MEASERR - A Program for Determination of Error in Individual Discharge Measurements
VERSION/DATE: Version: 2.5 1994/04/19
DOCUMENTATION (32 Pages):
Sauer, V.B., and Meyer, R.W., 1992, Determination of error in individual discharge measurements: U.S. Geological Survey Open-File Report 92-144, 21 p.
The uncertainty, or standard error, for individual measurements of stream discharge is computed based on a root-mean-square error analysis of the individual component errors. The component errors include errors in the measurement of width, depth, and velocity, and in computation procedures. The error in measurement of velocity include the uncertainties introduced by the measurement instrument, the pulsation of flow in natural rivers, the vertical distribution of measurements, and oblique flow. This error analysis can be used to evaluate the uncertainty for most discharge measurements made with the vertical axis, cup-type current meter. The procedures do not apply to other types of current meters or other methods such as dilution gaging or ultrasonic methods. A USGS study indicates that standard errors for individual discharge measurements can range from about 2 percent under ideal conditions to about 20 percent when conditions are poor and shortcut methods are used. Some conditions, such as wind, ice, boundary effects, flow obstructions, improper
equipment, as well as incorrect measuring procedures and carelessness, can result in larger errors than indicated by the error analysis.
The standard errors computed are at the 68 percent level of significance (one standard deviation from the true value). That is, 68 percent of the measurements made under similar conditions are expected to differ from the true value by less than one standard error. The MEASERR program computes an estimate of the standard error by computing the standard error of the individual error components, and then combining these errors in a root-mean-square error analysis to compute the overall standard error of the current-meter discharge measurement.
The program prompts the user for all necessary input and alerts the user to acceptable ranges of input data. The required input data include: 8- or 15- digit station identifier, optional textual description of the measurement station, the measurement number, the channel width, the cross-sectional area, the measured mean velocity, the current-meter type, if horizontal flow were present, the average observation time, the number of verticals measured, the number of points measured in the vertical, the measurement type (waded, suspended, or acoustic), and the channel-bed type. MEASERR can also retrieve input from discharge-measurement data from a WATSTORE OUTWAT retrieval. Required data not in the OUTWAT retrieval are prompted for. Input data are expected in the U.S. customary units.
The program displays the computed standard error and stores the results in a text file. This file contains a summary of the input data, computed standard error, and corresponding qualitative rating
(excellent, good, fair, or poor) for each discharge measurement.