The Critical Role of Ground Control Points and Checkpoints in Drone-Based Scene Investigations

Drones have become transformative tools in crash and crime scene investigations. They capture sweeping aerial imagery, speed up data collection, and keep investigators safe. But without two critical components—ground control points (GCPs) and checkpoints—that high-tech footage can fall apart in court.

“Without ground control points and checkpoints, all you have are pictures,” says retired South Dakota Highway Patrol Sergeant Kevin Kinney, who now serves as a dealer manager for Leica Geosystems. “It’s not a problem until you go to court and get lambasted by a defense attorney, or until someone who should be in jail walks free because of a technicality. It happens all the time.”

The lack of ground control points and checkpoints can impact the admissibility of your data due to inaccurate evidence and the inability to validate its accuracy. This affects not only the legal outcome but also the reputation of law enforcement.

Kinney warns, “It puts a black eye on the law enforcement community.”

How Ground Control Affects Drone Data

GCPs are marked locations on the ground with known geographic coordinates, measured with high-precision GPS/GNSS equipment. Strategically placed throughout the investigation area to cover the full extent of the scene, GCPs are used during the processing of drone imagery (e.g., photogrammetry) to align the model to real-world coordinates and create an accurate representation of the scene.

“Without ground control, your accuracy in the Z [geometric height] goes way off,” Kinney explains. “As a result, that flat surface in the end may look more like a banana.”

Ground control prevents inaccurate elevation data from distorting the reconstruction of crash or crime scenes and compromising the integrity of the evidence. Ground control also enables the integration of data from multiple sources, such as UAVs, laser scanners, and total stations, providing a comprehensive analysis of complex scenes.

“Despite its importance, many agencies fail to implement ground control effectively. Often, they simply don’t know it’s needed. In some cases, the agency might already be using drones with laser scanning and other scene documentation technology, and they’re just starting to integrate data sets. Other agencies might look to drones as their first piece of technology in creating a crime lab or crash team.

“Awareness is the No. 1 factor,” says Kinney. “But once you know you need ground control, you need an easy, inexpensive way to achieve it.”

Ground Control Points and Checkpoints: A Dual Approach to Accuracy

While GCPs align drone imagery with real-world coordinates, you also need a way to independently validate the accuracy of reconstructed scenes. This is accomplished with checkpoints.

Like GCPs, checkpoints are surveyed points with known geographic coordinates. However, checkpoints are not used during the imagery alignment process. Instead, after processing the imagery, you can use the checkpoints to compare the coordinates and distances in the photogrammetry model against their known accuracies and distances from the GNSS rover.

Many good photogrammetry software solutions will do this automatically and provide error metrics, such as root mean square error (RMSE), which assess both horizontal and vertical accuracy. A basic comparison between the known values of the checkpoints and the position of the checkpoints in the photogrammetry model is a perfectly acceptable method for validating the accuracy of your model.

New GNSS Smart Antennas Provide the Ideal Solution

GNSS technology provides 3D data in a global coordinate system, which puts your data “on the face of the Earth right where the incident happened," Kinney says. This geospatial reference facilitates the overlay of maps and integration with other data sources.

GNSS is also fast. “You turn it on, get a network connection and RTK corrections, and then collect the points,” Kinney says. “You can be done in under 10 minutes.”

Caption: William Henningsen of Collision & Crime Forensic Solutions demonstrates using the Leica Zeno FLX100 plus smart antenna for ground control in crime and collision investigations. The antenna is simple to use, durable, reliable, and cost-effective, making ground control accessible to any agency.

While GNSS technology is not new in law enforcement—it has been used to map collision scenes for more than a decade—a new generation of lightweight, easy-to-use GNSS smart antennas is bringing the technology back into the spotlight for its ability to make ground-based GNSS measurement simple and cost-effective.

What About RTK/PPP Drones?

