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Geopotential height approximates the actual height of a pressure surface above mean sea-level.
Therefore, a geopotential height observation represents the height of the pressure surface on which the observation was taken. That means, at every point along a given contour, the values of geopotential height are the same. An image depicting the geopotential height field is given below. Height contours are represented by the solid lines. The height field is given in meters with an interval of 60 meters.