GPS Surveying
Electric utility pairs real-time DGPS and real-time mapping for leading edge solution
By Peggy Ammerman

Because data is the very basis of a GIS (geographic information system) and ultimately determines the system's effectiveness, acquiring data is perhaps one of the most critical of all the processes involved in the construction of a GIS. In acquiring data on utility facilities such as poles, meters, and valves, conducting a field inventory offers several distinct advantages as a data acquisition solution. In addition to verifying utility facilities and producing a comprehensive structure-by-structure inventory, a field inventory can take advantage of global positioning systems (GPS) technology to create geographically precise data. Through GPS data collection which is the main task of field inventory, point feature data is produced. Point features are geographically defined by coordinates, usually x and y, at specified ratios of accuracy, often within +/- 3 feet (ft.). This field collected, highly precise data can then be placed on a land base through GIS data conversion and mapping processes.
   Data that is acquired using GPS technology comes with a certain degree of error. Once collected, an error correction technique called differential GPS (DGPS) is applied. As with many of the tasks involved in the construction of a GIS, DGPS as well as GIS mapping can each be performed in real time or as post processing tasks. While field inventory typically relies on real-time DGPS, few solutions that involve both field inventory and GIS mapping perform the mapping component in real-time or link the two processes in an integrated manner.
   In recognizing the value of performing both these processes in real-time, the MSE (Indianapolis) customer service center of Analytical Surveys, Inc. (ASI) has developed a leading edge field inventory solution. Based in Colorado Springs, Colorado, ASI is a world leader in a full range of advanced computerized mapping services. Through ASI's broad-based experience with hundreds of utility and municipal GIS clients and substantial expertise in GIS data translation, conversion, photogrammetry and GPS surveying, a solution was developed that maximizes the capabilities as well as integrates real-time DGPS and GIS mapping technologies, the core components. As a result, field inventory becomes a continuous process that is performed in the field and produces reliable, accurate data.
   At the core of the innovative solution are real-time DGPS and GIS mapping components. In addition, various technologies and processes enable, extend, and link the capabilities of these core components. For example, real-time differential corrections for GPS are extended to technicians working in the field by broadcasting the data via a licensed radio frequency (RF) transmitter system. The technicians are equipped with roving GPS receivers that accept real-time corrections and are then integrated with the real-time mapping process. To take full advantage of global positioning systems technology, data resources from both NAVSTAR (U.S.) and GLONASS (Russian) constellations are used. As another example, to improve on conventional GPS collection, ASI has developed a unique offset mapping method. This method uses 2 laser measurements to calculate a GIS offset which has proven more reliable and accurate. Real-time mapping also takes place on a pen-based PC and uses a Visual Basic application that acts as the "control panel." This automated process links to ASI's proprietary conversion environment called GIS Neutral Object Manipulation Engine (GNOME), which brings together point and feature data on a digital map in real-time. In addition to these technology-driven tools, the solution also relies on the ingenuity of the field inventory team who has built several custom-made devices like telescoping poles for mounting GPS equipment and a mobile radio antenna set-up called mobile antenna radio repeater system (MARRS) for boosting the broadcast of GPS corrections.
   The field inventory solution was originally developed for a facility survey and data conversion project at Gainesville Regional Utilities (GRU). ASI was presented with GRU's existing graphical maps and tabular data as a basis to place additional attributes and data for new developments in ESRI Arc/INFO format. The additional attributes included unique pole tag numbers, pole heights, and (x,y) coordinates. The project specified a relative accuracy of +/- 1 meter. The GRU electrical distribution system covers 130 square miles and includes approximately 70,000 customers. Initially 120,000 points required GPS coordinates.
   To help define a solution for the remainder of the project, ASI conducted a pilot. Many discoveries were made at the conclusion of the test run. The pilot is where ASI can draw on the corporate-wide resources and substantial experience from similar projects to test and support the proposed solution. For example, from other electric utility inventory projects ASI knows that electro-magnetic fields (EMFs) surround electrical equipment and can cause compass errors. Since traditional laser rangefinder offsets are based on the accuracy of compass readings, one degree of error at a distance of 200 ft. can produce an offset mapping error of 3.5 ft. This is usually in excess of the client's standards of relative accuracy at +/-3 ft. for GPS data collection. Working around electrical equipment usually increases compass errors to more than +/-10¡, which can cause offset mapping errors of more than +/- 35 ft. Calibration of fluxgate compasses increases accuracy. However, this is a time-consuming task that can impede work flow in a fast-paced production environment. ASI abandoned the use of compasses and instead came up with a solution that allows field technicians to use laser rangefinders along with a method called trilateration for collecting GPS offset data. Two GPS points are obtained as well as two laser distance measurements from each of these points to a specific object. An offset position can then be calculated with consistent accuracy. The trilateration method and laser rangefinder are useful in situations where the facility being inventoried is simply inaccessible. For example, a tall building might block GPS reception or a pole could be in a restricted area. In these instances, the technician can position himself for adequate GPS reception and use the rangefinder and trilateration method to determine the GPS point for the facility.
