Relatively stringent accuracy and precision requirements for well casing surveys and associated water-level measurements are generally difficult to achieve. Field experience and research indicate that the achieved accuracy is commonly less than expected. Realistic accuracy expectations require that the investigator know what is achievable for a site and how to convey the proper information to the surveyor. The point-difference accuracy of vertical surveys is calculated, based on the measured inaccuracy of the vertical survey divided by the square root of the length of the survey, and therefore by the size of the study area and number of wells surveyed. The accuracy of water-level measurements and contour maps is determined primarily by the equipment and procedures used for measuring the depth to water in the well casings. When setting accuracy requirements for water-level measurements and contour maps, the investigator must consider the end use of the data. Composite error from the two primary sources of error determines how much precision and accuracy are reasonably achievable.