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Techniques and Methods 3—A19

Levels at Gaging Stations

By Terry A. Kenney

Thumbnail of and link to report PDF (17 MB)ABSTRACT

Operational procedures at U.S. Geological Survey gaging stations include periodic leveling checks to ensure that gages are accurately set to the established gage datum. Differential leveling techniques are used to determine elevations for reference marks, reference points, all gages, and the water surface. The techniques presented in this manual provide guidance on instruments and methods that ensure gaging-station levels are run to both a high precision and accuracy. Levels are run at gaging stations whenever differences in gage readings are unresolved, stations may have been damaged, or according to a pre-determined frequency. Engineer’s levels, both optical levels and electronic digital levels, are commonly used for gaging-station levels. Collimation tests should be run at least once a week for any week that levels are run, and the absolute value of the collimation error cannot exceed 0.003 foot/100 feet (ft).

An acceptable set of gaging-station levels consists of a minimum of two foresights, each from a different instrument height, taken on at least two independent reference marks, all reference points, all gages, and the water surface. The initial instrument height is determined from another independent reference mark, known as the origin, or base reference mark. The absolute value of the closure error of a leveling circuit must be less than or equal to equation ft, where n is the total number of instrument setups, and may not exceed |0.015| ft regardless of the number of instrument setups. Closure error for a leveling circuit is distributed by instrument setup and adjusted elevations are determined. Side shots in a level circuit are assessed by examining the differences between the adjusted first and second elevations for each objective point in the circuit. The absolute value of these differences must be less than or equal to 0.005 ft. Final elevations for objective points are determined by averaging the valid adjusted first and second elevations. If final elevations indicate that the reference gage is off by |0.015| ft or more, it must be reset.

First posted September 29, 2010

For additional information contact:
Chief, Office of Surface Water
U.S. Geological Survey
12201 Sunrise Valley Drive,
Reston, VA 20192
http://water.usgs.gov/osw/

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Suggested citation:

Kenney, T.A., 2010, Levels at gaging stations: U.S. Geological Survey Techniques and Methods 3-A19, 60 p.



Contents

Abstract

Introduction

Purpose and Scope

Differential Leveling and Leveling Equipment

Establishment of Gage Datum

Frequency of Gaging-Station Levels

Preparation for Running Levels

Running Levels

Office Procedures

Auxiliary Data to be Obtained During Level Runs

Summary

References Cited

Glossary


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