\subsection{Barometer}
	
	If available, the Smartphone's barometer is used to infer the likelyness of the current $z$-location%
	%	
	As ambient pressure readings are highly influenced by environmental conditions 
	like the weather, time-of-day and others \cite{Muralidharan14-BPS},
	we use relative pressure instead of absolute ones.%
	%
	Due to noisy sensors, more than one reading is used to estimate this relative base.
	The usual setup time of a navigation-system (route calculation, etc.)
	is used to average all barometer readings during this timeframe.
	This average $\overline{\mObsPressure}$ serves as relative base.
	Furthermore, we estimate the sensor's uncertainty $\sigma_\text{baro}$ for later use within the evaluation step.%
	%
	During each transition from $\mStateVec_{t-1}$ to $\mStateVec_t$, we need a corresponding, relative
	pressure prediction $\mStatePressure$ which is adjusted whenever a $z$-change happens within the transition.
	%		
	%	\begin{equation}
	%		\mState_{t}^{\mStatePressure} = \mState_{t-1}^{\mStatePressure} + \Delta z \cdot b
	%		,\enskip
	%		\Delta z = \mState_{t-1}^{z} - \mState_{t}^z
	%		,\enskip
	%		b \in \R
	%		\enspace .
	%		\label{eq:baroTransition}
	%	\end{equation}

