A method for measuring blood pressure of a patient continuously and non-invasively comprises measuring a first pressure signal being proportional to arterial blood pressure of the patient at a first location by a first pressure sensor (P1). In addition ambient pressure is measured by a third ambient pressure sensor (P3). Furthermore the method comprises subtracting the signal derived from the third ambient pressure sensor from the signal derived from the first pressure sensor to compensate for alterations induced by alterations in measurement point altitude and atmospheric pressure changes thereby providing a signal representing relative systemic arterial blood pressure of the patient
