A compressor map in the standard format cannot be used directly in a performance calculation program. It is not possible to read efficiency from such a map with given speed N/sqrt(T1) and flow Wsqrt(T1)/P1 because in a part of the map the speed lines can be vertical. It is also not possible to read the map with given speed N/sqrt(T1) and pressure ratio P2/P1 because it might be, that there are two values for corrected flow at a given pressure ratio P2/P1.

The solution to the problem is to introduce auxiliary coordinates (here called beta lines) which intersect the speed lines. This allows to read the map independent from the shape of the lines with given beta and speed N/sqrt(T1).

From principle the beta grid can be selected arbitrarily as long as there are unambiguous intersections of the beta lines with the speed lines. In Smooth C the beta-lines are equally spaced straight lines or parabolas with parameter values of beta = 0...1.