Policy Concerning Accuracy of Stage Data


In Reply Refer To:                                    February 5, 1996
Mail Stop 415



OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 96.05

Subject:  Policy Concerning Accuracy of Stage Data

The purpose of this memorandum is to reaffirm and clarify the U.S. 
Geological Survey (USGS) policy on accuracy goals for collection of 
surface-water stage data, which was previously stated in Office of 
Surface Water (OSW) Technical Memorandum No. 93.07.   In summary, the 
previous memorandum states that stage data are to be collected with 
sufficient accuracy to support computation of discharge from a stage-
discharge relation and that procedures and equipment used are to be 
capable of sensing and recording stage with an uncertainly of no more 
than 0.01 ft or 0.20 percent of indicated reading, whichever is 
larger.

This accuracy goal addresses the intended uses of the stage data 
collected by the USGS and applies to the final result of the stage-
data collection process.  The goal expresses the combined accuracy of 
the total assemblage of instruments, equipment, and procedures to be 
used in any USGS stage-data collection system.  The goal is not 
intended as a specification for accuracy of any single component of 
the measurement system and is not intended as a specification for 
procurement of any particular kind of instrumentation or equipment.

The accuracy statement refers to the complete stage-measuring system, 
which includes all the instrumentation and equipment used at a site 
to measure stage and record its value.  In addition to the stage-
sensing component, a number of other components of instrumentation 
and equipment are needed for this purpose.   These components include:

o  The means by which the river stage is brought into communication 
with the stage-sensing component, such as a stilling well or 
bubbler system

o  The means by which the response of the sensing component is 
converted into a form suitable for recording, such as conversion of 
mechanical movement or electrical signal

o  The means by which the data are recorded and uploaded, such as 
paper chart, punched tape, or data logger

o  The means by which the recorded and uploaded values are verified 
and calibrated against direct field observations of river stages, 
such as by comparison with wire-weight, electric-tape, or staff-
gage readings

All of these components contribute incremental errors to the overall 
error or uncertainty in stage data.

In designing the installation of instrumentation and equipment for 
stage-data collection and recording at any particular site, the 
accuracy characteristics of all components of the system must be 
considered in combination.   That is, the accuracy specified for any 
component of the system should be considered in relation to the 
interconnections among the system components, the accuracies of the 
other components, and the overall accuracy goal. The accuracy 
requirement for any single component generally will be more stringent 
than the requirement for the system as a whole.

For procurement of non-submersible pressure sensors for use in 
surface-water stage measurement, the Office of Surface Water and the 
Hydrologic Instrumentation Facility jointly considered the errors 
inherent in the various components of gas-purge systems and agreed on 
the accuracy that would be required for the pressure-sensor component 
to enable the total stage-measurement system to meet the overall 
accuracy goal. In addition to pressure-sensor calibration errors, the 
errors considered include (but are not limited to) errors in reading 
reference gages and in setting pressure sensors in the field, 
movement of reference gages and gas-purge system orifices, 
obstruction of the orifice by sediment or debris, and exposure of the 
orifice to dynamic (velocity-head) pressure effects.  Qualitative 
assessment of error magnitudes and of achievable accuracies of the 
various system components led to an agreement between the Office of 
Surface Water and the Hydrologic Instrumentation Facility to adopt 
the larger of 0.01 ft or 0.10 percent of reading as the accuracy 
standard for the procurement of non-submersible pressure-sensor 
instruments.



                                    Thomas H. Yorke
                                    Chief, Office of Surface Water

WRD Distribution: A, B, FO, PO