For all those of you seeking the fast answer…
The USGA and R&A provide a “local rule” that with the exception of most major professional tours (PGA, LPGA etc), is adopted for almost all competitions. So unless you’re a high level professional golfer, this most likely applies for the events you play.
For all those of you who are interested in getting a full understanding of how the rule has evolved, let’s explore the joint USGA / R&A rule and decision that allows or disallows GPS and laser rangefinders to be used.
The local rule allows the use of distance measuring devices. If your device (laser rangefinder, smartphone, or GPS rangefinder) measures other conditions such as Slope or wind speed, these features must be turned off or disabled. If you can’t turn off these features, then it’s not legal, but if you can and do turn them off, it is legal.
Rule 14/3b prohibits a player from using any artificial device or unusual equipment for the purpose of gauging or measuring distance or conditions that might affect his play. That alone would make all golf GPS and laser rangefinders against the guidelines and the penalty for the breach is disqualification. However…
In 2006, the USGA and R&A decided to allow tournament committees to have the option of adding a local rule that allows “distance-measuring devices”.
If this local rule is in effect, artificial devices that measure distance ONLY, are permitted. Prior to 2016, devices that measured slope or provided other prohibited information were not allowed even if the features were turned off.
In 2016, the USGA and the R&A revised the rule (see Appendix IV-5) to allow devices with the capability of measuring conditions other than distance, as long as these features were turned off.
The exceptions for the earlier rule were the now retired Bushnell Tour X and the the Leupold GX-4i2. Both of these laser rangefinders had gotten special approval from the USGA by using a brightly colored faceplate to turn on the Slope feature, thus indicating to anyone paying attention that the device was not legal. When the bright faceplate was removed, the devices were (and still are) legal.
With the new rule, it’s more of an honor system. Faceplates are not required, and manufacturers have implemented a simple ON/OFF switch rather than the faceplate system. The player will simply switch the device out of Slope mode to make it legal.
As far as golf GPS devices go, most have been legal in competition since 2006. However, this new rule change did affect a few devices that were previously not allowed in competition. The Garmin Approach G7 and the Approach G8 both have a “Plays Like” feature and a “Club Advice” feature that are not allowed in competition, but these features can be turned off, which means that as of 2016, these devices can be used in competition.
Smartphone golf GPS apps also benefitted from the rule change. They may now be used in competition as long as no prohibited features are being used within the app OR on otherwise on the device.
Most organized golf tournaments utilize the local rule, but it’s always a good idea to read the guidelines sheet or check with the tournament organizer before using your golf rangefinder or GPS. Elite professional events and some high-level amateur events do not adopt the local rule. You won’t find these devices being used during a PGA or LPGA tournaments, but if you’re out there during the practice rounds, caddies and players will be using laser rangefinders and taking notes diligently.
So if you know that your golf tournament or golf group has adopted the local rule, then there is only one question remaining. Did you buy a GPS or laser that measures ONLY distance or has an on/off mode for prohibited features? If your laser binoculars measure “slope” or provide other prohibited information and these features cannot be turned off, your rangefinder won’t be allowed.
If you’re playing a friendly game, just make sure your group is in agreement about what guidelines are in effect. With regards to posting scores for your handicap, the local rule is in effect. You should post all scores when using your distance-only measuring device.
(Source http://www.golfrangefindershop.com/golf-talk/is-your-golf-rangefinder-legal/)