There are transformations that directly convert geodetic coordinates from one datum to another. Storm tide is measured using a geodetic vertical datum (NGVD 29 or NAVD 88). Commonly an ellipsoidal model is part of a more encompassing geodetic datum. Hellas Planitia is the lowest lying plain below the Martian geodetic datum. The importance of specifying the reference datum may be illustrated by a simple example. The new height is defined with the WGS 84 datum. HWMs on land are referenced to a vertical datum (a reference coordinate system). Lodi is flat terrain at an elevation of approximately above mean sea level datum. The datum of the space and the sheaf is called an affine scheme. A reference is distinct from the datum itself. A spot height may be expressed as AOD for "above ordnance datum". North is often defined as true north at the datum. Each value is known as a datum. The weighting for each older datum decreases exponentially, never reaching zero. The terms "prior" and "posterior" are generally relative to a specific datum or observation.