Typically, grading is done by the resort, and grades are relative to other trails within that resort. Australia and New Zealand also share the same rating system. Ski trail difficulty is measured by flaine resort map pdf slope, not degree angle.
However, this is just a general “rule of thumb. Although slope gradient is the primary consideration in assigning a trail difficulty rating, other factors come into play. A trail will be rated by its most difficult part, even if the rest of the trail is easy. Ski trails are measured by percent slope, not degree angle. Blue Square trails make up the bulk of pistes at most ski areas, and are usually very popular and stuffed with a lot of people. Amongst the most difficult slopes at a mountain.