3D Scanning: Field Productivity Factors in Laser Scanning, Part 1
Professional Surveyor Magazine - January 2007
Just as with traditional survey instruments, many factors contribute to the actual field productivity achieved for any given type of laser scanner and project that includes high-definition surveying. Certain scanner capabilities also influence office productivity in creating deliverables from captured data. This multi-part series examines this subject in detail.
Spec Readers Beware
Many novices investigating laser scanners may look simply at a scanner's specified scanning speed, usually expressed in points/sec, to try to determine "which scanner is most productive in the field." The simplistic thinking—that scanners with the highest scan speed specs are the most productive scanners for all types of projects—can lead professionals to incorrect conclusions.
Consider this fact: scan speeds of phase-based scanners (i.e. ones that use phase-differencing for distance measurement) are 100 or so times greater than scan speeds of today's time-of-flight scanners. Yet phase-based scanners (which have been in the commercial market since the 1990s just like time-of-flight scanners) currently comprise fewer than 10% of all the scanners in use. While higher scanning speeds can be advantageous, the actual field productivity of any type of laser scanner depends on many more factors than just scanning speed.
Various features and factors apply in assessing overall field productivity for each specific scanning system. There are two basic types of laser scanners: time-of-flight (TOF) and phase-based scanners, differing in how each measures distance. Some factors apply more to TOF scanners and some more to phase-based scanners.
Field productivity factors can be grouped according to a typical project workflow.
- The aspect of site reconnaissance and planning (if any).
- Simply getting the gear to the site and moving it around from setup to setup (a key element is how many setups are needed for the scanner and how many, if any, for scan targets).
- The actual scanning itself and associated QA steps.
1) Reconnaissance and Planning Time. For many projects, scanners with a limited field-of-view, ones with short-range capabilities, or ones that do not include accurate tilt compensation (and hence require the use of many more targets) may require time to recon the site and plan ahead where the scanner and targets should be placed. For some projects, users will add an extra day to their project schedules just for site recon. Users have told me that when true "full dome" field-of-view scanners with good useful range became available in the market, they stopped doing advance site recon altogether, as they found the new scanner capabilities gave them the freedom to use their scanner just like the logistical freedom they enjoy when using a total station.
2a) Time Needed to Carry Scanning System and Targets to and around the Site. In addition to the scanner itself, the vast majority of today's scanners require an external laptop (to control the scanner and collect scan data) and an external DC power supply (or AC supply). If users want to align digital images with the scan data in the field (and many do), some systems also involve external digital cameras while other systems have high-resolution digital cameras embedded in the scanner itself. There are also scanner mounts (often tripods) and sometimes even ladders or other devices to consider for those who want to elevate the scanner. Depending on the project and how many setups are involved for the type of scanner being used, the amount of equipment that needs to be carried from one setup to another can influence field time and crew size.
Likewise, the portability of each piece of equipment can also be a factor. Convenience features as obvious as carry- handles on the scanner and battery, carrying cases, and backpacks all factor in. Wheeled devices, including tripods with wheels, and the general ability to assemble a complete setup into an easily transportable arrangement can also help reduce field time. Wheeled devices have also proven to be a convenient way to avoid having to boot up the scanner at each setup and have become popular for use in and around commercial buildings and industrial plants.
Convenient "integrated" scanners: A recent advance for minimizing the time to carry gear is the arrival of "integrated" scanners. These have the battery, data storage, and a simple control panel all embedded in the scanner itself, greatly simplifying the carrying and setup task.
An added benefit of having an embedded controller in the scanner is that this can slash "boot-up" times from setup to setup. As convenient as integrated scanners are from a portability and boot-up standpoint, there are some field productivity trade-offs as well. For example, if users use only the built-in control panel and cannot see the scan data on a laptop as a QA check, then just to be safe they will tend to setup for scanning larger-than-needed fields-of-view and higher-than-needed scan densities. These, in turn, add to field time. As a result, users may elect to deploy a laptop in order to control the scanner with realtime field QA.
Targets: There are many types of scan targets, including bipod "twin-target poles" (that reference scanners to vertical); magnetic mount targets; tripod-mounted planar, spherical, or hemispherical targets; adhesive-backed, stick-on targets; and paper tape-on targets. Some targets can be left in place for convenient use later on, while others need to be placed, moved, and reused. Some targets can be placed conveniently, while others are placed with lifts or by climbing on ladders. Generally for any given site, the type of scanner, its field-of-view, and its useful range will determine which types of targets are best suited and where they need to be placed. Scanning system features that minimize the number of targets needed and facilitate placement in convenient locations will be described in Part 2 of this series.
2b) Number of Scanner Setups Required for the Project. As with any surveying project, the number of instrument setups required can also have a major impact on overall field productivity for high-definition surveying projects. As noted above, phase scanners can scan 100x faster than time-of- flight (TOF) scanners. Phase scanners, however, have much shorter useful range (for capturing the scene and scan targets) than many TOF scanners. The result is that phase scanners often involve many more set-ups. One user who had used both types of scanners for various projects told me that the time that his TOF scanner was actually scanning was typically around 6 to 6 ½ hrs out of an 8-hr work day; in contrast, the "actual time scanning" of a phase scanner in the same day was about 2 hrs. The rest of the time was spent setting up and surveying targets and moving the entire setup around the site. As the useful range of the phase scanners increases this should increase the amount of time the scanner is actually scanning in a work day.
