3D Laser Scanning for Common Surveying Applications

Transcription

1 Application White Paper 3D Laser Scanning for Common Surveying Applications Charles M. Coiner, PLS Anthony P. Bruno Cyra Technologies, Inc. A Leica Geosystems Company San Ramon, CA Abstract 3D laser scanning has been commercially available for several years, and has become well adopted for plant and facilities applications where accurate three-dimensional detail of complex facilities is critical for efficient design and construction projects. However, recent advances in point cloud processing software and office workflow options have made laser scanning attractive for many common surveying applications. In many respects, laser scanning follows the same general surveying process as other instruments: data is collected in the field, adjusted to the appropriate coordinate system, and relevant features can be extracted to produce deliverables ranging from topographic maps, coordinate values, 2D or 3D CAD drawings, etc. This paper describes typical scanning projects from field-to-finish, including several common surveying applications. Application White Paper Page 1 of 10

2 Introduction Surveying instrumentation has undergone a major transformation over the past 15 years, primarily due to the advent of the PC and advances in solid state electronics, including the microchip. The theodolite, steel tape, and field book have in many cases been replaced by electronic field instrumentation such as the EDM, GPS, hand-held data collectors, and automatic laser levels. In addition to this rapid evolution of electronic hardware there have been significant leaps in software tools and solutions for the surveyor. For example, in addition to delivering CAD drawings, surveyors now deal day-to-day with network computations, feature code processing, and TIN modeling. The technology advances have allowed the surveyor to take advantage of new tools to complete the same surveying tasks that have been performed for hundreds of years; for example, boundary surveying, topographic mapping, as-builts, volume calculations, etc. With few exceptions, these new technologies did not change the methodology, processes, or best practices of the surveyor. 3D laser scanning, also know as terrestrial LIDAR, is a powerful tool for the surveyor. Like other technological advances in surveying equipment, optimal use of scanning requires the use of new information. The EDM introduced the electronically reduced horizontal distance, GPS introduced the vector. Surveying with a 3D laser scanner generates a new set of information the point cloud. When compared to traditional methods, point clouds provide a significantly higher level of true geometric completeness and detail of the site virtually eliminating costly site re-visits to gather more detail or collect omitted features. photogrammetry in that it is derived from a remote sensing instrument, that is, the measurements are taken without physically contacting the target area. It can also compare with a total station in that it makes terrestrial measurements while sitting atop a tripod. Lastly, a comparison can also be made to remote sensing satellites, as additional non-positional data is collected from the raw measurements, such as the signal intensity of each point in the cloud, which will vary based on the reflectivity of the scanned object. Each point in the point cloud is measured with respect to the scanner position, and similar to photogrammetry, the position of the scanner (the camera) does not need to be known during the measurements. Aligning the point cloud to local control with laser scanning is similar to photogrammetric control, as overlapping targets can be used to join multiple scans (photos) together and to fit it to the desired coordinate system. Figure 1 shows a digital photo of an intersection and the corresponding point cloud consisting of 781,970 points. Since the intensity of each point in the cloud is mapped, the operator can easily interpret one surface from another while visualizing the project in all three dimensions. The Point Cloud Surveying s New Raw Data Measuring with a 3D laser scanner yields a point cloud. With a Cyrax (from Cyra Technologies Inc.), it is possible to collect as many as 1000 points per second (1Hz) to generate a point cloud that contains thousands and even millions of points, with relative accuracies of 6mm or better at a range of 50 meters. A point cloud can be compared with 1 The Cyrax 2500 is used as an example in this white paper, other scanners may have different specifications. Figure 1 Photo and Corresponding Point Cloud Application White Paper Page 2 of 10

