A z-value defines the vertical location of a phenomenon relative to a surface as given in a Cartesian coordinate system. To denote height (or depth), the z-value is stated relative to the plane in the x- and y-coordinate axes. z-values are extremely important in GIS for depicting surfaces in three dimensions (3D), in which every point has the pair of x- and y-coordinates in addition to the z-value. z-values can also be used to create 3D representations of phenomena that are not physically represented in the landscape, such as population or income. Note that this z-value is different from the z-score or Z-value, the measure of central tendency in standard deviation.