Alternate River/Valley Crossing Procedure for Theodolites Kendall Fancher Chief, NGS Instrumentation & Methodologies Branch

Currently two NGS approved procedures for conducting a river/valley crossing o Zeiss River Crossing Equipment Procedure o 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?

Zeiss River Crossing Equipment Instrumentation One equipment set includes: o (2) Zeiss Ni2 level instruments. o (2) rotary wedge attachments o (1) baseplate for mounting two levels on a single tripod head. o (2) targets o (1) target column o (1) 0.6m auxiliary scale

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

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

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

Measure Zenith Angles to Nearby Sighting Targets

Measure Zenith Angles to Distant Sighting Targets

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 compensation o Corrects for errors associated with an inability to perfectly level an Instrument Observing multiple data sets o Minimizes random error associated with pointing to targets at longer distances. Theodolite Procedure Error Management

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

Questions?