 The River Plym , which flows off Dartmoor to the north @-@ east , forms a smaller estuary to the east of the city called <unk> . Plymouth Sound is protected from the sea by the Plymouth Breakwater , in use since 1814 . In the Sound is Drake 's Island which is seen from Plymouth Hoe , a flat public area on top of limestone cliffs . The Unitary Authority of Plymouth is 79 @.@ 84 square kilometres ( 30 @.@ 83 sq mi ) . The topography rises from sea level to a height , at <unk> , of about 509 feet ( 155 m ) above Ordnance Datum ( AOD ) . 
