Free-air anomaly

Definition

Let $P$ be a station at height $h$ above sea level, and let $g_P$ be the magnitude of gravity measured at $P$. Let $\gamma_0$ be the normal gravity of the reference ellipsoid at the foot point of $P$ on the ellipsoid. Let $\partial\gamma/\partial h$ be the vertical gradient of normal gravity, which is about $-0.3086$ mGal per metre. The free-air anomaly at $P$ is $$\Delta g_{\mathrm{FA}} = g_P - \frac{\partial \gamma}{\partial h}\, h - \gamma_0 \approx g_P + 0.3086\, h - \gamma_0,$$ with $g_P$ and $\gamma_0$ in mGal and $h$ in metres.