   In addition to using the pilot for testing, several parts of the solution were innovated on an ad hoc basis. While the project presented technical challenges, there were factors like heat and a heavy tree leaf canopy that required innovation as well. The field inventory team worked for several months under normal conditions and then Florida was hit with a heat wave during the summer. The field team saw several weeks worth of daytime temperatures hovering over 100¡ F. The backpacks that carry the equipment are black, insulated, and designed to retain heat. Schedules required adjustment to allow for work during the cool of the day. Equipment was also affected by the heat wave. For example, the camcorder batteries used in the GPS backpacks have a safety device which is activated at 105¡ F. At that temperature, the battery shuts off to protect it from overheating. To continue working, the batteries had to be kept cool. (Note: Regardless of the distributor's labeling, most batteries are made by one manufacturer.) Other equipment was also affected by the heat wave. When exposed to intense heat and sunlight, the LCD screens on the pen-based PCs turned dark which impaired visibility.
   The process of gathering data on structures like poles which are located in a wide-open area is fairly straightforward. Where there are obstructions though, multipath reception or the reception of a signal along a direct and one or more reflected paths becomes an issue. The Gainesville area wasn't expected to have such a heavy tree canopy, which affected the technicians' ability to receive satellite signals. Similarly, the location of facilities such as meters limited the ability to acquire GPS data. In Florida, the most common roof style on homes includes a wide overhang. The combination of wide overhangs with meters mounted on walls under the overhangs made the reception of GPS data from satellites nearly impossible at times. In situations where data is being collected for wall-mounted meters, 180¡ of the sky and therefore, half your satellites are lost which means you have just half the opportunities to collect GPS data. To maximize our opportunities for GPS reception, data was collected from GLONASS satellites along with the NAVSTAR constellation. At a minimum, four satellites are required for data collection. So, when half the sky is being blocked by a building, securing the four satellites can be difficult using NAVSTAR only. By using both GLONASS and NAVSTAR, opportunities to secure satellite data were doubled.
   Improvisation also had a hand in increasing satellite reception opportunities. It was determined that because z coordinates (elevation data) were not required, the height of the GPS antenna could be extended to capture more satellites. The GPS antenna was mounted on a telescoping pole that was fashioned from camping tent poles. The expandable pole added 30 inches in height. For easy transport in the backpack, the pole collapsed back down. With our special extendable pole that reached above roof overhangs, another two to three satellites could be obtained for use in calculating a GPS position.
   There are two parts to GPS. One is capturing the satellite data. The other is securing and sending real-time DGPS corrections for the data to roving GPS units in the field. GPS error can result from noise, bias and blunders. Bias errors are usually corrected using differential GPS techniques. Through the process of differential correction, errors are corrected using data from a receiver called a base station at a known location to correct data on receivers known as roving receivers at other, unknown locations. With differential correction, a reference receiver or base station computes corrections for each satellite signal. Differential GPS can take place in real-time or by post processing. Real-time DGPS was chosen for the solution because it allowed the technicians to apply the corrections and check for quality while they were in the field capturing and mapping the data. Plus, the real-time mode of operation allowed all the technicians to concentrate on field inventory activities rather than post-processing tasks. With post processing, several additional steps are required to process even just one electrical circut
   For the real-time part of the DGPS solution, a UHF radio broadcast base station was established atop the GRU administration building. The U.S. Coast Guard offers public access to correction signals through continuous operating reference stations (CORS). However, the nearest CORS was 300 kilometers from Gainesville which could have introduced an additional 30 centimeters of error. Additionally, CORS cannot correct the GLONASS satellite positions. Considering these limitations, ASI decided to build our own FCC-licensed FM radio station to receive and broadcast correction signals. Securing a FCC license in a timely manner is key to the broadcast system. A broker handled filing the application and securing a temporary license.
   With the help of the University of Florida's survey department and that of a contracted licensed Ashtech Surveyor, a survey point of the GRU base station was established. That way, the base station could be used as an accurate reference point in conjunction with the DGPS transmission. To determine the survey reference point, GPS data was collected at the base station and at a highly accurate reference network (HARN) point located on the University of Florida campus. The data was collected at the two points over the same time period and using the same satellites. The data from the university and the base station was then compared for errors and a reference point was determined with relative accuracy of +/- 1 centimeter. The GPS antenna was then ready to receive as well as transmit correction signals from the reference point. For the most part, the base station and roving receivers are using the same satellites, so corrections can be determined with greater reliability. The broadcast system transmits the corrections to the roving receivers that apply the corrections in real-time to the GPS data.
