SLAM (Simultaneous Localization And Mapping) technology has revolutionized the creation of rapid 3D models by allowing for faster surveying with far lower costs even at greater frequency. Interestingly, one of the things that makes SLAM great for mobile reality capture -- no requirements for stationary scans, prior maps, or GPS -- also makes it vulnerable to uncertainty, or "drift" in the accuracy of a final 3D model. However, GNSS (Global Navigation Satellite System) technology, along with RTK (Real-Time Kinematic) technology, can address this shortcoming while not sacrificing the speed and flexibility of SLAM mapping.
Integrating these two technologies can improve your SLAM mobile mapping workflow and allow you to tackle more complex surveying scenarios without the need for completely new solutions or training. To help you get the most from this data fusion, we’ll outline how these technologies can be combined for different surveying tasks, along with the benefits.
SLAM technology uses sophisticated sensor fusion algorithms to localize a robot's position and orientation as it moves through an environment while also capturing a map of the entire environment. For every movement the robot makes, SLAM checks it against the unique geometric features in the environment and uses that to understand where it is. Then while its moving it saves all the data to build a map. This is where the “simultaneous” in SLAM comes from. The unit is mapping while it’s also determining its own position simultaneously, allowing for much faster surveying compared to stationary LiDAR (Light Detection And Ranging) units without SLAM.
However, SLAM uses relative position to create each map, essentially the coordinate frame of the scanner, in this case Nexys. This can sometimes cause an issue with data accuracy depending on the area that you are scanning and the 
acceptable tolerances for your industry/use case. However in most cases surveyors can simply georeference a SLAM point cloud into an existing coordinate frame and align the Nexys data with a real-world dataset.
Now with the added benefit of GNSS/RTK receivers like the Trimble DA2, surveyors can equip one while capturing data with Nexys to add a GPS data stream into the sensor fusion pipeline. This adds further accuracy constraints for when the final point cloud/3D model is rendered.
GNSS (or GPS) provides georeference location data for all types of applications, from driving directions to advanced military navigation. However, GPS is often limited by accuracy. For directions while driving, GPS signals alone work great to get you from point A to point B, but when placing a series of pylons for a bridge or doing more accurate mapping work for large scale construction, it’s simply not 
accurate enough.
Inertial GNSS/RTK technology fuses the onboard inertial systems within a SLAM unit like Nexys with an external RTK receiver (like a Trimble DA2 (seen right)). The RTK receiver constantly corrects the GNSS signals to within centimeter accuracy, and then relays that corrected information to the mobile mapping unit which stores that information along with the data capture.
With Nexys, our proprietary ExynAI sensor fusion algorithm combines all of the inertial, GNSS, and RTK data throughout a data capture and uses it during post-processing to provide further map constraints.
SLAM mobile surveying technology works best in feature-rich environments that have a variety of angled surfaces. The more angled surfaces, the more light that can be reflected back to the SLAM unit, and the more unique geometric features the robot can "see" as it moves throughout an environment. Each point can be compared against each other by the SLAM algorithm to maintain accuracy and double-check for accumulated drift.
However, in more sparse environments with fewer geometric features, drift can increase and overall accuracy can start to decrease if the scan persists for long enough.
Using GNSS/RTK receivers in these sparse environments allows you to maintain the speed and flexibility of mobile SLAM mapping while still capturing a centimeter-accuracy 3D model.
Learn more about using Nexys + Trimble DA2 for digital elevation modeling.
A core design philosophy behind our Nexys systems has always been to provide the most modular and flexible mobile mapping solution possible. The addition of GNSS/RTK integration through the Trimble DA2 receiver builds on that philosophy to give you even more freedom to tackle a variety of surveying missions without the need for multiple systems.
GNSS/RTK receivers require a direct line of sight to the open sky so they can maintain communications with GNSS/RTK signals. This makes them perfect for outdoor environments, which is why Nexys is also capable of processing the onboard DJI GNSS/RTK data stream for further map constraints .
However, indoor environments or GNSS/RTK denied areas can pose a challenge. Our Nexys has industry-leading accuracy and autonomous capabilities even when outside signals are not present, such as indoors or below ground. This allows you to choose the best surveying method for the task at hand while still ensuring you get the most accurate 3D twin possible.
If you want to learn more about how Nexys with RTK integration can significantly accelerate surveying while also increasing accuracy, book your personalized demo today or submit your information below for a personalized quote.
