alternate river/valley crossing procedure for theodolites kendall fancher chief, ngs instrumentation...
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![Page 1: Alternate River/Valley Crossing Procedure for Theodolites Kendall Fancher Chief, NGS Instrumentation & Methodologies Branch Kendall.Fancher@noaa.gov](https://reader031.vdocuments.mx/reader031/viewer/2022012919/5697bfd61a28abf838cae006/html5/thumbnails/1.jpg)
Alternate River/Valley Crossing Procedure for Theodolites
Kendall FancherChief, NGS Instrumentation & Methodologies [email protected]
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• Currently two NGS approved procedures for conducting a river/valley crossingo Zeiss River Crossing Equipment Procedureo Ordinary Leveling Instruments Procedure
• Both procedures require specialized equipment no longer readily available.o Zeiss River Crossing Equipment. o Optical level, such as the Fischer, possessing an
instrument horizontal line of sight tilting screw.
• There is still the need to conduct this activity.
Why Develop an Alternate River Crossing Equipment Procedure?
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Zeiss River Crossing Equipment Instrumentation
• One equipment set includes: o (2) Zeiss Ni2 level
instruments.o (2) rotary wedge
attachmentso (1) baseplate for
mounting two levels on a single tripod head.
o (2) targetso (1) target columno (1) 0.6m auxiliary scale
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Ordinary Leveling Instruments
• One equipment set includes: o (1) Fischer level or level
with tilting line of sight screw
o (1) Target assembly fabricated to NGS provided specifications
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Theodolite Instruments Equipment
• One equipment set includes: o (1) Electronic theodolite
with rotating telescope and automatic vertical tilt axis compensation
o (1) Target assembly fabricated to NGS provided specifications
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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
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Measuring Height Difference from BM to bottom Sighting Target
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Measure Zenith Angles to Nearby Sighting Targets
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Measure Zenith Angles to Distant Sighting Targets
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• Simultaneous reciprocal observations of all zenith angles to distant target assemblies.o 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)o Corrects for instrument collimation errors
• Automatic instrument vertical tilt axis compensationo Corrects for errors associated with an inability to perfectly level an
Instrument
• Observing multiple data setso Minimizes random error associated with pointing to targets at longer
distances.
Theodolite Procedure Error Management
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• 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.
Status of Procedures Development
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Questions?