Real-time kinematic (RTK) drones have an onboard GNSS RTK receiver that corrects image location in real time as they fly. Post-processed kinematic (PPK) drones have an onboard GNSS RTK receiver that corrects image location after the flight by attaching geocoordinates to each image. While each can provide accurate results without GCPs in some instances, there are some significant considerations.

One is that the RTK positions are embedded in the imagery's metadata or added as a companion file. Firmware and software updates can jeopardize the compatibility of RTK drone images with your photogrammetry software.

Another consideration is that corrections with RTK drones are much shorter in duration, much less frequent, and less reliable than RTK GNSS data on the ground. Drones can remain in RTK float for long periods, and many images might not be as well-positioned as you think.

Additionally, connections with existing RTK correction services can be very complex or unavailable. A proprietary GNSS base station and proprietary correction networks may be the only option.

If you choose to use an RTK or PPK drone without GCPs, be aware of the limitations, risks, and supplemental hardware and subscriptions that might be necessary to make it work. Even with RTK/PPK drones, well-distributed GCPs provide the most reliable accuracy.

And you will still need checkpoints. “Checkpoints provide vital quality assurance that the RTK data from the drone is reliable and can be trusted against good GNSS data on the ground,” Kinney explains.

What to Look for in a GNSS System for GCPs

When selecting a GNSS system for GCPs and checkpoints, there are several key features to consider to ensure optimal performance and reliability.

• Ease of Use: Single-button operation and compatibility with iOS and Android devices, as well as third-party software platforms.
• Durability: Robust construction with an IP65 rating for all-weather operation.
• Reliability: A pole mount for consistent data collection and long-lasting batteries for extended field operations.
• Efficiency: Tilt functionality for faster data collection without leveling the pole for each shot.

Kinney says affordability is also a factor. At the right price point, “you can put an antenna in every reconstructionist patrol vehicle and have that technology available to them at any scene they need it.”

Court-Ready Evidence, Every Time

For crime and crash scene investigators who use drones, understanding and implementing ground control points and checkpoints is crucial for ensuring the accuracy of data collected during investigations.

As Kinney emphasizes, it’s not enough to have compelling visualizations. “The data has to be correct and verifiable to stand up in court.”

By adopting reliable technologies and the correct procedures, you can enhance the precision of investigations, ultimately improving public safety and maintaining the integrity of the law enforcement community.

CONTRIBUTORS

Name: Kevin Kinney

Job title: Dealer Manager

Company: Leica Geosystems

 

Biography: Kevin Kinney is a former sergeant with the South Dakota Highway Patrol, where he served for nearly 20 years before retiring in 2023. He was director of the agency’s Crash Reconstruction Programs for more than 12 years and was instrumental in launching the Crash Assistance Program (CAP), which assists individuals and families impacted by motor vehicle crashes. Kevin also served as the assistant coordinator for the Officer Wellness Program, helping oversee the Peer Support and Critical Incident Response Team. Kevin is an ACTAR accredited traffic accident reconstructionist holds a Bachelor of Science degree in Mechanical Engineering from South Dakota School of Mines and Technology. Currently, Kevin applies his leadership skills and technical knowledge as a dealer manager for Leica Geosystems, continuing his passion for safety and innovation in the field.

Name: William Henningsen

Job title: Forensic Manager, Equipment Consultant and Trainer

Company: Collision & Crime Forensic Solutions

 

 

Biography: William Henningsen has been with the Omaha Police Department’sForensic Investigations Unit (FIS) since 1996. He has been involved in training and consulting with Collision & Crime Forensic Solutions (CFS)since 2015. He is currently certified as a Forensic Video Technician by the Law Enforcement and Emergency Services Video Association (LEVA). William is certified by Leica Geosystems as a Forensic Laser Scanner and has been previously certified as a Crime Scene Analyst by the International Association for Identification (IAI). He serves as a board member of the International Association of Forensic and Security Metrology (IAFSM) and has been a member of that organization since its founding. William is also a member of the Professional Society of Forensic Mapping (PSFM).