Pacific Islands Water Science Center
ABOUT THE PACIFIC ISLANDS WSC
USGS IN YOUR STATE
USGS Water Science Centers are located in each state.
Possible Errors Associated with Measured Water Levels
Measured water levels may contain errors associated with (1) the measuring tapes used, (2) the measuring-point elevations at the top of the well, and (3) the plumbness of the well.
Measuring tapes that are excessively stretched or kinked may have errors that exceed 0.1 feet per 100 feet of tape. To avoid errors associated with the different measuring tapes, water-level measurements are best made using measuring tapes that were calibrated with a reference steel tape.
Measuring-point elevations of wells are the reference elevations from which groundwater levels are determined. Errors associated with measuring-point elevations have exceeded 1 foot in some cases in Hawaii. Recent advances in the portability and operation of traditional surveying equipment, and in Global Positioning System (GPS) technology, have simplified the process of obtaining a fast, accurate survey of well location coordinates and elevations.
Plumbness of the well
If the tape used to measure a water level does not hang vertically because the well is out of plumb, then the measured depth to water in the well will be too great relative to the true depth from the measuring point. For example, if the top and bottom of a small-diameter well are displaced horizontally by 1 foot relative to each other, and the true vertical distance from the top to the bottom is exactly 100 feet, a measuring tape would indicate an incorrect distance of 100.005 feet. Reported water levels reflect measured vertical-alignment errors for the few cases in which this type of information is available. For shallow or large-diameter wells, vertical alignment errors generally do not cause any error in measured water levels because measuring tapes are able to hang freely and vertically in the well.