3 At first glance, viewing the shear volume of points in a point cloud can be overwhelming to the surveyor, as it is difficult to imagine capturing over a million points in a topographic survey. With a conventional total station or with GPS, the experienced surveyor typically captures the minimal amount of points to represent the target surface, collecting and annotating data about features in the field. As with all measurement techniques, this process is prone to costly errors and/or omissions in the data, and can sometimes be impossible to collect on the site due to traffic, toxic or prohibited areas, and inaccessible regions. With 3D laser scanning, many of these errors are eliminated based on the fact that scanning blankets the site with 3D points at a user-specified resolution. If the desired objects and features can be seen by the scanner, it can be represented accurately in the point cloud and extracted with software such as AutoCad. To collect data with the Cyrax scanner, a laptop PC is connected by a network cable (for maximum transfer rate). The laptop is used to interface and control the areas to be scanned and to visualize the data in 3D as it is collected. As the laser is sweeping across the site, the operator can see exactly what is being scanned and can navigate in the virtual scene to ensure completeness. This interface is also used to select objects of interest within the scene to capture with targeted, high-density scans. It is not uncommon to collect over 1 million points in a single scan, which takes about minutes to accomplish. With a 40X40 degree field of view (typical scanner specification), this covers a potential scan area of 20,000 cubic meters. Since there are usually objects (e.g. buildings, bridges, etc.) in the field of view, the scan area can be reduced based on the distance from the scanner to the object(s). Figure 2 - Scanner Field-of-View The number of points in a scan is specified by first selecting the area to scan. This is accomplished by using a digital photo image of the scan site, taken by the scanner, and then selecting a window of the area using the cursor. After the area is selected, the operator then sets the total number of points, in both the vertical and horizontal (e.g. 200 points vertical X 300 points horizontal). Alternatively, the user can set the specific point spacing desired, for example,.01ft. vertical by 0.01 ft. horizontal, at a specific range from the scanner. Specific areas that require special detail (e.g. a control target) can be selected and scanned with a higher density of points. Accuracy of Data Points As previously mentioned, the scanning process produces thousands of data points, each with their own unique x, y, z, and intensity level. The accuracy of each point in the cloud, relative to one another, is dependent on the spot size of the laser beam. Accuracy improves as the spot size is reduced. The most accurate laser scanners available today, such as the Cyrax 2500, operate at accuracies as good as 6 mm (0.019 ft.) within a 50 m (164 ft.) of the scanner. As the distance to the scanner is increased, the spot size increases, and accuracy between points is influenced. Most surveying tasks, including topographic mapping and as-builts, are typically done at close range (1-200 ft.) to obtain the appropriate level of detail. Regions that are not shown in one scan can be scanned with another setup, and these point clouds will be merged in the software. Application White Paper Page 3 of 10

4 In most cases the scan(s) can be tied to the local control system, in laser scanning this process is called registration. As with all surveying, the accuracy of the registered point cloud depends on the accuracy of the control network that is used as truth. The registration process calculates the appropriate transformation parameters and displays statistics. Point Cloud Utilization One of the first things a surveyor asks about 3D laser scanning is what do I do with all those points, or, why do I need all those points. Dealing with a point cloud containing millions instead of a few hundred specially selected points for a site survey does cause some to ponder. The good news is: It s good news. Point cloud data gives the surveyor more control of the job site -- common surveying functions such as topographic mapping can be done quickly and efficiently, and with more confidence than ever before. Here are just a few examples of how a point cloud can be quickly utilized to produce a variety of deliverables: also collected, including people, vegetation, trees, traffic, parked cars, etc. As traditional surveys do not typically include such noise, this is a step in the process that is new to the surveyor. Fortunately, due to recent major advances in point cloud processing software; powerful tools have emerged to allow the surveyor to quickly and easily eliminate noise. An automated process called region grow utilizes algorithms that crawl across contiguous surfaces to parse out the noise from the surface. Figure 3, below, shows the same road scene used in the previous example to illustrate the region grow process. The top image shows the roadway including noise caused by passing cars, the bottom image shows the region grow interface parameters and the selected road surface. The white region represents points on the road surface without traffic noise. Work with the raw point cloud in AutoCAD, MicroStation, or other CAD software, create a 2D or 3D drawing. Extract feature codes directly from point cloud, export to feature code processing software (Figure 6 shows an example of feature extraction) Calculate volumes or surface areas directly from the point cloud Create a DTM, generate contours of a selected area automatically Export DTM points to 3 rd party contour package Measure between any points in the point cloud, including elevation differences, slope, or horizontal distance Removing Noise Scanning a site captures everything in the selected field-of-view, including ground points, traffic signs, road surface, telephone poles, fences, etc. Depending on the requested deliverable, these features are highly relevant to the surveyor. However, because the scanner blankets the entire site, undesired objects are Figure 3 - Removing Traffic Noise Application White Paper Page 4 of 10

