Highly variable polarimetric signatures caused by complex structures in built-up areas make interpretation of these scattering behaviors intractable for polarimetric synthetic aperture radar (PolSAR) remote sensing. This article proposes a fine polarimetric decomposition method and derives several products to finely simulate the scattering mechanisms of urban buildings, thus fulfilling its use for effective surveillance. First, through theoretically establishing the roll-invariant condition for a completely general scatterer, a roll-invariant cross polarization (RICP) scattering model is constructed, which characterizes the cross polarization scattering in the manner of planar structure distribution. Second, by designing a root-discriminant-based parameter inversion strategy, a fine seven-component decomposition is proposed, which achieves the complete physical interpretation of matrix elements and reasonable inversion of model parameters. Third, by analyzing the external and internal scattering difference, the derivative products, i.e., scattering contribution synthesizers, are derived for built-up area monitoring. On the one hand, the experimental results derived from real PolSAR data confirm the superiority and effectiveness of the constructed descriptors. On the other hand, the extensibility of fine polarimetric decomposition in specific remote sensing is also explicitly demonstrated.