PROGRAMS AND PLANS--Policy Statement on Stage Accuracy

In Reply Refer To:                                June 2, 1989
WGS-Mail Stop 415


SUBJECT:  PROGRAMS AND PLANS--Policy Statement on Stage Accuracy

The U.S. Geological Survey (USGS) collects water-level or stage
data for many purposes.  A common purpose is to obtain a flow
characteristic that can be related directly to discharge.  Other
uses are to determine stage in estuaries, lakes, reservoirs,
streams, and ground-water levels.

This memorandum discusses policy as it relates to the measurement
of stage for the purpose of determining stream discharge at regu-
lar daily discharge gaging stations.

The USGS has traditionally used a stage-accuracy goal of + or - 0.01
foot (ft).  In recent years, many stage-sensing devices have been
marketed which are incapable of meeting this accuracy objective.
Extensive testing and evaluation of a variety of sensor systems
has been carried out at the USGS Hydrologic Instrumentation
Facility (HIF), and some pressure-based sensing systems have been
identified that offer acceptable alternatives to mercury manome-
ters and stilling wells.

Surface Water Branch Technical Memorandum 85.08 acknowledges the
difficulty of obtaining high accuracy stage measurements at sites
with unstable channels or other problems and allows for relaxing
normal accuracy goals for these stations.  Additionally, data
needs such as reconnaissance, special studies, and similar activi-
ties sometimes may be met with less accurate stage observations.
In these cases, District management is responsible for determining
acceptable accuracy requirements.

The intent of this memorandum is to reaffirm the present stage
accuracy goal of + or -  0.01 ft for daily discharge stations and
also allow for cases where lower accuracy is appropriate.  HIF's
efforts to procure new pressure-sensor systems for stage measure-
ment are a step towards achieving this accuracy goal.

Data may be used for purposes not foreseen at the time of collec-
tion, and the possibility of other uses should be considered
before modifying the general accuracy criteria.

                                       Ernest D. Cobb
                                       Acting, Chief, Office of Surface

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