|Accuracy Classification||95% Confidence
(less than or equal to)
From:Federal Geographic Data Committee FGDC-STD-007.2-1998
Draft Geospatial Positioning Accuracy Standards
Part 2: Standard for Geodetic Networks
The Federal Geodetic Control Committee (FGCC) wrote accuracy standards for GPS relative positioning techniques. These were preceded by older standards of first, second, and third order that then became subsumed under the group C in the newer scheme. Until the last decades of the twentieth century, the cost of achieving first-order accuracy was considered beyond the reach of most conventional surveyors. Besides, surveyors often said that such results were far in excess of their needs, anyway. The burden of the equipment, techniques, and planning that is required to reach its 2σ relative error ratio of 1 part in 100,000 of the old first-order was something most surveyors were happy to leave to government agencies. But the FGCC’s standards of B, A, and AA are respectively 10, 100, and 1000 times more accurate than first-order. With the advent of GPS, the attainment of these accuracies did not require corresponding 10-, 100-, and 1000-fold increases in equipment, training, personnel, or effort. They were now well within the reach of private GPS surveyors, both economically and technically. These accuracy standards are now superseded.
In 1998, the Federal Geodetic Control Subcommittee under the Federal Geographic Data committee published the Geospatial Positioning Accuracy Standards Part 2: Standards for Geodetic Networks. These standards, shown, supplant the earlier standards of 1984 and 1989.
These upgrades in accuracy standards not only accommodate control by static GPS; they also have cast survey design into a new light for many surveyors. Nevertheless, it is not correct to say that every job suddenly requires the highest achievable accuracy, nor is it correct to say that every GPS survey now demands an elaborate design. In some situations, a crew of two, or even one surveyor on-site may carry a GPS survey from start to finish with no more planning than minute-to-minute decisions can provide, even though the basis and the content of those decisions may be quite different from those made in a conventional survey.
In areas that are not heavily treed and generally free of overhead obstructions, sufficient accuracy may be possible without a prior design of any significance. But while it is certainly unlikely that a survey of photocontrol or work on a cleared construction site would present overhead obstructions problems comparable with a static GPS control survey in the Rocky Mountains, even such open work may demand preliminary attention. For example, just the location of appropriate vertical and horizontal control stations or obtaining permits for access across privately owned property or government installations can be critical to the success of the work.
An initial visit to the site of the survey is not always possible. Today, online mapping browsers are making virtual site evaluation possible as well. Topography as it affects the line of sight between stations is of no concern on a static GPS project, but its influence on transportation from station to station is a primary consideration. Perhaps some areas are only accessible by helicopter or other special vehicle. Initial inquiries can be made. Roads may be excellent in one area of the project and poor in another. The general density of vegetation, buildings, or fences may open general questions of overhead obstruction or multipath. The pattern of land ownership relative to the location of project points may raise or lower the level of concern about obtaining permission to cross property.
It is now relatively easy to do GPS surveying, that is when everything works the way it's supposed to. However, if there are going to be troubles like access to the points, overhead obstructions, work in trees, or helicopter transport, planning needs to be part of the process.