A rectangular lot running N-S and E-W is 150 by 100 ft. To locate contours it is divided into 50-ft square blocks, and the following horizontal rod readings are taken at the corners successively along the E-W lines, proceeding from west to east, and beginning with the northernmost line: 6.8, 5.6, 3.6; 6.3, 6.9, 5.1; 5.9, 4.7, 2.6; and 3.4, 4.9, 6.3. The HI is 323.9 ft. Sketch 2-ft contours.