Hyraulic head is a specific measurement of water pressure or total energy per unit weight above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, but represents the energy at the entrance (or bottom) of a piezometer. In an aquifer, it can be calculated from the depth to water in a piezometric well (a specialized water well), and given information of the piezometer's elevation and screen depth. Hydraulic head can similarly be measured in a column of water using a standpipe piezometer by measuring the height of the water surface in the tube relative to a common datum. In both cases, although the measurement is made at the water surface, the hydraulic head measurement represents the total energy at the entrance (base) of the piezometer. The hydraulic head can be used to determine a hydraulic gradient between two or more points.