Yield monitoring is one of the essential steps in the implementation of precision farming. Yield maps derived from yield monitoring system, clearly show crop yield variations. Research reports on preparing hay and forage yield maps are fewer in number and extent than those on grain crop. In this research, a method was developed for estimating alfalfa hay yield from a map developed for a small square baler. To obtain the crop yield map, two series of input data are needed namely: location and crop yield. In this research, location of the baler in the field was determined through using a local coordinate system. Since hay into the square baler is gradually accumulated to form the bale, spatial measurement of the hay is done by calculating the fractional volume accumulation. This fractional volume is converted into mass, by using two bulk density scales: bulk density of the forming bale and average bulk density of total field bales. Results of this study revealed that the output of system's shaft encoders were linearly related to the number of shaft revolutions (R2>0.998). By using each bale bulk density to convert volume data into mass, the error of system's yield measuring section was reduced from 22/12% to 1/74%. The error of the coordinate determination system used for length measurement was lower than that of the regular GPS (4.01% versus 14.27%).