Spatial Technologies-Surveying for a GIS World31 Oct, 2005 By: James L. Sipes Cadalyst
Tools for collecting real-world spatial data.
THINGS HAVE CERTAINLY changed since I learned surveying some twenty years ago. Where are the Philadelphia rod, the plumb bob and the 100' measuring tapes? Today, surveyors are probably more likely to be using theodolites, GPS units, electronic digital and automatic levels, electronic total stations and data collectors.
Some of the conventional uses of surveying include ground control surveys, boundary and cadastral surveys, total station topographic surveys, mineral surveys, hydrographic and drainage surveys, utility locations, construction surveys, road alignments and other similar uses. The type of surveying technique used depends on specific applications. Traditional ground surveys are effective for smaller areas because of the detail and accuracy they provide, while aerial mapping is often used for large areas where standard surveying techniques are not feasible.
The surveying industry works in collaboration with architects, engineers, landscape architects, planners, land managers and other professionals who work with the land. Surveyors also work closely with GIS professionals in an effort to create more accurate geospatial information.
Surveying and GIS
The integration of GIS and surveying is important for both professions. Every GIS user needs spatial data that is tied to a specific location, and data collection is still a major part of many GIS applications. GIS professionals also use data generated by surveyors to make maps that serve as the foundation for a range of analytical processes. The geometry of GIS features can be improved by linking them to survey features. By continuing this process, an entire dataset can quickly be upgraded to a much higher degree of accuracy.
Surveyors can use GIS to access, store, manage and retrieve surveys and survey control information (figure 1). Surveyors can use GIS to integrate accurate measurements into a geospatial database and tie this data back to the real world. GIS is starting to be the framework used to integrate surveying, engineering and GIS processes. Because survey data can be stored in a GIS database, it can be linked directly to GIS features.
Figure 1. Trimble's GPS Pathfinder ProXH receiver consists of a GPS receiver, antenna and all-day battery. It provides an accuracy of 30 centimeters, and this can be improved to 20-centimeter accuracy with the addition of an extra antenna.
The Need for Detail
In the past, surveyors and GIS professionals both produced maps, but their objectives were very different. Surveyors were more concerned with accurately and precisely documenting what is on the surface of the earth, while GIS professionals were more concerned with data, and how that data fits together. In recent years, as the ability of GIS professionals to work with and analyze data has increased, so has the desire to work at a greater level of detail. Municipalities, utility companies, designers, planners, engineers and resource managers are eager to increase the accuracy of their GIS data.
This interest in high-resolution images and detailed data is nothing new. It has been only in the last couple of years, though, that technology has improved to the point where we can obtain not only this type of data, but also the software and hardware to handle it. Computer processors are powerful enough to access, edit and save large satellite images and LIDAR data with millions and millions of points. Linux clusters let users string together desktop computers for parallel processing. Data storage has improved in performance at the same time that storage size has increased and cost has decreased.
A fundamental question is the level of accuracy needed for a specific GIS application. Technology has developed to the point where the biggest limitations are time and cost. We have the technology to achieve an incredible level of detail, and we know how to work with the data once we have it. But acquiring highly detailed data can be expensive, and the resulting files may be too large to manage. Too many large datasets can create a bottleneck that shuts down a system.
With today's technology, surveyors can achieve accuracy within a millimeter or two, but is that level of detail needed? The accuracy needed to define a property boundary is very different than that needed to delineate a wetlands or a river's edge. Do we really need to generate topographic data with 6" resolution? For an urban area with a lot of elevation change, such as Seattle, Washington, the answer is probably yes.
A variety of software packages seek to make it easier to integrate GIS and surveying data. Most strive to provide an interface that works with different CAD, GIS and design programs so data can be easily transferred within an organization or to clients. ArcCadastre is an all-in-one, multipurpose tool that handles geographic data. Developed by Lantmäteriet (National Land Survey of Sweden) in collaboration with Leica Geosystems and AED Graphics, it provides a package of tools to capture, process, maintain and use survey and cadastre information within ArcGIS applications.
ArcCadastre uses ArcGIS as a base, ESRI's Survey Analyst and Leica Geosystems AG for survey and computation functionality and FME Objects from Safe Software to import and export data. The software is used for large and small-scale mapping, public utilities and infrastructure development.
ESRI's ArcGIS Survey Analyst, an extension to the ArcGIS family of products, was developed in partnership with Leica Geosystems. Survey Analyst helps improve workflow between survey and GIS applications and is used to store and manage survey points, measurements and computations. It can also display survey measurements and observations on a map. Surveyors can incorporate their measurements and calculations into GIS databases that provide a control framework within an organization. Performance in computations and enterprise geodatabases, import and export tools and overall quality and stability are all improved in the latest version of ArcGIS 9 Survey Analyst.
Survey Analyst provides tools to make it easier for surveyors and GIS technicians to work together. Advanced editing tools provide a great deal of functionality and control, and its COGO can be used to create survey points that are not affected during GIS editing.
