Laser Scanners Improve Survey Workflow

3D Scanning is a powerful technology that uses advanced laser measurement technology to obtain measurements at many thousands of points per second. Surveying professionals are looking to adopt this new technology due to the dramatic productivity benefits that can be obtained. However, until now, surveyors have cited the lack of scanner versatility together with unfamiliar workflows as limiting factors in the widespread adoption of the technology.

Today, innovative laser scanning technology is providing new options for surveyors. By providing increased portability and robustness as well as a workflow more similar to a surveyor's instrument, laser scanning technology now available is enabling professionals to easily incorporate 3D scanning into their business portfolio to obtain greater productivity.

Today, 3D laser scanners for the survey industry use two main technologies, either phase-shift or time-of-flight measurement. Phase-shift technology works by sending a laser beam with a sinusoidal wave into a rotating mirror's center, which deflects the laser around the environment being scanned. The beam is then reflected back into the scanner and the "phase shift" of the laser is measured giving the distance of the laser from the object. Using encoders to measure the mirror rotation and the horizontal rotation of the laser scanner, the X, Y, Z coordinates of each point can be recorded. Phase-shift scanners are limited to shorter distances, up to about 10 feet for optimal use.

Time-of-flight technology is based on the principle of sending out a laser pulse and observing the time taken to reflect from an object and return to the instrument. Advanced electronics are used to compute the range to the target. The distance range is combined with angle encoder measurements to provide the three-dimensional location of a point. These scanners are able to capture measurements at long range, to more than 100 feet, and are the most popular with surveyors.

Time-of-flight technology is similar to the Direct Reflex (DR) or reflectorless technology used in many of today's most advanced total stations. However, the difference between 3D scanners and DR (reflectorless) technology used in total stations is the speed of measurement. Total stations can provide approximately four distances per second in DR mode. In contrast, time-of-flight laser scanners are capable of measuring up to 5,000 distances per second!

The ability to accurately position objects at more than 1,000 times the speed of a total station allows a 3D scanner to quickly produce large amounts of survey data. This data, commonly referred to as a "point cloud," can provide a three-dimensional shape, or visualization, of the feature being measured.

For surveyors who are more familiar with measuring discrete points to identify a feature, the sheer amount of data that can be produced from a 3D laser scanner may seem overwhelming. However, a 3D scanner is still providing 3D positional information in a similar way as a total station. The main difference is that the speed of measurement allows a 3D scanner to provide more positions in a shorter amount of time. This ability allows users to either significantly reduce survey field time or to collect a denser amount of points, which results in more accurate detail of the survey site.

In a similar way to early GPS technology, the first commercially available 3D scanners were generally used for specialized applications rather than typical survey tasks. As the technology has become more accessible and the benefits of such fast data acquisition have been realized, surveyors have started looking towards 3D scanners as a new tool for the future. As the interest by surveyors has grown, manufacturers have begun offering 3D scanning technology that is simple and efficient to use. Today, new 3D scanning technology is available that provides a workflow designed specifically for the surveying industry.

Laser Scanning Workflow

To understand the importance of a survey workflow with a 3D scanner it helps to first understand the workflow that has typically been used with a scanner.

The traditional scanning methodology is to use measurements to a number of common targets or spheres to relate multiple scans together or to relate measurements to an existing control network.

This workflow involves placing the scanner at locations about the survey site and measuring to a number of targets (either flat or spherical), as well as the actual feature of interest (e.g., a building). The scanner is then moved to the second location and at least three common targets or spheres from the first scanner location are measured (Figure 1).

The common measurements to the targets or spheres are then used to relate the scans together in a manner very similar to a GPS calibration workflow. This process is typically performed in the office once the field data collection has been completed (Figure 2).

While the scanning methodology is suitable for many applications, it does have limitations:

  1. the scanner cannot be easily set up over a known point or directly related to a known point;
  2. measurements have to be observed to multiple targets from each scan location, which requires careful survey planning and additional occupation time to measure each target or sphere;
  3. multiple scans require field postprocessing to relate the scanned data together into a single homogeneous data set and to relate the data to an existing control network.

These limitations can be efficiently resolved and simplified by using a traditional total station survey workflow (Figure 3), whereby the user:

  • accurately positions the instrument on a known ground point;
  • accurately levels the instrument using a plate bubble or electronic level;
  • measures an instrument height to relate the location of the instrument to the known ground coordinates;
  • accurately positions a backsight (BS) target over a known ground point;
  • measures a target height of the BS to relate the target location to the ground coordinates;
  • observes the BS target to orientate the survey;
  • observes an additional target (foresight) to easily relate data from multiple scans in the field.

With the traditional total station workflow, the scanner can be set up over known points to provide a direct relationship to existing ground control. Measurements between stations, commonly referred to as traverse measurements, provide an instant relationship between multiple stations and allow the user to view homogenous data in the field without additional post processing. The quality and extent of the survey data can be verified in the field, reducing survey time or costly re-measurement.

Scanning Survey Workflow

To support the survey workflow, a modified approach to 3D scanning hardware, software and survey methodology is required. Survey operations and workflows must be built into the scanner to provide a familiar, versatile and efficient tool for survey applications.

