1. An apparatus for locating an object having a predetermined projection figure on a projection surface, comprising: first means for linear scanning in a raster pattern, said first means including means for providing uni-directional non-repetitive longitudinal linear relative motion between the object and a straight line perpendicular to the longitudinal motion, a sequentially scanned linear array of radiation detectors arranged along said straight line, and means for repetitively scanning said linear array, whereby said projection figure has relative motion perpendicular to said array, second means for determining first tangent points of a first pair of tangents parallel to the line scan of said raster upon an initial detection of a projection figure by a line scan during the frame scanning motion, said initial detection being a detection of a farthest extending point of the projection figure in response to said linear scanning; said first tangent points being applied to opposite sides of said projection figure, and being the interception points of the first and last line scans to intercept said figure, third means for determining second tangent points of a second pair of tangents to said projection figure, said second pair of tangents being parallel to the frame scanning motion of said raster and being applied to opposite sides of said projection figure, said second tangent points being the interception points of the line scans which intercept said figure earliest and latest from the start of their scans, and fourth means for determining, on the basis of the relative positions of the four tangent points, first, the orientation of the object with respect to said line and frame scanning motions and, second, the position of a reference point on the object relative to the tangents.