Interpreting Marine GPS Data Expert Article

A version of this article was published in the ABA TIPS Admiralty & Maritime Law Spring 2022 Newsletter. The PDF is available for download at the link on the side of this page.

GPS units and chartplotters potentially contain a myriad of useful information for the reconstruction of marine mishaps. Our experts can export, interpret, and analyze this data within the context of specific events to reliably answer questions involving speed, location, and many other relevant issues.

This article provides an introduction to marine GPS navigation data as it relates to litigation and insurance claims. Concepts explored include how experts use data to determine vessel locations and operator actions/inactions of interest such as slow speed zone transits, waterfront bar/restaurant visits, and evasive maneuvers.

Marine GPS Forensics Expert Witness

Interpreting Marine GPS Data - Expert Article

Track Lines

A track line is a recording of where a boat has been. As the boat moves, it generates track points which can be viewed on a chartplotter. The series of track points create a track line like a trail of breadcrumbs. Tracks can range from just a few, to thousands of points and hold navigational data that can be critical to an investigation.

Track points may provide detailed insight into operator behavior such as compliance with navigation rules, speeds, distances from shore and provide insight to water depths, obstructions and/or navigation aids such as beacons, lights and markers. Did a boat operator turn to port (left) or starboard (right), did a vessel slow down, speed up, maintain speed, or remain stationary? Did an operator observe water depth? If the track line shows the vessel going aground, or over water too shallow for the operation, it suggests the operator was not paying attention to advertised water depths. Whatever your marine incident case may be, GPS data retrieved from chartplotters can provide critical insight and evidence.

Fig 1: The blue arrows in this extract are track point and may provide navigational insight to historic proceedings.


A waypoint is a planned geographical location which has been entered into a chartplotter. Waypoints, when connected together, form course lines and display an intended planned route or points of interest such as a destination. Planned course lines may be compared to track lines in order to gain an understanding of intent and actual proceedings. Waypoints may be also used for other purposes such as marking an SOS or MOB (man over board) location.

Fig 2: A planned route formed by numbered waypoints, connected by course lines.

Additional Chartplotter Features & Insights

Many chartplotters provide features which can be great tools for vessel operators. Boat operators should be familiar with all available navigational equipment and their use. The Navigation Rules for International and Inland Water references general marine equipment use in collision avoidance:

“Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists.”

Additional devices may be integrated into chartplotters and provide valuable retrievable data. For the purposes of a forensic investigation, you may be able to streamline your team by retaining a marine operations expert who can download the data and interpret operator actions.

Some examples of systems integrated in chartplotters are integrated RADAR (Radio Detection and Ranging) systems and depth finders. A thorough review of RADAR data may provide insight as to whether a collision alarm was utilized while an integrated depth finder may provide insight in to the settings and proper use of a shallow water alarm.

RADAR can be a great tool for boaters and may provide excellent features to assist in the safe proceedings of a vessel. When a RADAR system is installed, and (if applicable) integrated into a chartplotter, boat operators are required by the Navigation Rules to properly use these devices:

The Navigation Rules for International and Inland Water reference proper use of RADAR equipment:

“Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent observation of detected objects.”

GPS forensics may provide data beyond what is electronically downloadable from a chartplotter and requires a holistic approach to the treasure trove of system settings and data.

For more detailed information on how marine GPS data is extracted, preserved, and analyzed, read our article on Preserving Marine GPS Data.

Maritime Incident Investigations

The Maritime practice at Robson Forensic is made up of experts from every facet of the industry, from mariners to marine engineers, naval architecture and with experts with on-board and on-shore maritime backgrounds. They are frequently retained to investigate collisions and other mishaps that occur on and around the water. From commercial shipping to recreational boating and everything in between, the maritime industry is multi-faceted and diverse. Our experts have the education, training, and hands-on experience to investigate the matter at hand.

For more information, submit an inquiry or call us at 800.813.6736.

Featured Expert

Hendrik J. Keijer, Marine Operations Expert

Hendrik J. Keijer

Marine Operations Expert
Captain Henk Keijer is a retired Master Mariner and Marine Engineer. For more than 30 years Captain Keijer operated a variety of vessels ranging from cruise ships to cargo ships and smaller boats. He… read more.


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