Journal of Surveying Engineering

For over 30 years Sercel has been studying and manufacturing high accuracy commercial electronic equipment used for maritime radiolocation and land surveys. To answer the present and future needs in these fields, Sercel developed two types of GPS receivers intended for high accuracy, real time dynamic positioning (TR5S) and geodesy or survey (NR52). Software has been developed that allows the user, thanks to very accurate GPS data gathered by these receivers, to work out either fix position solutions with a decimetric precision within a few minutes, or with a centimetric precision within a few tens of minutes. This software is very flexible (multisite and multisession solutions) and offers a very high computation speed using light processing means. This paper addresses Sercel's latest software, which allows accurate determination of the trajectory of any mobile (land mobile, maritime mobile, or plane). This software has been set up and tested under severe conditions. The accuracies obtained are about 10 to 50 cm, three‐dimensional, and have a fast data sampling rate and a very low noise measurement. Original and effective answers have been brought to the difficult problem of temporary satellite masks. This software and equipment has been proved operational in various fields. This paper describes the equipment and methods used and the results obtained from field and flight tests.

Since 1982, the Geodetic Survey of Canada has been carrying out precise trilateration by electro‐optical distance measurements (EDM) incorporating atmospheric data recorded along the EDM ray path. These networks have been established in seismically unstable areas of Vancouver Island, British Columbia, and the Charlevoix region in Quebec. The three‐dimensional adjustments of EDM measurements show standard deviations of the order of 1 cm in horizontal position or relative accuracies of 0.5 parts per million on average. Replacing trilateration with GPS observations, to monitor these networks, would provide substantial savings in time and money, as well as eliminating many logistic constraints inherent to terrestrial survey methods. In 1986, in order to evaluate the GPS measurement as a potential alternative to precise trilateration, ten stations of the 16‐station network near Port Alberni, on Vancouver Island, were positioned using four TI‐4100 GPS receivers immediately following the precise EDM survey. This network covers an area of $50\mathrm{km}\times 75\mathrm{km}\text{,}$ with interstation distances varying from 15–45 km. The data collected were processed using DIPOP, software developed at the University of New Brunswick, and with PHASER developed by USNGS. Solutions using triple, double, and nondifferenced phase observations in baseline mode and network mode are compared. Details of the observing procedures are discussed. Solutions from the different processing methods are compared against each other and against the terrestrial values.

Global Positioning System (GPS) surveys were undertaken in 1984 and repeated in 1985 along level lines in southeastern Arizona where subsidence induced by groundwater withdrawal was occurring. The objective of these surveys was to determine the accuracy with which subsidence could be determined both from comparison of repeat GPS measurements and from comparison of GPS measurements with existing leveling. Repeatability of GPS determinations of differential ellipsoid height during a single survey was at the 1–2‐cm level. Differencing results of repeat GPS surveys performed 15 months apart gave subsidence values that agreed at the 2–4‐cm level with those obtained from leveling. Also, it was found that, after using gravimetric geoid heights to convert GPS‐derived ellipsoid heights to orthometric heights, agreement between the GPS/geoid orthofnetric heights and leveling orthometric heights was in the 3–7‐cm range. These results show that repeated GPS measurements can be used to monitor future subsidence in southeastern Arizona and that comparison of GPS results with existing leveling can give valuable information on past subsidence in the area.

Accuracy of a GPS baseline determination is a complex function of satellites selected and the duration and timing of the data collection interval. Satellite selection can be extremely important if the GPS receiver employed is one that tracks only four satellites simultaneously. Even with receivers that track all visible satellites, the timing and duration of the data collection interval can be significant. For short data collection intervals and short baseline observations, an order of magnitude difference in accuracy may result, depending on whether or not the whole cycle ambiguity can be resolved. The ability to resolve whole cycle ambiguities is in turn determined by which satellites are selected and the timing and duration of the tracking interval. Magnavox has developed a “Differential GDOP” program that provides a measure of the ability to resolve whole cycle ambiguities as a function of these parameters. This paper describes a method and rationale for “Differential GDOP” computations. A practical example is used to illustrate its benefits in generating a data collection strategy.