5 The noise removal process typically takes only a few seconds: First, the user selects a point on the road surface and starts the region grow function. Second, the user enters the parameters of the region grow, which include the thickness, angular tolerance, allowable gap, and size of the region. Once these parameters are set, the software then grows from the origin point, selecting only those points on the desired surface based on the tolerances. The user can interactively iterate this process until the desired surface is selected. Finally, the user simply deletes the noise or stores it in another drawing layer. Figure 4, below, shows the clean roadway surface. The surveyor can now perform other actions directly on the road surface such as TIN creation, line/feature extraction, generation of contours, etc. point clouds in several common surveying applications. These applications include: Use of point cloud in common CAD packages Extracting survey features from the point cloud (Feature Coding) Creating a TIN, generating contours, exporting points Volumetric calculations Basic distance measurements Registration of point cloud to local control Use of point cloud in common CAD packages Most modern surveying and engineering practices make use of the latest computer aided design and drafting (CAD or CADD) tools to generate representations of a project site throughout the design and build process. Although some rely on the old way using paper and the drafting pen, advances in CAD have streamlined and optimized this process of generating final deliverables. Figure 4 - Roadway With Traffic Noise Removed 3D Laser Scanning for Common Surveying Applications There are hundreds of potential uses and applications of point cloud data that benefit the surveyor, the engineer, and the CAD operator. 3D laser scanning provides the surveyor with an accurate and efficient way to collect and present field data; it provides the engineer with rich and reliable information to base design, accurately monitor changes, check construction fabrication, etc. Although the benefits may vary based on the discipline, one thing remains common: The point cloud is an up-to date, accurate, and unambiguous measurement of what exists in the real world. The remainder of this paper describes, in detail, the work-flow process of taking advantage of 3D Figure 5 - Display of point cloud in AutoCAD For surveying and civil engineering projects, there are two prevalent packages being used today; AutoCAD by AutoDesk Inc., and MicroStation by Bentley Systems Inc. Both AutoCAD and MicroStation have limited capabilities when it comes to utilizing very large datasets such as a point cloud. This is due to the fact that these packages are focused on Application White Paper Page 5 of 10

6 creating polylines, blocks, and CAD objects, which are relatively easy to view, manage, and store. Loading and manipulating large point clouds containing thousands or millions of points in these environments is not practical or efficient at this point in time. Using a proprietary plug-in by Cyra Technologies, a Leica Geosystems company, called CloudWorx, it is now possible to easily work with point clouds within the CAD environment. The plug-in allows read-only access to point cloud database, thereby preserving raw data integrity. The plug-in s commands work and appear the same as any other command accessed from within CAD. There are several advantages of this approach, including: CAD users can work with point cloud data to extract and draw features as well as create 2D drawings and 3D models. Information on coordinates and distances can be obtained using familiar CAD measuring tools. Engineers and designers can propose designs based on accurate, comprehensive, real world information. Users have the ability to compare 3D scan data against proposed 2D or 3D designs. Drawings can be redlined in CAD as appropriate. With 3D laser scanning, there is one major difference in the data collection process: After a scan is completed, feature coding is accomplished in the processing software, not during occupation of each point. There are many advantages to this methodology, including the fact that features are extracted using a mouse click. Feature coding directly inside a point cloud is a revolutionary new way to extract point and line features within the same workflow, as surveyors can use the same exact feature code libraries to identify and label features of significance. Drawing layers, symbols, colors, and other features already configured within the existing feature code library are maintained. To capture point and line codes efficiently in the desired feature coding format, Cyra s Cyclone software includes a feature called Virtual Surveyor that controls the collection of features and ASCII export formats. Figure 6, below, shows the top of curb (TC) coded along approx. 150 ft. of curb line in one viewpoint (viewed in orthographic mode). The graphic on the right shows the same curb, in perspective view, from the scanner position. Since the scan was registered to the local survey control, the resulting code list is automatically numbered and annotated in that coordinate system. Extracting Survey Features from the Point Cloud (Feature Coding) One of the most familiar aspects of collecting topographic data for the surveyor is coding point and line features in the field, such that linework and symbols can be automatically generated in the office. Feature coding with GPS or terrestrial total station instruments is performed at the job site, where each point is physically occupied by the surveyor for seconds and sometimes minutes. Reflectorless instruments allow these features to be coded at the instrument and a code is typically entered for the measurement. Feature code libraries are typically used to store a list of specific point and line codes, these libraries may differ based on the deliverable or the equipment/software being used by the surveyor. Figure 6 - Feature Coding using Cyra s Virtual Surveyor function Application White Paper Page 6 of 10