   To broadcast the signals, a FM radio broadcast system consisting of the GPS receiver unit linked to a licensed RF transmitter was built. The base station calculates autonomous positions every second from available satellites. These positions are compared with the known survey coordinates or reference point of the base station antenna and the error is then compiled. The comparison of autonomous versus survey coordinates - the error - is put in a packet of binary information and then transmitted to roving GPS receivers in the field. While a field technician is collecting a GPS point the binary packet is read, calculations are made, and the point is differentially corrected in real-time.
   The base station antenna was placed at a height of 80 ft. The signal is transmitted through a 35-watt amplifier, which gives an approximate broadcast range of 6 miles. Because there are some landscape features such as hills and low spots that obstruct transmission, a repeater was used to send the information even farther. The repeater is an antenna that is semi-permanently installed on a 250-foot radio tower. The repeater receives the signal and then boosts it with another 35 watts of power so it reaches another 10 miles. There were still some gaps in the broadcast system, so Pacific Crest 2-watt radio modems were programmed to function as repeaters. These additional modem repeaters are mounted on vehicles and help boost signals to dead spots in the project area. Originally, the field team thought that using the repeater as a stationary unit on a tower would suffice. However, the correction signal was still blocked by trees and other obstructions. So, a 35-watt mobile repeater that can be left in parks or open areas to rebroadcast signals into dead zones was added. The mobile unit consists of an antenna at a height of 20 ft. supported by down guys and tapped into the ground with tent stakes. The unit is powered by a car battery and placed in a 50-gallon cooler with a fan for ventilation. This device was named MARRS, short for mobile antenna radio repeater system.
   What sets the solution apart as a leading edge model for field inventory is joining real-time DGPS with real-time mapping. Feasibly, a field technician could capture 100 points a day. However, the real value of acquiring corrected point data is being able to immediately associate the point with a feature on a map. Real-time mapping, like real-time DGPS, also affords the benefit of quality control and being able to instantly verify data. Through our field inventory solution, field technicians have all the tools to associate and map corrected GPS points with the correct structures while they are standing in the field conducting the inventory.
   The mapping component uses pen-based PCs and links several software applications, which convert and map the point feature data. During the first part of the process, the GPS NMEA code, a standard electronic communications string used with GPS, is read by the Geographic Tracker, a Blue Marble Geographics product. Basically, the Tracker allows a pen-based PC to read, accept and translate the NMEA string, and also recognize NAVSTAR and GLONASS data. Next, the position data is run through Blue Marble's Geographic Calculator which reads the latitude and longitude, translating the data to: U.S. survey foot, NAD 1983, state plane coordinate system - Florida north zone. The information is then used by ASI's Visual Basic application. This application allows the user to display the northering and easting coordinates. On demand, the field technician can then initiate the collection of 10 points worth of data. Once the position data is captured, the application performs a root mean square calculation that gives an average position for the 10 points. Overall, the Visual Basic application functions as the "control panel" of the point collection process by controlling the GPS unit and data collection process. There is a link between the Visual Basic application and the final mapping step that is anchored by ASI's robust and flexible data conversion environment, GNOME. The GIS neutral object manipulation engine or GNOME is an object-oriented program that enables highly accurate and efficient processes such as the manipulation and placement of objects according to the specifications of the target GIS. More importantly, the GNOME conversion environment promotes the efficient and precise placement of features according to the exact x,y position. GNOME also operates according to prescribed rules, promoting quality control in real-time.
   To build control as well as confidence into the solution and also satisfy project requirements, all GPS equipment is set to certain industry-accepted and project-specified standards. Equipment is programmed with a text initialization file that specifies certain conditions. For example, the GPS receiver is programmed to only accept signals for PDOP, a measure of satellite-receiver geometry, of no more than 6. Initialization also flags unhealthy satellites and set the elevation mask which specifies that acceptable satellites are those located at least 10¡ above the horizon.
   Through ASI's novel real-time solution, other technology tools and processes; the reliability and accuracy of data is ensured from the outset. With GIS construction, the quality of the data starts with the process used to acquire the data. As a data acquisition strategy, the field inventory solution becomes a critical factor in the overall success and value of the GIS.

About the Author:
Peggy Ammerman is a writer with the Indianapolis, Indiana, office of Analytical Surveys, Inc. (ASI) and specializes in GIS and geospatial information technology issues. She can be reached at 317-634-1000, ext. 285 or by e-mail at [email protected].

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