The rating team spends time with the facility's staff going over the course, and spends a lot of time on the course itself taking measurements of various things. The USGA recommends that the rating team play the golf course it is rating either before or after the rating visit, too.
Based on the information gleaned during the visit(s), the course rating and course slope are calculated, certified by the appropriate overseeing golf associations, and given to the club, which then posts the ratings on its scorecard and elsewhere.
Course rating used to be based almost solely on length. The longer the course, the higher the rating. But obstacles (degree of difficulty), in addition to distance, are now part of the consideration.
A scratch golfer, in this use, is defined by the USGA as a male golfer who hits his drive 250 yards and can reach a 470-yard hole in two; or a female golfer who hits her drive 210 yards and can reach a 400-yard hole in two (and, of course, plays to scratch).
A bogey golfer, in this use, is defined by the USGA as a male golfer with a handicap index of 17.5 to 22.4, who hits his drives 200 yards and can reach a 370-yard hole in two; and a female golfer with a handicap index of 21.5 to 26.4, who hits her drives 150 yards and can reach a 280-yard hole in two.
So, for example, on a 400-yard hole, the rating team goes 200 yards down the fairway to analyze the landing area for a bogey golfer; and 250 yards down the fairway to analyze the landing area for a scratch golfer. What obstacles were encountered along the way? What is the state of the fairway at each spot for each golfer - narrow or wide, hazards close by or no hazards? What angle is left to the green? What obstacles still await - water, sand, trees? How far is the approach shot from the scratch golfer's landing area and from the bogey golfer's landing area? And so on.
Taking into account length and obstacles, and experience gleaned from playing the course, the rating team evaluates the overall difficulty of the golf course under normal playing conditions and issues the course rating for scratch golfers.
But the team also computes a "bogey rating," something many golfers don't know exists for each golf course. The bogey rating is similar to course rating, it's just an evaluation of how many strokes a bogey golfer will take to play the course rather than an evaluation of strokes needed for scratch golfers.
And the bogey rating has in important role: it is used in the calculation that produces the slope rating.
Slope, remember, is a number representing the relative difficulty of a course for bogey golfers compared to scratch golfers. The calculation that determines slope is this: bogey course rating minus USGA course rating x 5.381 for men or 4.24 for women.
Effective playing length is exactly that - not the actual yardage on a hole or a shot, but how long the hole plays. A 400-yard hole will play shorter if it is downhill from the tee; or longer if it is uphill from the tee. Altitude affects playing length, and does the firmness of the fairways. Does the course produce a lot of roll-out on shots? Are there forced lay-ups?
Obstacle stroke value is a numerical rating of the difficulty presented by obstacles on the course. The course is rated in 10 categories: topography; ease or difficulty of hitting the fairway; propability of hitting the green from the fairway landing area; difficulty of bunkers and probability of hitting into them; probability of hitting out of bounds; how much water will come into play; how trees affect play; speed and contouring of the greens; and the psychological effect of all these things.
The rating team looks at all these things for both scratch golfers and bogey golfers, and from every set of tees. And then following the USGA's four formulas (male scratch golfer, female scratch golfer, male bogey golfer, female bogey golfer), some adding, subtracting, multiplying and dividing, the rating team produces its numbers.
And you thought rating a golf course was easy!