# Dictionary:Relative ellipse area

In telluric surveying, electric-field vectors are observed at a field station (${\displaystyle E_{u}}$ and ${\displaystyle E_{v}}$) and also simultaneously at a base (${\displaystyle E_{x}}$ and ${\displaystyle E_{y}}$). The vectors are related to each other in linear combinations:
${\displaystyle E_{u}=aE_{x}+bE_{y}}$
${\displaystyle E_{v}=cE_{x}+dE_{y}.}$
The determinant of this transform, ${\displaystyle J=ad-bc}$, called the ‘‘relative ellipse area,’’ is a measure of the electrical properties at the station relative to that at the base.