Course ratings and slope ratings are calculated for a course on the basis of a visit to the course by a USGA rating team.
The rating team will spend time with the course's staff going over the course and will spend a lot of time on the course taking measurements of various things. The USGA recommends that the rating team play the course either before or after the rating visit, too.
Based on the information gleaned during the visit(s), the course rating and course slope will be 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. Distance and obstacles now come into play.
The rating team will go over the course with an eye to how both scratch golfers and bogey golfers will play it.
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 drives 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 instance, on a 400-yard hole, the team will go 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? What angle is left to the green? What obstacles still away? How far is the approach shot for each? What is the height and reach of trees? And so on.
Taking into account length and obstacles, and experience gleaned from playing the course, the rating team will evaluate the overall difficulty of the course under normal playing conditions and issue the course rating for scratch golfers.
Likewise, they will issue the bogey rating for bogey golfers.
The bogey rating is then used to calculate slope, which, 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.
The "effective playing length" and "obstacle stroke value" are the determining factors in course rating and bogey rating.
Effective playing length is exactly that - not the actual yardage on a hole or a shot, but how it actually plays. Is there a downslope that will add yards? An upslope? Is the course at altitude, which will produce more length? What's the height of the fairway grass, which will affect roll? 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; easy 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 will look at all these things for both scratch golfers and bogey golfers, and from every set of tees. And then follow the USGA's four formulas (male scratch golfer, female scratch golfer, male bogey golfer, female bogey golfer), some adding, subtracting, multiplying and dividing, and produce its numbers.