Terrestrial Mobile LiDAR Scanning (TMLS) and Static Scanning Combined – Part 1

Much has been written, debated and discussed about the pros and cons of both Terrestrial Mobile LiDAR (Light Detection and Ranging) Scanning – TMLS and Static Laser Scanning. This is especially evident in the world of transportation survey. However, less information seems to have been presented on the topic of combining these two technologies for documentation of buildings and structures. While each method has its advantages and disadvantages, it begs the question; When would combining these technologies be advantageous for documenting buildings and structures and when would it not?

Why Use Terrestrial Mobile LiDAR Scanning?

The Mobile LiDAR industry is emerging at a rapid pace. Within only a few short years, mobile scanning has come to represent the promise of significant gains in the speed of data capture, increased productivity, accuracy and efficiency, and substantial improvements in safety. As a result, considerable cost savings in data acquisition can be expected.

The speed of data acquisition of Mobile LiDAR compared to Static Laser Scanning and/or traditional survey methods is significantly faster. Whether the mobile system is mounted on a car, truck, railroad high-rail, a cart, a backpack, etc., the rate of data capture is astonishing. Project delivery times can also be cut substantially. In addition to quickly gathering tremendous volumes of point cloud data, mobile scanning instruments are often paired with digital video imagery to create colorized point clouds or color-fused, LiDAR mesh imagery.

Since mobile units are by nature mobile, they do not require the same labor intensive setup-scan-reposition workflow that static scanners do. As a result mobile systems are very efficient and can be expected to result in a tremendous increase in productivity. (Photo below courtesy of Applanix)

Mobile LiDAR surveying techniques also provide increased safety for surveyors and the general public since data can be acquired remotely, day or night. Mobile scanning methods allow a surveyor to get off site much faster. The reduction in the number of hours spent surveying means a smaller chance for an accident to occur. A reduction in time also means cost savings through lower labor costs, lower travel costs and fewer mobilizations.

Why Use Static Scanning?

While mobile scanning has many advantages over static scanning, the same can be said about static scanning over mobile scanning. Static scanners can be used in many places mobile scanners cannot. They are also typically capable of higher resolutions, have improved positional accuracy and require a significantly lower capital investment.

For documentation of buildings and structures, static scanning is often the tool of choice simply due to the fact that mobile systems are limited to where they can be positioned. Crawl spaces beneath the floor, stairs, above ceiling plenum spaces, rooftops, congested mechanical rooms, facades blocked by landscape, etc. all can prove to be difficult or impossible for a mobile unit to access.

The density of the point cloud data obtained with static scanners is generally greater than that of mobile units. This can be very important when documenting buildings and structures. Structural connections, complicated MEP (Mechanical, Electrical, Plumbing) systems, ornate architectural elements as commonly found on building facades will often require very detailed data in order to represent things properly.

Static Laser Scanners dont change position while scanning, thus they do not require the same sophisticated systems to maintain their position as mobile scanners do. Mobile systems must rely on GPS (Global Positioning System) and an INS (Inertial Navigation System) which adds significantly to the complexity and cost of such systems.

While each of these technologies has its advantages and disadvantages, what can be learned by looking at two real life projects where both Mobile LiDAR and Static Laser Scanning were combined to document structures? The first project, H Street Bridge, was a bridge and tunnel renovation for a transportation project in Washington, D.C. The second project Industrial Arts Building, was for a HABS (Historic American Building Survey) of an old abandoned Industrial Arts Building located on the grounds of the Nebraska State Fair in Lincoln, Nebraska.

(To be Continued in the Next Newsletter)