Surveying with global positioning (GPS) satellite receivers affords many opportunities to solve previously difficult surveying problems with greater speed and accuracy than may have been possible with conventional survey techniques. When evaluating the practicality of GPS for a given business, however, there is a big difference between science of the technology, availability of the equipment, and the ability to apply and use the equipment correctly and profitably. Based on extensive firsthand experience, the purpose of this paper is to identify some practical aspects of using GPS.

Differential GPS phase‐measuring surveys yield accuracies of a few parts per million in all three coordinates. To achieve the best results for the least cost requires careful planning of the measurement scheme. Determination of accurate elevations from GPS measurement schemes is possible if the measured vectors are properly adjusted and rotated into the geoidal surface. A discussion of the basics of this procedure is presented.

Out of an increasing need for a reliable survey control system throughout metropolitan Kansas City, Missouri, grew the concept of regional cooperation among the area's municipalities and counties. With such regional cooperation and the assistance of the Missouri Department of Natural Resources, Division of Geology and Land Survey, Land Survey Program, a global positioning system (GPS)‐based control network was designed from six existing first‐order stations and seven benchmarks. The new control network includes 48 additional first‐order control stations and several upgraded NGS second‐order marks and benchmarks. This is the first phase of a multiphase project that covers over $1\text{,}000\mathrm{sq}\mathrm{mi}\left(2\text{,}400{\mathrm{km}}^2\right)$ of the metropolitan area. Participation of local land surveyors from private practice and government led to the success of the project. These surveyors, coordinated by the Metro Surveyors Association, aided in planning, and performed the correct monumentation and sketching of points to be surveyed. The survey was completed in two weeks in June 1987 using Macrometer II instrumentation. After delivery of the GPS data, local surveyors were contracted for astronomical observations on azimuth marks. This paper presents planning and logistics of the regional project and a cost analysis.

GPS‐guided stereo, solid‐state camera systems can perform real‐time surveying and mapping functions that are vital to the automation of many engineering processes. While GPS provides point positioning of the cameras in real time, a stereo vision system can perform 3‐D mapping of the scene, as well as recognition and location of survey targets. GPS has already demonstrated the positioning accuracy required for such a system. Vision systems with pointing accuracy of $\pm 1\sec$ of arc or better have also been built. Current research and developments in vision systems for surveying are cited. Two conceptual applications of GPS‐guided vision systems are given.

Surveying results with the Global Positioning System (GPS) have impressed those in the surveying industry because of the potential for a drastic reduction in the time and effort required for many projects. The Motorola Eagle Geodetic Receiver System has achieved a major step in the direction of meeting the requirements of a practical GPS surveying tool that can be used with a minimum of support personnel. By combining the small size and light weight of the popular Motorola Eagle GPS Receiver with a modest laptop computer, a costeffective system for performing geodetic survey work has been developed. The menu‐driven operation of the system is described, and is designed to be easy to use with a minimum of training. Features also described include a self‐contained baseline adjustment program, as well as other features that enhance the efficiency of dealing with data from large survey projects.

Analysis of Global Positioning System (GPS) survey data has shown that GPS can be used to establish precise relative positioning in a three‐dimensional system. The results of many tests and operational projects have clearly shown that GPS survey methods can replace classical horizontal terrestrial survey methods. Comparable accuracies have also been achieved for GPS‐derived ellipsoid height differences. The problem of converting these ellipsoid height differences to orthometric height differences remains to be resolved. Can the accuracies achieved for these orthometric height differences provide a viable alternative to classical geodetic leveling techniques? Some results of analyses performed by the authors in estimating orthometric heights from GPS surveys indicate that with appropriate planning, consideration of GPS survey specifications for connection to bench marks, proper field observing procedures, and proper strategy for estimating geoid heights and final orthometric height values, it is possible to use GPS survey methods to estimate orthometric heights to meet a wide range of engineering requirements for vertical control. Therefore, another question needs to be addressed. What are the accuracy requirements of most engineering and mapping applications? This is best answered by users of the data and will influence how much effort should be directed toward determining more accurate geoid heights.