7 Surface Calculations Mapping irregular surfaces such as drainage ditches, landfills, stockpiles, or common site surveys typically have involved the collection of tens or even hundreds of ground points to generate and accurate digital terrain model (DTM). A DTM can be used to generate several types of measurements, including volumes, surface areas, profiles, cross-sections, etc. Contours can also be generated directly from the DTM. A DTM is typically created from a digital elevation model (DEM) or a triangulated irregular network (TIN). A DEM consists of a gridded elevation data set that is oriented grid "North-up." This form of data storage is commonly encountered with government agency data and with smaller scales of map information (generally less than 1:25,000). A triangulated irregular network (TIN) is a form of ungridded elevation data storage in which "mass points" and "breaklines" are used to describe the terrain surface in a generally more accurate manner than DEM s. With 3D laser scanning, the process of generating contours is greatly improved in both the field and in the office, due to the rapid collection of a massive amount of ground data and specialized software tools to generate a TIN. Once the TIN is generated, it is possible to perform several surface-related functions, including: Surface area calculations Volumetric calculations Cross sections and profiles Contour creation Export of TIN or TIN points Figure 7 - Calculating Volume Figure 5 shows the volume of a hazardous pit as derived from a single scan of the site. In this calculation, the user can define a geometric plane to which the volume measurements are made, for example, a plane that represents the top of the pit or the existing grade at the base of a landfill. It is also possible to make measurements from a specified elevation. Once this reference plane is established, a sampling rate is selected that indicates the density of the virtual rod readings made relative to the reference plane. Based on this information, the volume is displayed in the desired units. With 3D laser scanning, field-to-finish volume calculations are possible in a matter of minutes, compared to several hours with traditional methods. In addition, the amount of detail generated by thousands of survey-grade points yields a more accurate representation of the earth s surface. Application White Paper Page 7 of 10

8 Registration and Geo-Referencing of Point Cloud to Local Control Another important question surveyors ask about laser scanning is how do I tie the point cloud into my local control system? This process, called registration, is very easy to comprehend by the surveyor, as the procedures and best practices are very similar to setting photogrammetric control or establishing GPS control networks. As is typical with most surveying sites, a 3D coordinate system is either established (assumed) or is tied into an existing formal system (e.g. state plane system). transformation from one system (scanner) to the other (local control). To make a simple analogy, this transformation is similar to the common translation from GPS(WGS-84) to state plane coordinates. Assuming a redundant set of known coordinates (more than 3) was used to complete the registration, the system will provide details as to how well the transformation fit on a point-bypoint basis. The surveyor can then eliminate or weight points based on these results or other local planar projections. In order to geo-reference the point cloud(s) to an existing local coordinate system, at least 3 known points in Northing, Easting, and Elevation (NEE) are required on the site. As with most surveying practice, the minimum will provide the answer but will not allow for any checks; typical field procedure suggests setting up and locating more targets than the minimum to isolate and account for uncertainty. The surveyed targets must also be geometrically positioned on the site such that the registration produces an accurate result. Similar to photogrammetry, it is important to place targets that form a strong geometric configuration across the project site. Modelled vertex of planar (flat) target Scanned point cloud of standard spherical target Once the targets are placed in the scene, terrestrial measurements are then made with GPS and/or an optical total station to measure and record the local coordinates of specialized scan targets. These targets are then scanned at a high density. A fine scan is done on each of the targets to ensure accurate modelling of the vertices, or geometric centers. Figure 8 shows point clouds and modelled vertices of two target types, a flat target, and a spherical target. Modelled vertex of spherical target The default coordinate system of the laser scanner is based on the scanner center, which corresponds to 0,0,0 in an x,y,z coordinate system. We will call this the Scanner Coordinate System (SCS). All raw measurements taken by the scanner are with respect to this arbitrary system. As previously mentioned, there is no requirement to know the x,y,z of the scanner center, or to level the scanner, as the registration process will transform the SCS to the local coordinate system. Since the targets now have known N,E,E from surveying in control, the system performs a mathematical Figure 8 Target Examples Once a registration is complete, all points from the scan(s) are now viewed, stored, and exported in the local coordinate system. Data such as TIN models, cross sections, and feature codes can be created and exported using the local coordinate reference. Application White Paper Page 8 of 10

