The base elevation for any structure is the vertical distance above Mean Sea Level (MSL) of the bottom point from which its architectural height is measured. This point corresponds to the lowest grade level around the structure's perimeter, normally defined as sidewalk level.

In order to determine any structure's absolute height above sea level, its height can simply be added to its base elevation (with the exception of CTBUH heights, which are measured from entrance level which sometimes differs from ground level).

The reference point is the base of the structure. Adding a height measured from another starting level requires accounting for the difference between that level and the base.