USEFUL RELATIONS Lineal feet x .00019 = miles Lineal yards x .0006 = miles Square inches x .007 = square feet Square feet x .111 = square yards Square yards x .0002067 = acres Acres x 4840 = square yards Cubic inches x .00058 = cubic feet Cubic feet x .03704 = cubic yards Links x .22 = yards Links x .66 = feet Feet x 1.5 = links 360° = 21600' = 1296000" Radius = arc of 57.2957790° Arc of 1° (radius = 1) = .017453292 Arc of 1' (radius = 1) = .000290888 Arc of 1" (radius = 1) = .000004848 Curvature of Earth's surface = about 0.7 feet in 1 mile Curvature in feet = 0.667 (Dist. in miles)² Difference between arc and chord length, 0.05 feet in 11½ miles Probable error of a single observation = 0.6754 √(Σv² / n - 1) Error in chaining of 0.01 feet in 100 feet: Due to— 1. Length of tape error of 0.01 feet 2. Alignment. One end 1.4 feet out of line 3. Sag of tape at center of 0.61 feet. 4. Temperature difference of 15° 5. Difference of pull of 15 lbs. SQUARE MEASURE 144 sq. inches = 1 sq. ft. 9 sq. ft. = 1 sq. yard 30¼ sq. yds. = 1 sq. rd. 40 sq. rds. = 1 rood. 4 roods = 1 acre 640 acres = 1 sq. mile. SURVEYORS' MEASURE 7.92 inches = 1 link. 25 links = 1 rcd. 4 rds. = 1 chain. 10 sq. chains or 160 sq. rods = 1 acre. 640 acres = 1 sq. mile. 36 sq. miles (6 miles sq.) = 1 township.