2.3.2 Applications of GPS

GPS SATELLITE SURVEYING

 


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

 

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:


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


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:

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:

 

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