In the late 1970’s, the U.S. Department of Defense (DoD) developed a satellite-based system for locating and positioning men and equipment in battle. This system became known as Global Positioning System or GPS. Early in its development, GPS technology was slow, coverage was limited, and accuracy was highly dependent on field conditions. By the late 1980’s, reliability, accuracy, and coverage improved, and the technology became available to the public for private and commercial use. These advances were the result of improvements to hardware, software, and, most importantly, the deployment and positioning of additional satellites in orbit around the globe.
Today GPS technology is reliable, accurate, and a common part of everyday life. Families use it to navigate to vacation destinations, shipping companies use it to monitor the locations of their trucks, and geologists use it to measure the minute changes in elevation and position of a mountain top. The applications for GPS data are almost limitless. It was inevitable that someday GPS would be used as a design tool. As designers we are always looking for better ways to understand and graphically represent the physical dimensions and spatial relationships of both existing and proposed elements on our sites. GPS technology provides us with just such a tool.
The modern day GPS is made up of three segments. The first of these segments is the Space Segment. This segment is comprised of 24 NAVSTAR satellites (SVNs) orbiting the earth every twelve hours at an altitude of approximately 12,600 nautical miles. This enables uninterrupted GPS coverage worldwide. Each satellite contains several high precision atomic clocks that constantly transmit radio signals to earthbound receivers using its own unique identification code. The second segment of the GPS System is the Control Segment. This segment consists of a group of four ground-based monitor stations, three upload stations, and a master control station. These stations continuously track the satellites and accurately calculate their positions in space, using complex algorithms and clock correction coefficients. These first two segments are controlled and maintained by the DoD. Out of a concern for U.S. national security, the DoD has the capability to turn off or on any satellite, or to introduce deliberate errors into the GPS measurements. This is known as Selective Availability or S/A. The third GPS segment is the User Segment. This segment consists of various public, private, and military GPS receivers on land, sea, and air.
There are two basic “grades” of GPS receiver technology currently available to the public. The first is “Survey Grade” GPS technology which can be as accurate as 1/100th of a foot in both the vertical and horizontal planes. This technology is expensive, complex to operate, and is normally reserved for use by registered surveyors for whom that level of accuracy is critical. The more commonly used GPS technology is known as “Mapping Grade” GPS technology. Mapping Grade technology is consistently accurate to <1 meter in the horizontal plane, and is usually sufficient for the purpose of design development. Vertical accuracy is not consistent with this grade of GPS unit.
The “Mapping Grade” units are user-friendly, and data collected in the field may be recorded as either DGPS (Differential GPS) or as Real-time DGPS. DGPScollected field data must be “post processed” in order to achieve sub-meter accuracy. Simply put, the computed positions of the field data, which are unknown locations, are compared to the position of a known location, a “Base Station”, which records the error for each satellite in a computer file. These offset distances are used to compute and process the data to increase the accuracy of the data logger (Rover) positions to less than one meter. The computed output is a differentially corrected file. This is done in the office. Many DGPS community base stations are located and maintained by universities, municipalities, and federal agencies. Most are available for use by the general public.
In Real-Time DGPS the Base Station calculates and immediately broadcasts the error for each satellite as it receives the data. This “correction” is received by the Rover unit which then immediately applies the correction to the position it is calculating. The result is the position you see on the Rover is your differentially corrected position with an accuracy equal to the data had it been post-processed. These Real-Time GPS stations are maintained primarily by the U.S. Coast Guard and U.S. Corps of Engineers. With both the Community and Real-Time Base stations the distance from the Rover is an inherent factor in the accuracy of the position. Generally a distance of no more 250 miles between the Base Station and Rover is required to ensure sub-meter accuracies.
Trimble Navigation currently makes two GPS units which are excellent Mapping Grade GPS units, the Trimble Geo Explorer and Trimble ProXL. These “Rover” units are either hand-held or backpack field units that are rugged and capable of withstanding adverse environmental conditions. Each unit is battery-operated and comes with a receiver, antenna, data collector and data processing software. Both the Windows-based data collection and data processing software function together seamlessly to efficiently and accurately collect and process point, polygon, and line segment data.
To more efficiently collect field data and to also ensure that the data collected is consistent within a given dataset, a user may elect to develop a “data dictionary”. Data dictionaries are data collection “menus” that allow the user to collect specific attribute information about a particular feature. They are used in the field to control the collection of features (objects) and attributes (information about those objects). A data dictionary includes a list of features for data that will be collected in the field and, for each feature, a list of attributes that describe the feature. A data dictionary prompts the user to enter information; it can also limit what is entered to ensure data integrity and compatibility with a GIS or CAD system. Although a data dictionary is not always required for field work, having one does make both data collection and processing faster and easier. The data dictionaries you create depend on the intended applications. Since different users have differing collection requirements, each user can design data dictionaries to suit their needs.
The collected and edited data can be exported using the data processing software in formats compatible with AutoCAD, Arcview, Excel, Microstation, and MapInfo file structure to name a few. The new layers of collected GPS data can be registered to existing base information for a better interpretation and understanding of a site’s design constraints and opportunities.
For designers, what are some examples or types of GPS data that might be useful in the design process? Line data could be used by recreational designers to locate, map, and lay out foot trails, boardwalks, fitness courses, and bike paths. Environmental designers may use point, line, and polygon data as a tool to develop, map, and lay out wetland and stream bank restoration projects, or to map locations and attribute data of threatened and endangered species and critical wildlife habitat. Urban land planners may use GPS to capture data on the location, species, size, and condition of street tree plantings, or to locate and map utilities such as manholes, transformers, water meters, and other utility infrastructures. In summary, GPS as a tool for design is invaluable for accurately recording the spatial and attribute data of existing and proposed site features where traditional surveying might be cost prohibitive or unrealistic. It allows the designer to efficiently locate and/or lay out existing and proposed site features in a spatially accurate, real world relationship, while enhancing the design process.Richard D. Powers, ASLA, is Vice President of Earth Design Inc., in Pickens, South Carolina. He can be reached at firstname.lastname@example.org.