A number of companies have developed software for specific surveying applications. For example, Trimble provides specialized software for specific needs, such as the HYDROpro Navigation and HYDROpro Terramodel software to create application-specific marine systems.
An amazing array of surveying equipment, from manufacturers such as Trimble and Leica, can be linked with GIS data.
Many companies have been providing GPS survey services since the early 1990s. In recent years, these companies have also started using GIS technology as a way to manage data. GPS surveying technology has become commonplace because it helps surveyors increase productivity, improve efficiency and lower costs. GPS units can provide horizontal accuracy up to three millimeters, but many on the market have accuracy closer to a meter or so. Not surprisingly, there is a direct correlation between the cost of a GPS unit and its accuracy. We can also expect to see the use of GPS technology increase as the US government makes more GPS satellites available. New GPS receivers entering the market today will be able to take advantage of new capabilities as they become available.
In addition to GPS, other technologies are used in surveying. Differentially corrected GPS navigation can be connected to boats and used to survey the bottom of a lake or river. 3D HDS (high-definition surveying) laser scanners can create highly detailed spatial datasets, and techniques such as RTK (real-time kinematic) data collection can produce high-quality maps much more quickly than can traditional surveying methods.
Laser scanners measure the dimensions of an object by shining a laser light and determining the amplitude of the reflections off the object. The scanners can take thousands and thousands of measurements per second. Laser scanners are typically either phase-based or pulse-based devices. Phase-based scanners send out a continuous signal that is reflected off an object. Distance is determined by measuring the phase differences between modulated waves. These scanners are typically effective up to a couple of hundred feet. Pulse-based scanners emit a pulse of light along a line and measure the time it takes the light pulse to get to an object and return. They are slower than phase-based scanners, but effective over a much longer distance.
Trimble, one of the leaders in the survey industry, provides the software and equipment to address virtually any surveying need. The company is also the government's largest supplier of surveying, leveling and grade-control systems. It offers a range of high-precision centimeter level (RTK) and submeter (DGPS) positioning solutions that can be integrated with other marine sensors.
Trimble's Direct Reflex EDM technology is used for precise surveying of building facades and interior spaces. Its Virtual Reference Station system uses RTK solutions to provide accurate GPS positioning in real time.
Leica produces a wide range of products as well, including its Geosystems Total Station and the Leica Laser Tracker, a mobile PCMM (portable coordinate measuring machine; figure 2).
Figure 2. Handheld GPS units, such as the Trimble Recon, have become commonplace because they help surveyors increase productivity, improve efficiency and lower costs. They typically are compact and rugged, and can handle even the toughest outdoor conditions.
GPS units need to be able to withstand extreme temperatures, site conditions and weather. Strata is one company that provides rugged software and hardware for even the most demanding field applications. Its PenMapPC range of products combine surveying, CAD and GIS functionality. Penmap is an integrated system that provides a common interface to survey techniques such as GPS, Total Stations and leveling. The PenmapGPS-RTK is a tablet PC with integrated GPS capabilities that can tolerate extreme environments.
Topcon's GPT-7000 series is a hands-free surveying device that includes a built-in CCD camera, a Windows CE touch screen and TopSURV on-board data collection software. It provides up to 250 meters of reflector-less operation and can measure up to 3,000 meters with a prism. The built-in camera can capture images from a job site.
Gyro-stabilized camera mounts are used for most aerial surveying because they significantly improve the quality of the aerial photography. The stabilization helps reduce any loss of quality that may result from aircraft movements such as turbulence, pitch and rolling. Leica's ASCOT Aerial Survey Control Tool is a GPS-based flight management system for aerial survey flights. It handles flight planning, navigation and camera control during the flight, and produces flight reporting and GPS data that is ready for postprocessing.
Technologies such as LiDAR (light detection and ranging) can help address these types of data needs. LiDAR uses powerful laser transmitters and receivers, a GPS receiver and a flight management system to calculate the altitude of the plane. The level of detail that can be obtained from LiDAR is staggering. LiDAR systems can provide a horizontal accuracy of better than 18" and a vertical accuracy of better than 6". Commercially available LiDAR systems typically advertise vertical accuracy of around 15 centimeters if ground control points or stereo superposition is used to help verify the data. A network of ground control points can provide an even greater level of accuracy for LiDAR systems. In addition, LiDAR technology can produce high-quality DTMs faster and less expensively than traditional stereo-compilation methods. Traditional processing with stereo pairs can collect 1,500 to 2,000 points per hour. LiDAR, on the other hand, can collect around 10,000 points per second.
In recent years we've seen an increased integration between land surveying and GIS, and that trend is expected to continue. The two professions will continue to become more integrated because more GIS applications are requiring the level of resolution that land surveyors can provide.
IN THIS ARTICLEArcCadastre • www.arccadastre.com/
ESRI • www.esri.com
Leica • www.leica.com
Topcon • www.topcon.com
Trimble • www.trimble.com
James L. Sipes is the founding principal of Sand County Studios in Seattle, Washington. Reach him at firstname.lastname@example.org