Systems and methods to register patient anatomy or to determine and present measurements relative to patient anatomy

Systems and methods are disclosed for use in electronic guidance systems for surgical navigation. A sensor is provided with an optical sensor, to provide optical information, and a measuring sensor, to provide measurements for determining a direction of gravity. The sensor communicates optical information and measurements to an inter-operative computing unit. In an embodiment, the inter-operative computing unit receives first optical information for a registration device and a patient anatomy and a measurement to determine a direction of gravity to perform a registration step. The inter-operative computing unit receives second optical information for the patient anatomy and an object and determines and presents measurements relative to the anatomy. The measurements relative to the anatomy are determined from the second optical information, and in relation to the registration of the anatomy of the patient.