( ( For ( the tiltmeter ) ) ( , ( it ( ( is ( an ( angle array ) ) ) . ) ) ) )
(ROOT (S (PP (IN For) (NP (DT the) (NN tiltmeter))) (, ,) (NP (PRP it)) (VP (VBZ is) (NP (DT an) (NN angle) (NN array))) (. .)))
For the tiltmeter, it is an angle array.
(ROOT (S (NP (NNS Tiltmeters)) (VP (VBP are) (VP (VBN used) (S (VP (TO to) (VP (VB measure) (NP (NP (DT the) (NN angle)) (PP (IN of) (NP (NP (DT the) (NN slope)) (PP (IN of) (NP (DT a) (NN volcano))))))))))) (. .)))