Align all parts of the equation array and comment it

Question

I would like to align all parts of the equation (Not just the '=' sign) to make it looks better and comment it afterwords. Can someone please help me.

Thanks S.L.

\documentclass[12pt]{article} 
\usepackage{amsmath}


\begin{document} 
\begin{eqnarray*}
\Delta h_{100,A} + 100.00 &=& H_A\\
\Delta h_{A,200} - 107.500 &=& -H_A\\
\Delta h_{200,C} +107.500 &=& H_C\\
\Delta h_{C,100} - 100.00 &=& -H_C\\
\Delta h_{A,B} &=& H_B -H_A\\
\Delta h_{200,B} + 107.500 &=& H_B\\
\Delta h_{B,C} &=& H_C - H_B
\end{eqnarray*}
\end{document}

Answer 1

% arara: pdflatex 

\documentclass{article} 
\usepackage{mathtools}

\begin{document} 
\begin{alignat*}{2}
&\Delta h_{100,A} + 100.00 &&= \phantom{-}H_A\\
&\Delta h_{A,200} - 107.500 &&= -H_A\\
&\Delta h_{200,C} + 107.500 &&= \phantom{-}H_C\\
&\Delta h_{C,100} - 100.00 &&= -H_C\\
&\Delta h_{A,B} &&= \phantom{-}H_B -H_A\\
&\Delta h_{200,B} + 107.500 &&= \phantom{-}H_B\\
&\underbrace{\Delta h_{B,C} \phantom{{}+ 107.500}}_\text{Observation Vector} &&= \phantom{-}H_C - H_B
\end{alignat*}
\end{document}