scanner?
HDR images should be photo quality. Images that are too dark or too light can obscure important scene details. To evaluate image quality,compare images from the scanner with images captured by a high-quality DSLR camera.
4. How is accuracy determined?
Information on accuracy is easily found on spec sheets, but more important than the numbers is how that accuracy is achieved. If the scanner requires onsite compensation, this has to be done at every scan position. Keep in mind that 3D positional accuracy is only known if the angular accuracy is known. Without true onsite compensation, you are scanning with unknown 3D positional accuracy. Also be aware that if measurements are only done before and after the scan with an average tilt applied to the complete scan during import, this is tilt measurement, not real-time dual-axis compensation. Be sure you understand how the accuracy numbers for the scanner are determined before jumping into your purchase.
5. How does the scanner handle data management?
High-speed, high-quality laser scanning gives you the ability to capture a lot of scene data very quickly. If the scanner relies on an open-format SD card to store and transfer this data, you will quickly max out your storage space.SD cards are also limited in temperature stability. Look for a data managementsystem that enables you to store hundreds of gigabytes of data, transfer hundreds of megabytes of data per secondand can withstand extreme low and high temperatures.
6. How does the scanner perform in-field registration?
If the scanner provides in-field registration, what does that process look like? Is it based primarily on software, or does the scanner hardware also play a role? Ask about the computing requirements – can you use an iPad in the field, or do you have you use a high-performance tablet or notebook? Can you optimize the registration and enrich the scan data by adding geotags and other information, or does the software limit your interaction with the data? Can you track the live position of the scanner in the field and visualize the scan data to make sure you’ve captured everything you need before you leave the scene? How long does it take to process the scan data before it’s registered?
Keep in mind that automatic in-field registration is only as good as the complete data flow of the system. A software-based system that isn’t optimized for data processingcan create bottlenecks in your scene documentation workflow as you wait for the system to register scans, especially when scanning in color at high resolution. Without the ability to perform some manual adjustments, you might have to rescan some setups if they don’t register correctly in the automated process. To optimize your efficiency and results, look for a scanner with an in-field registration system that integratessoftware with hardware, such as internal cameras and an inertial measurement unit (IMU).
7. How does the scanner integrate with other reality capture and digital scene mapping solutions?
The need to capture complex and sometimes inaccessible scenes is often critical to support investigations. Whileagencies increasingly consider high-speed 3D laser scanning to be a critical piece of reality capture technology, it’s not the only tool for this purpose. Easy-to-use and highly portable terrestrial laser scanning systems that operate at the touch of a single button provide a simple way to move from 2D to 3D workflows. Mobile mapping systemsenable reality capture of large-scale environments. Unmanned aerial systems (UAS, also known as drones or UAVs) provide a way to capture alternative scene perspectives and provide access where physical entry is restricted. Innovative GNSS RTK rovers enable you to perform photogrammetric measurements on the fly. And purpose-built software solutionsstreamline investigative efforts by turning digital data into accessible visualizations and intelligent diagrams and maps.
Do you need a 3D laser scanner? Or do you need a clear and effectivepathway to the future of digital scene mapping and documentation?
Our consultants can help you make sense of reality capture so you canmake the best decisions for your agency. Contact us today to discuss your needs.