One overlay is placed on each photograph near theobject to be measured. Theoverlays are connected by a threaded rod with a micrometer attached. Themapper varies thespacing of thetwo dots until their parallax shift appears equal to that of theobject being measured. As seen through a stereoscope, thedots merge into a single dot that appears to hover at thesame height as thetop of theobject. A reference mark whose apparent location and elevation on themodel can be controlled is called a floating mark.
Most photogrammetric instruments use a floating mark to determine location.
In these devices themark consists of a dot or cross within theviewer for each eye. Thelocation is controlled by moving theviewers across thephotographs, while theelevation is controlled by slightly varying thespacing between theviewers and thereby varying theparallax shift of thedot. Features like roads and rivers can be traced directly by following them with thefloating mark. In this way, themapper always follows a line of constant elevation. It might seem that such a technique is subjective and prone to error, since it is based on themapper's judgement whether a dot appears to rest on thesurface of a three- dimensional image.