For the most part skis have three centers- the core center, the waist, and "true" center. The naming convention isn't official but the concepts are real. The core center is the thickest part of a ski, or in the case of a profile with a plateau, it is the center of the plateau. The waist is the narrowest part of the shape of the ski for a classic symmetrically shaped ski. For, an asymmetrical ski, there could actually be two sidecut centers, but that's a bizarre tangent and not relevant- not in 2017 at least. Then there is the "true" center, which is the important one for people spinning into the future. True center can be determined with a tape measure, right in the middle of the length measurement. Finally, there is the longitudinal inertial center. If you really care about your knees and lose sleep over tenths of millimeters, you'd actually find the balance point of the ski and call that inertial center. If you are doing that you also are probably spending sleepless evenings thinking about where on the boot you should be aligning these marks anyways, because using the midsole line on your boot probably doesn't optimize your anatomical dreidel-ness.