The ELA, sometimes denoted ELA0, of a glacier with a climatic mass balance equal to zero on average over a number of years. The Balanced-budget ELA is usually estimated as the altitude at which a curve fitted to an observed relation between Annual ELA and Annual mass balance Ba crosses the axis Ba = 0. The uncertainty in such estimates can be substantial, especially when mass-balance sampling is sparse or the equilibrium zone occupies a large fraction of the glacier surface. The Balanced-budget ELA may differ from the steady-state ELA because it is estimated from observations made in conditions that may not approximate to steady state. In particular, most measurements of mass balance published over the past several decades have been negative.