An elevation accuracy of 5 ft. is adequate for most nodes; therefore, a USGS topographic map is typically acceptable. However, for nodes to be used for model calibration, a higher level of accuracy is desirable. Consider a situation where both the model and the actual system have exactly the same HGL of 800 ft. at a node (see figure below). The elevation of the ground (and model node) is 661.2 ft. while the elevation of the pressure gage used in calibration is 667.1 ft. The model would predict a pressure of 60.1 psi while the gage would read 57.5 psi even though the model is correct.

A similar error could occur in the opposite direction with an incorrect pressure appearing accurate because an incorrect elevation is used. This is one reason why model calibration should be done by comparing modeled and observed HGL values and not pressures.