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