Hardware

The first component of the 3D scanning system to consider is the physical scanner itself. Many 3D scanners have traditionally been large, not easily maneuverable, or have required a ground power supply. For today's competitive market, newer 3D scanners are becoming robust, portable and have flexible power solutions. While most scanners have required a PC or laptop for operation, new technology allows scanners to also be operated with a rugged field controller, thereby greatly enhancing the field portability.

In order to support a survey workflow, scanners must provide:

  1. A tribrach with laser or optical plummet to ensure that the scanner is precisely positioned on a known point.
  2. A mark for accurately measuring the instrument height. The field software must also correct the instrument height measurement for the slope to obtain a true vertical measurement.
    • A dual-axis compensator for leveling the instrument over a known point. In addition, the dual-axis compensator must actively correct the horizontal and vertical angles for mislevelment to ensure that accurate measurements are obtained.
    • A centric standard 5/8" thread mount in the top of the scanner. The thread allows a survey prism or GPS receiver to be placed on top of the scanner for positioning as part of an integrated survey solution, even during scanner operation.

In addition to these specific survey workflow features, it is critical that 3D scanners be equipped with advanced laser technology to provide accurate focused measurements and enhanced long-range operation. This technology ensures that the scanner can be used for a variety of survey applications and environments.

3D Scanning Field Software

An integral part of a survey workflow is to be able to efficiently relate measurements between stations and known ground points. 3D scanning software needs to support the survey workflow for effective use.

One of the most recent 3D scanners currently available is the Trimble GX 3D Scanner. Offered with PocketScape field software, the Trimble GX provides an example of how hardware and software can work together to support a survey workflow in today's advanced 3D scanners.

Field Software

PocketScape software is designed to operate on a rugged field controller. While PocketScape fully supports a typical scanning workflow it also fully supports a survey workflow by providing familiar operation that is almost identical to that of a modern total station.

When a user first connects to the scanner and selects to start a station setup, an electronic level form is displayed (Figure 4).

The electronic level enables the scanner to be fully leveled and positioned over a known point. The user selects Next, enters atmospheric information to correct for refraction of the distance measurement. The software then enables the user to enter station setup information, such as the instrument point name and height, as a surveyor would during a total station survey.

Since the laser scanner does not have a telescope like a total station, it is necessary to scan the BS target to accurately aim or define the BS orientation. The button is used to scan a BS point, target or sphere, typically in less than one minute. Once the target is measured users can assign a point name and BS height to complete the BS definition and orientation.

The azimuth to the BS is displayed for confirmation of the orientation. The user selects "Ok" to complete the station setup and then measure to additional targets or features from the fully referenced station setup. The wizard-style approach and support for the survey workflow ensures that the surveying professional can easily operate the scanner.

Integrated Surveying Solution

Today, 3D scanners that use the survey workflow easily complement GPS and total station techniques as part of a Connected Survey Site. For example, a professional can use traditional survey techniques to establish a control network of known ground points and then use a 3D scanner for efficient data collection over the survey site. Maximum survey efficiency can be ensured by having the right tools for each surveying job close at hand.

In fact, a respected professor of surveying thinks that all 3D scanners used by surveyors will incorporate the survey workflow in the future. The Graduate School of Science and Technology (INSA Strasbourg) is currently using the Trimble GX 3D scanner for teaching and research purposes in their program.

"The workflow available in 3D scanners such as the GX is close to the standard fieldwork methods where a network of control points measured by GPS or total stations are now also used as reference stations for the 3D scanner," said Prof. Dr.-Ing. Pierre Grussenmeyer, INSA Strasbourg, Graduate School of Science and Technology, Photogrammetry & Geomatics Group. "The leveling of the GX and the direct georeferencing of the point cloud is of great interest to surveyors, especially for outdoor projects. This allows a survey workflow to be integrated throughout the project, regardless of what technology or technique is used. I believe 3D scanners in the future will all incorporate a survey workflow, at least for most of the outdoor scans where georeferencing of project data are required. It makes it easy to check to merge the point clouds and the consistency with other external data, from terrestrial surveys, LiDAR, photogrammetry, and other methods."

Conclusion

3D laser scanning is a powerful technology that offers many benefits to surveyors due to the increased speed of measurement. However, the lack of versatility of 3D laser scanners together with unfamiliar workflow processes has, until recently, limited a broad adoption of the technology by surveyors.

The latest innovations in 3D scanning for the surveying industry point the way to the future of scanning for surveyors: portability, robustness and the use of a typical survey workflow so that professionals can quickly incorporate 3D scanning into their business portfolio. The short learning curve allows surveyors to increase productivity, create new business opportunities, and enjoy significant returns. 3D scanning is opening a whole new world for surveyors.

About the Authors

Tim Lemmon is an Applications Engineer for Trimble. His experience includes optical and scanning-based products. Tim has a BS and MS in Applied Science from RMIT University, Melbourne, Australia.

Paul Biddiscombe is a senior 3D scanning product manager for Trimble. Paul has worked in product management and marketing for many years and has a BA (Hons) in Economics from Kingston University, London and a Masters in International Marketing from Staffordshire University, Stoke-on-Trent, UK.

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