1. An apparatus for measuring the elevation of a three-dimensional foreground subject by first and second images photographed at two points of known height and having a distance therebetween, which comprises: photoelectric conversion means for generating electric signals representing said photographed first and second images each divided into a plurality of minute areas; a first memory coupled to said photoelectric conversion means for storing electric signals representing said first image supplied from said conversion means; a second memory coupled to said photoelectric conversion means for storing electric signals representing said second image supplied from said conversion means; a correlator coupled to said first and second memories for determining from output signals supplied from the first and second memories a minute area of said second image which has a shade most similar to that of a certain minute area of said first image; calculating means coupled to said correlator for calculating displacement of said determined minute area of the second image and said certain minute area of the first image; correcting means coupled to said calculating means for correcting a pattern of one image to the extent of said calculated displacement, said memory for said one image, correlator, calculating means and correcting means constituting a feed back circuit which is operated until the value of said displacement comes to have a prescribed level; adding means coupled to said calculating means for algebraicly summing all calculated displacements when operation of said feed back circuit is completed; and measuring means coupled to said adding means for measuring by said algebraic sum, said elevation of said three-dimensional foreground subject, said height of said two points and said distance therebetween.