1. Introduction

2. Principal Component Analysis (PCA)
2.1. Method
2.2. Definition of spatial scales

3. Simulations

4. PCA applied to hydrodynamic simulations

5. PCA applied to fBms: Sensitivity of the PCA calibration to the density spectrum

6. Effects of the density dispersion

7. Application to molecular clouds identified in the Galactic Ring Survey

8. Summary and conclusion