The GPS Observer
Today, there are no specifications for land surveying. There are state standards, usually dealing with the error of closure of a traverse. Most surveyors know to double every observed angle except for topographic surveys. There are specifications for ALTA surveys, and they are rigorous.
SpecificationsGeodetic leveling is a measurement system comprised of elevation differences observed between nearby rods. Leveling is used to extend vertical control.
New surveys are required to tie to existing network bench marks at the beginning and end of the leveling line. These network bench marks must have an order (and class) equivalent to or better than the intended order (and class) of the new survey. First-order surveys are required to perform check connections to a minimum of six bench marks, three at each end. All other surveys require a minimum of four check connections, two at each end. "Check connection" means that the observed elevation difference agrees with the adjusted elevation difference within the tolerance limits of the new survey. Checking the elevation difference between two bench marks located on the same structure, or so close together that both may have been affected by the same localized disturbance, is not considered a proper check. In addition, the survey is required to connect to any network control points within 3 km of its path. However, if the survey is run parallel to existing control, then the following table specifies the maximum spacing of extra connections between the survey and the control. At least one extra connection should always be made.
InstrumentationOnly a compensator or tilting leveling instrument with an optical micrometer should be used for first-order leveling. Leveling rods should be one piece. Wooden or metal rods may be employed only for third-order work. A turning point consisting of a steel turning pin with a driving cap should be utilized. If a steel pin cannot be driven, then a turning plate ("turtle") weighing at least 7 kg should be substituted. In situations allowing neither turning pins nor turning plates (sandy or marshy soils), a long wooden stake with a double-headed nail should be driven to a firm depth.
Calibration ProceduresCompensator-type instruments should be checked for proper operation at least every two weeks of use. Rod calibration should be repeated whenever the rod is dropped or damaged in any way. Rod levels should be checked for proper alignment once a week. The manufacturer's calibration standard should, as a minimum, describe scale behavior with respect to temperature.
Field and Office ProceduresDouble-run leveling may always be used, but single-run leveling done with the double simultaneous procedure may be used only where it can be evaluated by loop closures. Rods should be leap-frogged between setups (alternate setup method). The date, beginning and ending times, cloud coverage, air temperature (to the nearest degree), temperature scale and average wind speed should be recorded for each section plus any changes in the date, instrumentation, observer or time zone. The instrument need not be off-leveled/re-leveled between observing the high and low scales when using an instrument with a reversible compensator. The low-high scale difference tolerance for a reversible compensator is used only for the control of blunders.
With double scale rods, the following observing sequence should be used:
- backsight, low-scale
- backsight, stadia
- foresight, low-scale
- foresight, stadia
- off-level/relevel or reverse compensator
- foresight, high-scale
- backsight, high-scale
Office ProceduresThe normalized residuals from a minimally constrained least squares adjustment will be checked for blunders. The observation weights will be checked by inspecting the post adjustment estimate of the variance of unit weight. Elevation difference standard errors computed by error propagation in a correctly weighted least squares adjustment will indicate the provisional accuracy classification. A survey variance factor ratio will be computed to check for systematic error. The least squares adjustment will use models that account for:
This completes the standards and specifications. Much to my delight, the next article of this series (which will be the last) will be written by David Zilkosky, deputy director of the National Geodetic Survey. Dave and I attended a conference together in June 2000, and he agreed to write the datum part of this series, specifically the North American Vertical Datum of 1988 (NAVD 88).