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Alternate River/Valley Crossing Procedure for Theodolites. Kendall Fancher Chief, NGS Instrumentation & Methodologies Branch Kendall.Fancher@noaa.gov. Why Develop an Alternate River Crossing Equipment Procedure?. Currently two NGS approved procedures for conducting a river/valley crossing
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Alternate River/Valley Crossing Procedure for Theodolites Kendall Fancher Chief, NGS Instrumentation & Methodologies Branch Kendall.Fancher@noaa.gov
Why Develop an Alternate River Crossing Equipment Procedure? • Currently two NGS approved procedures for conducting a river/valley crossing • Zeiss River Crossing Equipment Procedure • Ordinary Leveling Instruments Procedure • Both procedures require specialized equipment no longer readily available. • Zeiss River Crossing Equipment. • Optical level, such as the Fischer, possessing an instrument horizontal line of sight tilting screw. • There is still the need to conduct this activity.
Zeiss River Crossing Equipment Instrumentation • One equipment set includes: • (2) Zeiss Ni2 level instruments. • (2) rotary wedge attachments • (1) baseplate for mounting two levels on a single tripod head. • (2) targets • (1) target column • (1) 0.6m auxiliary scale
Ordinary Leveling Instruments • One equipment set includes: • (1) Fischer level or level with tilting line of sight screw • (1) Target assembly fabricated to NGS provided specifications
Theodolite Instruments Equipment • One equipment set includes: • (1) Electronic theodolite with rotating telescope and automatic vertical tilt axis compensation • (1) Target assembly fabricated to NGS provided specifications
Theodolite Method Observing Procedure • Measure height differences from nearby benchmarks to bottom sighting targets • Measure 8 sets of D/R zenith angles to lower and upper sighting targets of nearby target assemblies • Simultaneously measure 8 sets of D/R zenith angles to lower and upper sighting targets of distant target assemblies • Break the instrument tripod setup and repeat measurements • Measure height differences from bottom sighting target to nearby benchmarks
Measuring Height Difference from BM to bottom Sighting Target
Theodolite Procedure Error Management • Simultaneous reciprocal observations of all zenith angles to distant target assemblies. • Corrects for the effects of curvature and refraction associated with unbalanced observations and varying atmospheric conditions • Double centering, or observing all zenith angles with telescope in both direct and reversed phases (face 1/face 2) • Corrects for instrument collimation errors • Automatic instrument vertical tilt axis compensation • Corrects for errors associated with an inability to perfectly level an Instrument • Observing multiple data sets • Minimizes random error associated with pointing to targets at longer distances.
Status of Procedures Development • Procedures have been field tested by different observer’s over a range of crossing distances using a variety of equipment with good comparison against height differences determined uisng direct leveling and the Zeiss River Crossing Equipment procedure. • Document is currently under technical peer review within NGS.