Rangefinders allow a golfer to measure exact distance to the hole from their current position; they are illegal according to Rule 14-3 of the rules of golf, but the USGA allows individual course clubs to institute a local rule permitting rangefinders, and they are common among recreational golfers. The typical rangefinder is an optical device that is aimed by sighting the scope on the flag and using the calibrated gauge in the optics to estimate the distance based on the flagstick's apparent height. Other rangefinders estimate range using a calibrated focus or parallax control; the user sights the target, brings it into focus, and reads the distance mark on the control. Newer laser rangefinders operate by simply sighting any target and pressing a switch to take a very precise distance reading using an invisible laser. Newer golf carts often include GPS tracking which, combined with an electronic map of the course, can serve a similar function.
The minimum allowed diameter of a golf ball is 42.67 mm and its mass may not exceed 45.93 g. Modern golf balls have a two-, three-, or four-layer design constructed from various synthetic materials. The surface usually has a pattern of 300-450 dimples designed to improve the ball's aerodynamics by reducing the ball's drag-inducing wake and allowing spin on the ball to create lift. The method of construction and materials used greatly affect the ball's playing characteristics such as distance, trajectory, spin and feel. Harder materials, such as Surlyn, usually result in the ball's traveling longer distances, while softer covers, such as urethane, tend to generate higher spin, more "feel" and greater stopping potential. Golf balls are separated into three groups depending on their construction: two-, three-, or four-piece covers. Generally four-piece golf balls tend to be the most expensive, though price is no assurance of quality.
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