9 Topographic Site Survey One of the most common deliverables for an engineering survey is a topographic map that depicts planimetric features, legal boundaries, objects of interest, and contours of the terrain. It is not only possible but now very practical to utilize 3D laser scanning to help complete this sometimes labor-intensive, detail-oriented task. By combining the various workflows described in this document, it is possible to create final deliverables based entirely, or in part, on point cloud data. Existing property line data might exist in the CAD environment, along with road alignment information. Point cloud data can be overlaid on top of this information in CAD, and can be manipulated to trace or even plot point cloud elements right in the final drawing. As-Builts Surveyors commonly produce accurate as-built surveys of structures such as buildings, bridges, or roads, usually for the purposes of checking engineering or building code compliance. In addition to these purposes, as-builts are also created for preservation/conservation, construction archiving, fabrication inspection, interference design checking, etc. As-built surveys that require a significant amount of detailed data capture of certain features or inaccessible areas can be quickly identified, located, and mapped directly from a 3D point cloud. Fig below shows an as-built point cloud of a new bank drive-thru with several measurements of overhang clearance. Figure 10 Point Cloud As-built of Bank Drive-Thru Figure 9 - Point Cloud and corresponding 2D map in AutoCAD Application White Paper Once a point cloud is registered to the local coordinate system, measurements can be made between points or objects, checking against setbacks, property lines (for encroachment) or clearances between structures. Engineers can make compliance measurements between any points or surfaces of a structure directly in the point cloud. Redline notes can also be attached to communicate proposed design changes or areas of potential interference. Alternatively, the surveyor can use the point cloud to generate an accurate 2D map detailing features important to the as-built plan. To complete this task, there are several work-flow options available to the surveyor as described in the next section. Page 9 of 10

10 Multiple Work-Flow Options Like many surveying instruments, 3D laser scanning is accompanied by its own specialized software for capturing, processing, and visualizing field data. The utilization of point cloud data within this environment will differ depending on one of more of the following factors: The specific application (as-built, topo, etc.) The required deliverable media Your existing work processes The capabilities of your customer As described earlier, raw point clouds can be used as a final deliverable to the client who is taking advantage of cost-effective CloudWorx plug-ins for their existing CAD packages. Earthwork and contouring functions can be performed in the 3D processing software or DTM points can be exported to a 3 rd party software for further processing and design. Linework can be generated directly on top of the point cloud and exported to CAD, can be generated in CAD, or can be generated using feature codes and your surveying software s feature code processor. These are just a few examples, in reality, 3D point clouds can be utilized and adapted to most work-flows. Figure 11 shows some examples of field-tofinish work-flows originating with a 3D scan of the site. Raw Data Points CAD w/plugin 2D or 3D Map Linework Plot/Format Field Scan Point Cloud(s) CAD Feature coding Survey SW 2D or 3D Map Create TIN Civil Design SW Topo Map Raw Point Cloud Conclusion The surveyor has historically selected the most appropriate, most economical equipment to get the job done. Today, the typical surveyor has multiple microchip-based instruments at their disposal, including GPS, laser levels, and robotic total stations. More and more, surveyors are complementing this suite of measuring equipment with 3D laser scanning instruments, with the goal of enriching their tool chest with the unique measuring capabilities that the scanner represents. One end result is an expansion of potential projects available: In addition to common topographic and route surveys, a surveyor can now measure geometries of complex plants or large facilities. Other general benefits 3D laser scanning offers the surveyor are: Added value services to current clients o 3D deliverables o More detailed topography o More detailed as-builts Higher productivity, lower costs for certain projects Improved safety Reduced errors in data capture Reduced errors due to omission Ability to return to the site without leaving the office As the design-build process becomes more advanced, requirements for better accuracy and more detail in the construction process will be necessary. As demonstrated in this paper, it is clear that the 3D laser scanning provides a detailed, reliable, and accurate solution to many surveying and measurement problems. For more information on 3D laser scanning, please visit or contact: Cyra Technologies Inc Norris Canyon Road San Ramon, CA Deliverable Figure 11 - Examples of Work-Flows & Deliverables Application White Paper Page 10 of 10