2.3.2 Applications of GPS

GPS SATELLITE SURVEYING

Adopting the broadest definition of "GPS surveying", the following classes of surveys can be identified:

• Land Survey: applications which for the most part are associated with surveying and mapping operations.
  
• Marine Survey: applications related to hydrographic, oceanographic and exploration geophysics survey operations.
  
• Airborne Survey: applications associated with aerial mapping, scientific and exploration geophysics surveys.

What are the criteria for deciding if an application belongs to "surveying", "navigation", or "other"? This is not as easy as it may first appear. In general (and there are exceptions), a "surveying" application is a positioning task that:

• Is of comparatively high accuracy. This is, of course, a subjective judgement, but in general "high accuracy" is interpreted as being of an accuracy beyond that originally intended for a positioning system operated in a standard routine manner. As GPS is a navigation system which is intended to deliver only 100m absolute positioning accuracy, there is the potential for applications requiring a higher accuracy than this to be legitimately regarded as "survey" applications. However, "differential navigation", or DGPS, can deliver accuracies of several metres. Hence the accuracy threshold for "surveying" may be arbitrarily set at the sub-metre level.
  
• Requires the use of unique observation procedures, measurement technologies and data analysis. The development of specialised procedures, instrumentation and sophisticated software is the hallmark of GPS "surveying". There are two main types of observations that can be made on the GPS signals, each with very different "noise" characteristics: measurements of phase on the L-band carrier signals have millimetre random error, while pseudo-range measurements made with the aid of the time signals modulated on the carrier waves are between 100 and 1000 times noisier.
  
• Is not required "urgently". That is, it is an application in which if positioning information is not available in real-time, a tragedy is unlikely to occur. "Navigation" is concerned with the safe passage of vehicles, ships and aircraft.
  
• In general permits post-processing of data to obtain the highest accuracy possible.
  
• Has as its reason d'entre, the production of a map, or the establishment of a network of coordinated points which support the traditional functions of the surveying discipline, as well as new applications such as GIS.
  
• Is generally a static positioning task. This is, of course, not the case with marine and airborne surveying operations. (There is however a strong trend towards "kinematic" GPS surveying as described in section 5.5.1.)

In the case of land surveying applications, the characteristics of GPS satellite surveying are less contentious:

• The points being coordinated are generally stationary.
  
• Depending on the accuracy sought, GPS data are collected over some "observation session", ranging in length from several seconds to several hours, or even days.
  
• Restricted to relative positioning modes of operation (those applications that can be satisfied with GPS operated in the single point positioning mode are therefore considered to belong to the "land navigation" category).
  
• In general (depending on the accuracy sought) the measurements used for the data reduction are those made on the transmitted L-band carrier wave, and not on the timing signals modulated on the carrier waves -- hence the requirement for specialised hardware and software.
  
• Mostly associated with the traditional surveying and mapping functions, but accomplished using GPS techniques in less time, to a higher accuracy (for little extra effort) and with greater efficiency.

Land Surveying Applications for GPS

A convenient approach is to adopt an applications classification on the basis of accuracy requirements. Three classes of applications can be identified on this basis, for which a range of relative accuracies (it is assumed that single receiver point positioning is not accurate enough to satisfy these applications) ranging from low-to-moderate, 1 part in 10⁴, through to the ultra-high 1 part in 10⁷ or better accuracies:

Category A (Scientific)	: better than 1 ppm
Category B (Geodetic)	: 1 to 10 ppm
Category C (General Surveying)	: lower than 10 ppm.

Category A surveys primarily encompass those surveys undertaken for precise engineering, deformation analysis, and geodynamic applications. Category B surveys include geodetic surveys undertaken for the establishment, densification and maintenance of control networks to support mapping. Category C surveys primarily encompass lower accuracy surveys, primarily undertaken for urban, cadastral, geophysical prospecting, GIS and other general purpose mapping applications. Users in the latter two categories form the majority of the GPS user community, while category A users often provide the primary "technology-pull" for the development of new instrumentation and processing strategies, which may ultimately be adopted by the category B and C users.

Note that this classification scheme is entirely arbitrary, and does not reflect any "order" of survey as defined by Survey Authorities. It does, however, provide a convenient breakdown of GPS survey "type", enabling the similarities and differences between the categories to be highlighted. Below are listed the advantages and disadvantages of the GPS technology (in the context of land survey applications) in broad-brush terms only.

Advantages of GPS Over Conventional Surveying Methods

There are several advantages of the GPS satellite surveying technique:

• Intervisibility between stations is not necessary.
  
• Because GPS uses radio frequencies to transmit the signals, the system is independent of weather conditions.
  
