1. An apparatus for reading image data on a transaction document, said image data including a plurality of marking areas employed by a user to record marks, the apparatus comprising: sensor means for sensing, one row at a time, a plurality of rows of image data as said document is transported across said sensor means, each row including a plurality of columns, said sensor means comprising means for generating output signals representative of said pixel areas; transform means for receiving said output signals and generating bit signals representative of each pixel area, wherein said transform means comprises means for digitizing said output signals representative of said pixel areas and for providing gray scale digitized output signals, transform memory means for generating and storing bit signals for individual columns as a function of a threshold value, and means for addressing said transform memory means to select a bit signal representative of whether the gray scale digitized output signals are representative of generally dark or light pixel areas; image memory means for storing said bit signals, said image memory means being organized into rows and columns of bit signals corresponding to said plurality of rows and columns of said pixel areas of said image data; means for locating at least one portion of said image memory means representative of at least one of said plurality of marking areas of said transaction document, said means for locating being operative after rows of bit signals representative of said at least one marking area are stored in said image memory means and while additional rows of bit signals representative of other image data are being sensed, transformed, and stored in said image memory; and means for identifying marks made in said at least one marking area while said additional rows of bit signals are being stored and while other marking areas are being located by probing said bit signals within said image memory means.