The two most important scanner-related factors that determine the number of setups required for a project are its maximum field-of-view (FOV) and its useful range (for both scene capture and for capturing targets plus other fine features). In certain situations, dual-axis compensation can also reduce the number of scanner setups.
Maximum field-of-view (FOV): This capability can have a significant impact not only on the number of scanner (and target) setups but also on the feasibility of even being able to use a scanner for candidate projects. Basically, the bigger the maximum FOV per scan, then the smaller the number scanner setups and the more logistical freedom that a user will have as to where the scanner (and targets) can be placed.
For scanners with both a full 360° horizontal FOV and a large vertical FOV (e.g. 270° or more), users can place the scanner inside the scene and capture geometry both around and overhead with a single instrument setup. Likewise, targets can be placed both around and above/below the scanner location. Without this capability, the user may have to reposition the scanner setup location or try to reorient the scan head to try to capture the same geometry. In some cases, the scanner may have to be re-positioned or the scan head reoriented several times.
Today, all of the latest models of scanners have full horizontal FOV capabilities, but many have limited vertical FOVs. Consider the following common situation: A user wants to capture the facade of a multi-story building or other tall structure. However, the building or structure is within several meters of other structures or buildings, so there is insufficient room for the user to "back the scanner away" from the scene in order to capture it. In this case, the ability to scan upwards allows the user to scan the tall structure from a spot close to the structure.
Useful range: A scanner's "useful range" is the maximum practical range at which a scanner can be placed from objects in the scene and from scan targets and still meet the project's accuracy requirements. A scanner's useful range (with all other site factors such as surface reflectivity and angle of incidence to objects being equal) is a function of several factors:
- Maximum scan density at range
- Spot size at range
- Accuracy of each point at range (based on single measurement distance accuracy, vertical angle accuracy, and horizontal angle accuracy)
- Scan noise at range
- Maximum range at which a useful number of points with sufficient accuracy can be captured
A scanner's "useful range" is not currently specified by vendors. Today, vendors specify only "maximum range" for certain types of albedo surfaces (for example, 18 percent reflectivity). Maximum range in this specification context indicates simply whether the scanner detects some vendor-specified percentage of laser returns from the surface. It has no relation to the accuracy or utility of the points collected.
The best way to get a handle on a scanner's useful range is to ask others who have such scanners what range they use for specific types of applications. The topic of "Useful Range" was covered in depth in a previous article (ref. Professional Surveyor, November 2004); understanding this concept is highly valuable for appreciating its impact on the number of scanner setups for projects. In general, the higher the scan density, the smaller the spot size; the more accurate each measurement, the lower the scan noise; and the longer the maximum range at which scan data of sufficient accuracy can be captured, the longer the scanner's useful range will be.
Additional useful range factors for phase-based scanners: Two other factors specifically influence the useful range of phase-based scanners. One is whether the scanner is used indoors or outside in bright sunlight conditions. These types of scanners are sensitive to sunlight, which effectively reduces their useful range. Phase-scanner capabilities have improved in this area over time, but useful ranges are still less in outside sunlight than they are in interior areas.
A second factor specific to phase-based scanners is their "ambiguity interval" limitation. This is akin to the resolution of ambiguity intervals for GPS. Scan data collected by phase-based scanners that is beyond their ambiguity interval range will show up as close to the scanner instead of far away. For example, consider a scanner with an ambiguity interval of 79m. If it receives returns from an object at 84m range, the object will show up as being at 5m range. The scanner cannot tell the difference between returns at 5m, 84m, etc. In many cases, these types of scanners do not receive returns beyond their ambiguity intervals, so it may not be an issue.
Site logistics factors: Of course, a key consideration is matching the useful range capabilities of a scanner with the types of sites for which it is being considered. For example, although phase-based scanners have a relatively short useful range, they are very popular for smaller, enclosed areas such as interior rooms in buildings and interior areas of industrial plants. In these cases, the line-of-sight distance from the scanner to the farthest visible object to be scanned is often well within the scanner's useful range.
Dual-axis sensing and compensation: Although this fairly new capability for laser scanners has a much greater impact on reducing the number of targets needed for high-definition surveying, for certain types of sites it can also reduce the number of scanner setups. On sites where targets cannot be spread out in the scanner's field-of-view, such as in narrow corridors or around building corners, the scanner can be used in traverse, backsighting, and resection modes. Users can snake in and around these challenging sites, just as they would with a total station. Without dual-axis compensation, more scanner setups would be needed around the perimeter of the site to accomplish the same task.
Various scanning system features influence field productivity. Some scanners require pre-project site recon while others may not. Some systems are more integrated and thus easier to carry around a site, while others, although not integrated, can be conveniently wheeled around buildings and plant sites. Some scanning systems involve fewer scanner setups than others. Additional factors, such as scanning speed, remote control capabilities, the ability to operate a scanner unattended, and others will be covered in a future article.
About the Author
Geoff JacobsGeoff is senior vice president, strategic marketing for Leica Geosystems, Inc.
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