• If the same field and data reduction procedures are used, position accuracy is largely a function of interstation distance, and not of network "shape" or "geometry".
  
• Because of the generally homogeneous accuracy of GPS surveying, geodetic network planning in the classical sense is no longer relevant. The points are placed where they are required (for example, in a valley), and need not be located at evenly distributed sites atop mountains to satisfy intervisibility, or network geometry, criteria.
  
• Because of the two advantages of not requiring intervisibility of stations, or following a conventional network design strategy, GPS surveying is more efficient, more flexible and less time consuming a positioning technique than using terrestrial survey technologies.
• GPS can be used around-the-clock.
  
• GPS provides three-dimensional information.
  
• High accuracies can be achieved with relatively little effort, unlike conventional terrestrial techniques. The GPS instrumentation, and to some extent the data processing software, is similar whether accuracies at the 1 part in 10⁴ or 1 part in 10⁶ level are sought.

Disadvantages of GPS Surveying

It would be remiss not to also mention the disadvantages, some of which will no doubt be overcome in time, others with some additional effort, while others cannot be dismissed so easily:

• High efficiency has its price. Efficient use of GPS requires that travel times between stations are cut in order to match the savings in on-site time.
  
• Because station intervisibility is not necessary GPS is a particularly attractive technology for use in rugged, inhospitable terrain. However, the logistical problems of transporting and supporting several field parties are still formidable (and would have been even if conventional terrestrial techniques were used). If helicopters are necessary, the costs of the survey will rise substantially.
  
• GPS requires that there be no obstruction to the signals by overhanging branches or structures (though the antenna can be raised above the obstruction). It cannot, of course, be used underground, and may have limited application in densely settled urban areas.
  
• Because GPS surveys can be "optimised" (by appropriate selection of sites) to satisfy the specific needs of the particular survey, these may not be useful for other applications in the same area. Further GPS surveys may need to be carried out, as the need arises, for new applications. The extreme of this would be to dispense with a permanently monumented control network altogether, and to require the re-establishment of coordinates by GPS each time the need arises.
  
• Two intervisible stations would have to established by GPS in order to satisfy the requirement for azimuth data for use by conventional (line-of-sight) survey methods.
  
• GPS coordinates are provided in the earth-centred, earth-fixed coordinate system defined by the GPS satellite ephemerides (the WGS84 system when the broadcast ephemeris is used). Results may need to be transformed into a local geodetic system before they can be integrated with results from conventional surveys.
  
• GPS results are, in general, more accurate than the surrounding control marks established by terrestrial techniques over time. Comparison of GPS and terrestrial results will be the source of confusion, controversy and conflict for many years to come.
  
• GPS vertical information is not provided in the height system generally required. The GPS heights have to be reduced to a sea level datum (more precisely, the geoid).
  
• The GPS instrumentation is still comparatively expensive. Although the price of one receiver is likely to soon match that of a theodolite-EDM instrument, a minimum of two are required for survey work.
  
• GPS requires new skills to be learned, and new procedures and strategies for planning, field operation and data analysis to be developed. In addition, an understanding of how GPS results can be integrated with conventional horizontal and vertical networks is required.

Further Remarks

The prospect for increased acceptance of GPS satellite surveying is very good, particularly as the cost of GPS systems drops and new higher productivity techniques are developed. Although GPS was initially used for high-order geodesy and geodetic control surveys on the one hand, and geophysical exploration surveys on the other, adoption of the GPS technology for applications such as lower-order control densification, and even cadastral, engineering and detail surveys, has already commenced.

However, for all its technical advantages, there remain a number of significant differences between the results of the GPS surveying technology and that of conventional terrestrial techniques. To reconcile these differences, and in order to ensure that GPS will complement these other technologies (and hence maintain compatibility with the geodetic framework established in many countries over a long period of time), a significant amount of post-processing of GPS results is necessary. This tends to make the GPS technology less attractive, and has the effect of raising the threshold of acceptance slightly higher than it would otherwise have been.

In addition there is an investment in human resource development that must be taken into account. GPS manufacturers are striving to make equipment that is ever more "user-friendly", which will mean that many other professionals apart from "qualified professional surveyors" will be able to carry out high accuracy GPS surveys. The challenge, however, to surveyors is to maintain the "edge", by seeking to use their best judgement and skills not only to achieve high GPS accuracy, but also to ensure that the quality and reliability of the results are at the level demanded by the client. A further advantage that surveyors enjoy over other professionals is that they often are the only ones skilled in integrating GPS results into previously coordinated networks.

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© Chris Rizos, SNAP-UNSW, 1999