In Reply Refer To:
Mail Stop 415

April 18, 2011

OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 2011.05

SUBJECT: Policy on leveling procedures at streamgages and announcement of the publication Techniques and Methods Report Book 3, Section A19 “Levels at gaging stations.”

The Office of Surface Water (OSW) is pleased to announce the release of the U.S. Geological Survey (USGS) Techniques and Methods Report Book 3, Chapter A19 “Levels at gaging stations" by Terry A. Kenney. The report is available on-line at: http://pubs.usgs.gov/tm/tm3A19/ . This report describes procedures that are now OSW policy and are required for running levels at streamgages. All personnel responsible for running levels at streamgages should become acquainted with the information provided in this report.

The report describes leveling procedures that should be used to ensure that streamgages 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 report provide guidance on instruments and methods that ensure streamgage levels are run to the required precision and accuracy. A summary of leveling procedures contained in the report is as follows:

• Levels are to be run at streamgages

  • whenever there are unresolved differences in gage readings,

  • if the station has been moved or damaged, or

  • on a routine schedule according to policy requirements provided in the T&M.

• Engineer’s levels (both optical levels and electronic digital levels) can be used for streamgage 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 streamgage levels consists of a minimum of two foresights, each from a different instrument height, taken on the surveyed objective points (which must include 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 0.003*n^0.5ft, 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 the levels indicate that the reference gage is off by + 0.015 ft or more, it must be reset.
  

A summary of requirements and tolerances specified in Techniques and Methods Report Book 3, Chapter A19 is presented in Appendix E of the report.

With respect to previous OSW policy and guidelines on levels, Techniques and Methods Report Book 3, Chapter A19:

• Provides different guidelines on how to distribute closure errors,

• Provides clearer guidelines on use and scrutiny of side shots,

• Mandates compensation for temperature effects on rod-scale material,

• Mandates more frequent peg-tests, and

• Provides better guidance for assessing datum stability over time.

This new report supersedes U.S. Geological Survey (USGS) Techniques of Water-Resources Investigations Book 3, Chapter A19, 1990, “Levels at Streamflow Gaging Stations,” by E.J. Kennedy and USGS Open-File Report 81-1104, 1981 “Manual for leveling at gaging stations in North Carolina” by N.O. Thomas and N.M. Jackson, Jr.

Copies of the report have been mailed to each of the main Water Science Center offices. If you need additional copies of the report, please contact the Office of Surface Water.

Suggested citation: Kenney, T.A., 2010, Levels at gaging stations: U.S. Geological Survey Techniques and Methods 3-A19, 60 p. (Also available at http://pubs.usgs.gov/tm/tm3A19/).
Comments or suggestions about the subject report should be addressed to the Office of Surface Water.