The vertical distance of a point above a datum. The vertical datum is usually an estimate of mean sea level. Older measurements were often determined in a local coordinate system and were not tied to a global reference frame. Some were made not with surveying instruments but with barometers, in reliance on the decrease of atmospheric pressure with altitude. It is now usual to measure altitude or elevation using the Global Positioning System or an equivalent global navigation satellite system. Altitude and elevation are synonyms in common usage, although altitude is less ambiguous. The unqualified word 'elevation' can also refer, for example, to the act of elevating or to angular distance above a horizontal plane.